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DCT Construction Manual Issue 1 Page 1 DCT DSB/CW TRANSCEIVER CONSTRUCTION MANUAL
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DCT - ozQRP.comozqrp.com/docs/DCT_manual_V1.pdf · 2019. 8. 14. · DCT Construction Manual – Issue 1 Page 6 1 INTRODUCTION The DCT (Double sideband and CW Transceiver) is an inexpensive

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  • DCT Construction Manual – Issue 1 Page 1

    DCT DSB/CW TRANSCEIVER

    CONSTRUCTION MANUAL

  • DCT Construction Manual – Issue 1 Page 2

    CONTENTS

    1 Introduction .............................................................................................................................. 6

    2 DSB vs SSB .................................................................................................................................. 7

    3 DC receiver ................................................................................................................................ 8

    4 DSB Transmitter ...................................................................................................................... 9

    5 CW Transmitter ..................................................................................................................... 10

    6 DCT Block Diagram ............................................................................................................... 11

    7 Circuit Description ................................................................................................................ 12

    7.1 Microcontroller ........................................................................................................................... 12 7.2 Connector Board ......................................................................................................................... 12 7.3 VFO ................................................................................................................................................... 13 7.4 Receive Product Detector ........................................................................................................ 13 7.5 Receive Audio Amplifier .......................................................................................................... 13 7.6 Receive Audio Filters ................................................................................................................ 13 7.7 Headphone Amplifier ................................................................................................................ 13 7.8 Microphone Amplifier ............................................................................................................... 14 7.9 Balanced Modulator .................................................................................................................. 14 7.10 Transmit Pre-driver................................................................................................................... 15 7.11 Transmit Driver ........................................................................................................................... 15 7.12 Transmit Power Amplifier ...................................................................................................... 15 7.13 Power Supply and RX/TX switching ................................................................................... 16

    8 Parts List ................................................................................................................................... 26

    8.1 Main Board SMD Parts .............................................................................................................. 26 8.2 Main Board Through Hole Parts ........................................................................................... 29 8.3 Connector Board Parts ............................................................................................................. 30 8.4 Other parts .................................................................................................................................... 30 8.5 Band Specific Parts ..................................................................................................................... 30

    9 Construction ............................................................................................................................ 31

    9.1 General ............................................................................................................................................ 31 9.1.1 Recommended tool list ................................................................................................................... 31

    9.1.2 Winding toroids................................................................................................................................. 31

    9.2 Balanced Modulator .................................................................................................................. 32 9.2.1 Trimpot ................................................................................................................................................. 32

    9.2.2 Transformer ........................................................................................................................................ 32

    9.3 Transmit Low Pass Filter ......................................................................................................... 33 9.3.1 Capacitors ............................................................................................................................................ 33

    9.3.2 Coils ........................................................................................................................................................ 33

    9.4 Receive Bandpass Filter ........................................................................................................... 34 9.4.1 Capacitors ............................................................................................................................................ 34

    9.4.2 Coil .......................................................................................................................................................... 34

    9.5 Pre-Driver ...................................................................................................................................... 35 9.5.1 BD139 transistor .............................................................................................................................. 35

    9.5.2 Transformer ........................................................................................................................................ 35

    9.5.3 Filter Capacitors ................................................................................................................................ 36

    9.5.4 Filter Coil .............................................................................................................................................. 36

  • DCT Construction Manual – Issue 1 Page 3

    9.6 Driver .............................................................................................................................................. 37 9.6.1 BD139 transistor .............................................................................................................................. 37

    9.6.2 Transformer ........................................................................................................................................ 37

    9.7 LCD ................................................................................................................................................... 38 9.7.1 Trimpot ................................................................................................................................................. 38

    9.7.2 Male header ......................................................................................................................................... 38

    9.7.3 standoffs ............................................................................................................................................... 38

    9.7.4 Female header .................................................................................................................................... 38

    9.8 Connectors and Switches ......................................................................................................... 39 9.8.1 Relay....................................................................................................................................................... 39

    9.8.2 BNC ......................................................................................................................................................... 39

    9.8.3 DC ............................................................................................................................................................ 39

    9.8.4 Slide Switch ......................................................................................................................................... 39

    9.9 RF Power Amplifier ................................................................................................................... 40 9.9.1 Trimpot ................................................................................................................................................. 40

    9.9.2 Output Transformer ........................................................................................................................ 40

    9.9.3 IRF510 ................................................................................................................................................... 41

    9.10 Connector Board ......................................................................................................................... 43 9.10.1 Board Assembly ................................................................................................................................ 43

    9.10.2 Standoffs ............................................................................................................................................... 44

    9.10.3 Connector Board Installation ...................................................................................................... 44

    9.11 Controls .......................................................................................................................................... 45 9.11.1 Rotary Encoder .................................................................................................................................. 45

    9.11.2 Potentiometers .................................................................................................................................. 45

    10 Connector board jumper settings .................................................................................... 46

    11 Making connections.............................................................................................................. 47

    11.1 Power Supply ............................................................................................................................... 47 11.1.1 DC power supply ............................................................................................................................... 47

    11.1.2 Internal Battery ................................................................................................................................. 47

    11.2 Antenna .......................................................................................................................................... 47 11.3 Morse Key ...................................................................................................................................... 47 11.4 Headphones .................................................................................................................................. 47 11.5 Speaker/Microphone ................................................................................................................ 48 11.6 Microphone ................................................................................................................................... 48

    12 First Time Power Up ............................................................................................................ 49

    13 Configuration .......................................................................................................................... 50

    13.1 To access Configuration: .......................................................................................................... 50 13.2 Navigating Configuration ........................................................................................................ 50 13.3 Configuration Items ................................................................................................................... 50

    13.3.1 Firmware .............................................................................................................................................. 50

    13.3.2 Callsign .................................................................................................................................................. 50

    13.3.3 Frequency ............................................................................................................................................ 50

    13.3.4 Freq Steps ............................................................................................................................................ 51

    13.3.5 30S Freq Save ..................................................................................................................................... 51

    13.3.6 Freq Calibrate ..................................................................................................................................... 51

    13.3.7 Volts Calibrate .................................................................................................................................... 51

    13.3.8 Pwr Mtr Cal ......................................................................................................................................... 51

  • DCT Construction Manual – Issue 1 Page 4

    14 Testing and alignment ......................................................................................................... 52

    14.1 General ............................................................................................................................................ 52 14.2 Receive ............................................................................................................................................ 52 14.3 Transmit ......................................................................................................................................... 53

    14.3.1 PA bias setup ...................................................................................................................................... 53

    14.3.2 Carrier Balance .................................................................................................................................. 53

    14.3.3 Voice Test ............................................................................................................................................. 54

    14.3.4 CW Test ................................................................................................................................................. 54

    15 Final assembly ........................................................................................................................ 56

    15.1 Case holes ...................................................................................................................................... 56 15.2 Installing the lid .......................................................................................................................... 56 15.3 Mounting in the case ................................................................................................................. 56

    16 Operation ................................................................................................................................. 57

    16.1 LCD ................................................................................................................................................... 57 16.2 Menu System ................................................................................................................................ 58 16.3 DSB Receive .................................................................................................................................. 59

    16.3.1 Tuning steps ....................................................................................................................................... 59

    16.3.2 AF Gain .................................................................................................................................................. 59

    16.3.3 Attenuator ........................................................................................................................................... 59

    16.3.4 Filter ....................................................................................................................................................... 59

    16.3.5 RIT ........................................................................................................................................................... 59

    16.4 DSB Transmit ............................................................................................................................... 59 16.5 CW Receive .................................................................................................................................... 60

    16.5.1 Filter ....................................................................................................................................................... 60

    16.6 CW transmit .................................................................................................................................. 60 16.6.1 Semi break in ...................................................................................................................................... 60

    16.6.2 Answering A Call ............................................................................................................................... 60

    16.6.3 CW drive ............................................................................................................................................... 60

    16.6.4 Key delay .............................................................................................................................................. 61

    16.6.5 Sidetone ................................................................................................................................................ 61

    16.6.6 Automatic CQ Caller ......................................................................................................................... 61

    16.7 Getting on the air ........................................................................................................................ 61

  • DCT Construction Manual – Issue 1 Page 5

    List of Figures

    Figure 1 Direct conversion receiver ......................................................................................................................... 8

    Figure 2 DSB generation ................................................................................................................................................ 9

    Figure 3 CW Transmitter ............................................................................................................................................ 10

    Figure 4 DCT Block diagram ..................................................................................................................................... 11

    Figure 5 Microcontroller ............................................................................................................................................ 17

    Figure 6 Connector Board .......................................................................................................................................... 18

    Figure 7 Main Board Connector Board interface.............................................................................................. 19

    Figure 8 VFO and LCD .................................................................................................................................................. 20

    Figure 9 Power Supply ................................................................................................................................................ 21

    Figure 10 Product detector, Attenuator, Audio Filters and Switching .................................................... 22

    Figure 11 Headphone amplifier ............................................................................................................................... 23

    Figure 12 Mic Amp, Balanced Modulator, DC switch, Pre Driver and Filter ......................................... 24

    Figure 13 Driver, Power Amplifier, LPF and Relay.......................................................................................... 25

    Figure 14 Transmit Low pass Filter components ............................................................................................ 33

    Figure 15 BD139 Lead identification .................................................................................................................... 35

    Figure 16 Typical DC circuit voltages. ................................................................................................................... 55

    Figure 17 Case holes ..................................................................................................................................................... 56

    Change History Date Issue Comments

    10/8/2019 1 First release

    Acknowledgment.

    Thanks to Peter Parker VK3YE for his invaluable assistance with prototype testing and

    suggestions for improvement during the development of this kit.

  • DCT Construction Manual – Issue 1 Page 6

    1 INTRODUCTION The DCT (Double sideband and CW Transceiver) is an inexpensive and easy to build QRP kit for the 40M or 80M band. At the core of the transceiver is a microcontroller that controls a digital VFO and LCD, and the transceiver switching and control functions. Building the DCT is quick and easy. All components mount on the PCB and there is no discrete wiring required. Most parts are SMD and these come pre-installed on the PCB. Only a few through-hole parts need installing. The PCB is a high quality double sided type with ground plane, solder mask and silk screen. The receiver doesn’t require any alignment and the only setup required for the transmitter is setting the power amplifier bias and adjusting the balance modulator to null out the DSB carrier. A kit of parts for the DCT with everything you need, including an enclosure and silkscreened lid

    is available from www.ozQRP.com.

    DCT Specifications and features:

    1. Size 196mm x 112mm x 60mm excluding knobs and connectors.

    2. Sensitive direct conversion receiver.

    3. Double Sideband voice transmitter.

    4. CW transmitter with semi break-in.

    5. Power output variable from 0 to 5W.

    6. Stable digital VFO.

    7. Customizable frequency tuning steps.

    8. Tuning, AF gain, Mic gain and CW drive controls.

    9. Easy access menu system.

    10. Callsign entry.

    11. Automatic CQ caller.

    12. Use with low cost handheld speaker/mic or separate microphone and headphones.

    13. Internal battery or external supply. Range 11 to 17V

    14. Available in 40M or 80M versions.

    15. Selectable low impedance dynamic or Electret microphone.

    16. Compatible with Baofeng low cost handheld speaker/mic.

    17. Backlit LCD displays frequency, transmit power, receive signal strength, battery voltage

    and transceiver status.

    18. Selectable audio filters: narrow 800Hz BPF or 2.3KHz LPF.

    19. 3.5mm stereo headphone connector.

    20. 3.5mm straight Morse key socket.

    21. Power slide switch.

    22. 2.1mm DC power socket – centre pin positive.

    23. BNC antenna socket.

    24. Carrier suppression up to 50dB.

    25. All spurious transmit outputs better than -46dBc.

    26. Receive current approximately 80mA.

    27. Transmit current approximately 900mA at maximum power output.

    28. Reverse polarity protection included.

    http://www.ozqrp.com/

  • DCT Construction Manual – Issue 1 Page 7

    2 DSB VS. SSB It may seem unusual that the DCT is a double sideband (DSB) transceiver when most voice

    stations on the air use single sideband (SSB). The reason for this is simple – for the amateur kit

    builder a DSB transceiver is less expensive, less complicated and easier to build and align than a

    SSB transceiver. This is mainly because a DSB transceiver doesn’t have the complication of a

    crystal filter, IF amplifier and multiple mixers used in a SSB rig.

    While a DSB rig has many advantages for the home builder, there are some things to consider.

    Firstly, a DSB transmitter occupies twice the bandwidth of a SSB transmitter. On a quiet band

    this does not cause any problems but on a crowded band it may not be as easy to find a free spot

    to operate without interfering with nearby stations. Secondly, the direct conversion receiver has

    equal response to both sidebands. This means you hear signals on both upper and lower

    sidebands simultaneously. This results in a slightly higher noise level and the possibility of

    hearing two separate stations at the same time.

    There are, however, a couple of nice advantages when a direct conversion receiver is used with

    a DSB transmitter. Firstly, you can operate with SSB stations using Upper Sideband (USB) or

    Lower Sideband (LSB) without having to change controls or move frequency. Secondly, being

    able to hear both sidebands means that you can check for other stations on both sides of your

    frequency before transmitting and avoid interfering with them.

    Finally, DSB transmissions are entirely compatible with SSB transceivers, and in fact most operators won’t be aware unless you tell them.

  • DCT Construction Manual – Issue 1 Page 8

    3 DC RECEIVER The receiver in the DCT is a direct conversion (DC) type. A DC receiver can receive both SSB and

    CW signals. The main difference compared to a superhet receiver is that the VFO operates on

    the same frequency as the received signal and there are no intermediate frequency stages.

    A simplified example of a direct conversion receiver for 40M is shown in Figure 1.

    Signals from the antenna are presented to the product detector, and mixed with the VFO signal.

    The output of the product detector contains sum and difference frequency signals. The sum

    frequency of 7.101MHz + 7.100MHz (14.201MHz) is easily filtered out by a low pass audio filter.

    However the difference frequency of 7.101MHz – 7.100MHz (1KHz) can pass through the filter

    and be heard in the headphones. This is the upper sideband response as the antenna signal

    frequency of 7.101MHz is above the 7.100MHz VFO frequency.

    Note that there is also another antenna signal that can be heard. This is the lower sideband

    signal at 7.099MHz. This would also produce a 1KHz tone in the headphones.

    This ability to simultaneously detect both upper and lower sidebands is an important

    characteristic of a direct conversion receiver.

    Figure 1 Direct conversion receiver

  • DCT Construction Manual – Issue 1 Page 9

    4 DSB TRANSMITTER

    Figure 2 shows the simplified diagram of a Double Sideband generator as used in the DCT. The

    balanced modulator is a form of mixer and the term balanced refers to the fact that the original

    input signals are suppressed at the output due to the balanced circuit configuration.

    The first input to the balanced modulator is from the VFO. The second input is audio from the

    microphone amplifier. The dominant outputs are the sum and difference frequencies, that is, the

    sum and difference of the carrier and audio frequencies. In this 40M example, 7.101MHz

    (7.100MHz + 1KHz) upper sideband, and 7.099MHz (7.100MHz – 1KHz) lower sideband.

    Again, the important thing to note is that only the sidebands are present at the output as the

    carrier and audio signals have been suppressed by the action of the balanced mixer.

    The waveform in Figure 2 at top right shows the DSB output signal in the time domain, or how it

    would be seen on an oscilloscope. Note the overlapping envelope shape that follows the audio

    waveform. The diagram at bottom right shows the DSB output signal in the frequency domain

    and how it would be seen on a spectrum analyser. The horizontal axis is frequency and the

    vertical axis is amplitude. The dotted vertical line in the middle indicates the suppressed carrier

    frequency.

    By contrast, if this was a SSB transmitter, there would be a crystal filter placed after the mixer

    and one of the sidebands would be filtered out. However, it would then be necessary to add

    another mixer to move the SSB signal onto the wanted transmit frequency.

    Figure 2 shows a single 1KHz tone for the audio signal. This is done to make it easier to

    understand the process involved. In practice there will be a range of voice band frequencies in

    the microphone signal, but the same principle applies.

    Figure 2 DSB generation

  • DCT Construction Manual – Issue 1 Page 10

    5 CW TRANSMITTER The DCT as well as being a DSB voice transmitter can transmit CW. In this mode the balanced

    modulator is again used but the microphone amplifier is turned off. The VFO applies a carrier

    frequency signal but because the modulator is balanced there will be no output without any

    other input. A simplified diagram of the DCT CW transmitter is shown in Figure 3. In practice

    the key is not directly connected to the DC switch, but monitored by the microcontroller which

    in turn controls the DC switch.

    When the key is operated the DC switch injects a DC current into the modulator. This causes it

    to become unbalanced and generate a CW carrier wave. When the key is opened again the DC

    current goes to zero and the carrier stops.

    In Figure 3 the top waveform is the output carrier signal that gets amplified and passed to the

    antenna. The bottom waveform shows the DC switch output. When the DC switch is high the

    carrier is turned on, and when it is low it is turned off.

    Figure 3 CW Transmitter

  • DCT Construction Manual – Issue 1 Page 11

    6 DCT BLOCK DIAGRAM

    Figure 4 DCT Block diagram

  • DCT Construction Manual – Issue 1 Page 12

    7 CIRCUIT DESCRIPTION

    7.1 MICROCONTROLLER

    The microcontroller in the DCT (U2) is an Atmel ATmega328 running an internal 8MHz clock. It

    controls the LCD, VFO and transceiver switching and control functions. There are limited output

    pins on U2 so a 74HC595 port expander chip (U1) is used. U1 takes clk, data and latch lines from

    U2 and outputs up to 8 individual control lines. Not all are used in this circuit.

    The 16 character by 2 line LCD is controlled via a standard 4 wire data bus and control lines.

    There are two pulse width modulated (PWM) outputs from U2. The first on pin 13 generates

    sine wave audio tone frequencies. The high frequency 32KHz PWM carrier is filtered out by R3

    and C2. The tones are fed to the audio amplifier for system beeps and for CW sidetone. The

    second output on pin 14 is low pass filtered to provide a variable DC voltage and used to vary

    the gain of the headphone amplifier.

    The analog to digital converter in U2 receives two inputs. One on pin 22 from the receiver audio

    to display signal strength on the LCD, and the second on pin 19 from the RF power amplifier to

    indicate transmit power on the LCD.

    PTT and key presses are monitored on pins 24 and 23 respectively. After processing they are

    repeated by U2.

    A rotary encoder is used as the tuning control. It produces quadrature signals when rotated and

    one line (REA) is fed into the microcontroller interrupt input (pin 32) and becomes the

    reference input. Once an interrupt is detected the state of the other line (REB) is read and the

    direction determined. The rotary encoder also incorporates a press button switch input on U2

    pin 1 and is used for a number of functions as described later. U2 incorporates pull-up resistors

    for the encoder inputs so they are normally held high and capacitors C10 and C11 help limit

    encoder switch bounce.

    The Menu pushbutton is monitored on pin 30 and is normally pulled high by an internal pull-up

    resistor.

    7.2 CONNECTOR BOARD

    The connector board is a small sub-board that is soldered into the main board via an 8 pin

    header. It holds the microphone, headphone, CW key connectors and the Menu pushbutton. The

    connector board signals are shown below:

    Pin Signal Description

    1 GND common ground

    2 SPKR output from audio amp

    3 PTT_IN input from PTT

    4 KEY_IN input from CW key

    5 GND common ground

    6 MIC_SIG input from microphone

    7 PB1 input from Menu push button

    8 5VA 5V for Electret microphone

  • DCT Construction Manual – Issue 1 Page 13

    7.3 VFO

    A Si5351A clock generator chip (U5) is used as the VFO. A 25MHz crystal supplies the master

    clock on pins 2 and 3. The Si5351A is controlled over the I2C bus by microcontroller U2. The

    3.3V power supply rail is applied to pins 1 and 7. Capacitors C8 andC12 and a ferrite bead help

    guard against instability and decouple the IC from the power supply. The RX VFO appears on pin

    10 and the TX VFO output is on pin 9.

    MOSFETs Q1 and Q2 form a bi-directional level shifter between the 5 volt microcontroller logic

    level and the 3.3V logic level of the VFO.

    7.4 RECEIVE PRODUCT DETECTOR

    Signals from the antenna are passed through a bandpass filter comprised of L1, C18 and C19.

    The ratio of the two capacitors provides an impedance conversion between the 50 ohm antenna

    and the approximate 1K ohm single ended input impedance of the product detector U6. Diodes

    D1 and D2 protect U6 from overvoltage spikes.

    U6 is a SA612 and takes antenna signals on pin 1 and produces balanced audio on pins 4 and 5.

    The 3.3V pk-pk VFO signal is reduced through a resistive divider network (R16 and R17) to

    around 500mV pk-pk. Power for U6 comes from a 6.8V Zener diode and filtered by C23 and C24.

    7.5 RECEIVE AUDIO AMPLIFIER

    The output of U6 is input to differential amplifier U7a with a gain of around 10 times or 20dB.

    To keep high frequencies out of the audio, low pass filtering is applied through capacitors C29

    and C31.

    Non-inverting amplifier U7b is configured as a switchable gain amplifier. With transistors Q3

    and Q4 off the gain of U7b is unity or 0dB. When transistor Q3 is turned on capacitor C32 is

    connected to ground via R26 and the gain increases to around 10 or 20dB. When Q3 is off and

    transistor Q4 is turned on the gain is around 3.2 or 10dB.

    The switching is controlled by the microcontroller to act as an adjustable receive attenuator.

    7.6 RECEIVE AUDIO FILTERS

    The output of the audio amplifier is applied to a 2.3KHz low pass filter formed with U8a.

    Connected directly to the output of U8A is U8b configured as an 800Hz bandpass filter.

    A half rail power supply or pseudo ground for the op-amps is formed by R19 and R20.

    The filter outputs are switched through to the AF gain control via electronic switches. The low

    pass filter switch uses Q5 and Q7, while the band pass filter switch uses Q6 and Q8. The

    switches are under control of the microcontroller and selects which filter is in circuit. To enable

    a filter, the associated MOSFET gate is taken high which pulls the drain low and the base of the

    PNP transistor low. Audio can now pass via the low impedance of the transistor emitter to

    collector path via C38 to the AF gain control VR2. Sampled audio is taken via C37 and DC biased

    to half 5V for the signal meter A/D input of the microcontroller.

    7.7 HEADPHONE AMPLIFIER

    The headphone amplifier U9 is a LM4875. It is a bridge-tied load (BTL) output device normally

    powering a loudspeaker in balanced mode, but in this circuit it is used in a single ended

    headphone mode. This reduces the overall power output available but is adequate to drive

    headphones and also the small speaker in handheld speaker/mics.

  • DCT Construction Manual – Issue 1 Page 14

    Pin 2 is the audio input and receives the receiver audio as well as tones generated by the

    microcontroller.

    Power is supplied by a 5 volt regulator REG3 and filtered by C47 and C48. The audio output is

    via capacitor C49 to the connector board. A 100 ohm resistor (R2) is placed across the

    headphone line to guard against instability.

    A feature of U9 is that it has a DC voltage controlled gain pin (pin 4). A voltage of about 3.5 volts

    gives maximum gain and minimum gain is about 1.2 volts. The chip can be completely shut off

    (muted) if the voltage is between 0.5 and 1 volt.

    The gain is controlled by a microcontroller PWM output filtered by R44 and C46 to provide a

    smooth DC control signal.

    7.8 MICROPHONE AMPLIFIER

    Microphone signals are coupled via capacitor C50 into the common emitter amplifier transistor

    Q10. Capacitor C51 prevents RF getting into the base circuit. The gain is set to 20 mainly by

    resistor R61. The output is applied to a discrete 2.3KHz low pass filter formed around transistor

    Q12. The object here is to limit the transmitted bandwidth as we are occupying twice the

    bandwidth of a SSB signal. The Mic gain control is the emitter resistor of Q12. The output of the

    filter is amplified about 3 times and buffered by Q13. The low value collector resistor ensures a

    low impedance drive to the modulator.

    Power to the microphone amplifier is supplied via electronic switch Q5 and Q11. This allows the

    microcontroller to turn off the amplifier in DSB receive and CW modes.

    7.9 BALANCED MODULATOR

    The balanced modulator is formed with T1, D3, D4, VR5 and L2. The TX VFO signal is applied to

    the primary winding of T1. The secondary winding of T1 is made up of 2 windings where the

    start of one winding is connected to the end of the second winding to form a centre tap. The

    centre tap is effectively at ground potential to RF due to capacitor C60. The wiper of the

    trimmer resistor is at ground potential for audio frequencies and DC due to choke L2.

    The signal from TX VFO induces alternating current in the secondary of T1. During one half of

    the waveform the Anode of D3 will be positive and the Cathode of D4 will be negative with

    respect to ground. This causes both diodes to conduct equally resulting in the voltage at the

    wiper to be zero. When the TX VFO polarity reverses, the Anode of D3 will be negative and the

    Cathode of D4 will be positive and both diodes will be biased off. Again the voltage at the wiper

    will be zero. This is called the balanced state because no VFO signal is being output.

    In DSB mode an audio signal is injected into the centre tap and the mixer balance is upset

    because the audio also induces current in the diodes. Because the audio signal changes state

    much less frequently than the carrier signal the instantaneous diode currents are not equal. As a

    result a signal is now developed at the wiper of VR5 and is a double sideband suppressed

    carrier waveform.

    Due to variations in component parameters the balance is not exact and if not compensated for

    the some VFO signal (carrier) would be always present. Trimpot VR5 is used to balance the

    diode currents in each side and bring the modulator into balance. In practice up to 50dB of

    carrier suppression can be achieved.

  • DCT Construction Manual – Issue 1 Page 15

    In CW mode, when the key is pressed, the microcontroller takes the KEY_OUT signal high and a

    voltage appears across the CW drive potentiometer VR4. The wiper supplies current to the base

    of Q14 and in turn a current through R69. This current passes into the centre tap of T1 and to

    ground via choke L2.

    A DC current in the modulator upsets the balance because one diode is turned on more than the

    other and so RF is present at the trimpot wiper. Capacitor C59 is used to create a small ramp up

    and down of the base voltage and hence modulator current to prevent sharp carrier on off

    transitions that cause key clicks.

    7.10 TRANSMIT PRE-DRIVER

    The small signal from the modulator requires three stages of amplification to lift it to the 5 watt

    level at the antenna. The first stage is the pre-driver formed around BD139 transistor Q15. It is

    biased to 25mA collector current and has a large amount of negative feedback to stabilize

    performance. Its loaded gain is low but its main purpose is to act as a buffer stage between the

    modulator and the following filter.

    A 3 pole low pass filter is formed with C64, C66 and L1 and is included to reduce the harmonic

    content from the modulation stage. The addition of C65 across L1 forms a parallel circuit to

    create a deep notch at the second harmonic.

    7.11 TRANSMIT DRIVER

    Transmit signal from the pre-driver is applied to the driver stage built around transistor Q16. A

    BD139 works well here when biased with about 50mA of collector current. The design is well

    proven using both shunt and series feedback to provide low input and output impedance and

    good stable gain on the low HF bands.

    7.12 TRANSMIT POWER AMPLIFIER

    The power amplifier (Q17) is an IRF510 MOSFET and can provide in excess of 5 Watts PEP from

    a 13.8 V drain supply. The output from the driver is applied across resistor R82 and becomes

    the AC drive component for Q17 gate. Zener diode ZD2 and trimpot VR6 provides a stable and

    variable DC gate voltage to place Q17 just into conduction for linear service. There is a short

    ramp up of the gate voltage when switching to TX state as capacitor C71 charges and is included

    to provide a smooth gate voltage transition.

    The drain load for Q17 is a broadband bi-filar wound transformer (T4) and this configuration

    was found to provide maximum output into a 50 ohm load. The waveform from Q17 can be high

    in harmonics and so a 7 pole low pass filter is included to reduce the level of harmonic and other

    spurious energy to an acceptable level.

    The transmit signal is sampled by capacitor C88 and ground referenced by R88. The signal is

    rectified by D6 and filtered by C89. This voltage is reduced in resistive divider R86 and R87 and

    input to the microcontroller A/D to display power output on the LCD bar graph.

  • DCT Construction Manual – Issue 1 Page 16

    7.13 POWER SUPPLY AND RX/TX SWITCHING

    When the PTT is operated the TX/RX relay is energized and the transmit signal is passed to the

    antenna. When the PTT is not operated the relay switches the antenna through to the receive

    circuits. The relay also switches power to the TX and RX sections as required.

    A ‘P’ channel power FET (Q19) protects the components in case the power supply is connected

    in reverse. It works like this. For a P channel MOSFET to be turned on the gate must be at least

    4V negative with respect to the source. The internal structure of a MOSFET has a diode

    connected between the drain and source terminals and is reverse biased in its usual

    application. However in this circuit if power is applied with the correct polarity, the internal

    diode will be biased on and the source voltage will be around a volt below the drain voltage.

    The gate is effectively at ground potential because of the 10K resistor and so the gate voltage is

    more than 4V negative with respect to the source. This turns on the MOSFET and the drain to

    source becomes a very low resistance, bypassing the diode, and supplying power to the board.

    If the power supply is connected with reverse polarity the internal MOSFET diode will not

    conduct. As a result the gate to source voltage never exceeds -4V and the MOSFET remains off

    protecting the board. Zener diode ZD3 protects the gate from voltage spikes.

    Un-switched 12V is distributed around the board to linear regulators to power the

    microcontroller, VFO and headphone amplifier.

  • DCT Construction Manual – Issue 1 Page 17

    Figure 5 Microcontroller

  • DCT Construction Manual – Issue 1 Page 18

    Figure 6 Connector Board

  • DCT Construction Manual – Issue 1 Page 19

    Figure 7 Main Board Connector Board interface

  • DCT Construction Manual – Issue 1 Page 20

    Figure 8 VFO and LCD

  • DCT Construction Manual – Issue 1 Page 21

    Figure 9 Power Supply

  • DCT Construction Manual – Issue 1 Page 22

    Figure 10 Product detector, Attenuator, Audio Filters and Switching

  • DCT Construction Manual – Issue 1 Page 23

    Figure 11 Headphone amplifier

  • DCT Construction Manual – Issue 1 Page 24

    Figure 12 Mic Amp, Balanced Modulator, DC switch, Pre Driver and Filter

  • DCT Construction Manual – Issue 1 Page 25

    Figure 13 Driver, Power Amplifier, LPF and Relay

  • DCT Construction Manual – Issue 1 Page 26

    8 PARTS LIST

    8.1 MAIN BOARD SMD PARTS

    Capacitors

    Des Value Des Value

    C1 100nF 50V 10% Ceramic MLCC C41 10nF 50V 10% Ceramic MLCC

    C2 10nF 50V 10% Ceramic MLCC C42 10nF 50V 10% Ceramic MLCC

    C3 1nF 50V 10% Ceramic MLCC C43 100nF 50V 10% Ceramic MLCC

    C4 1nF 50V 10% Ceramic MLCC C44 10uF 25V electro

    C5 1nF 50V 10% Ceramic MLCC C45 1uF 50V 10% Ceramic MLCC

    C6 100nF 50V 10% Ceramic MLCC C46 1uF 50V 10% Ceramic MLCC

    C7 100nF 50V 10% Ceramic MLCC C47 100uF 25V electro

    C8 100nF 50V 10% Ceramic MLCC C48 100nF 50V 10% Ceramic MLCC

    C9 100nF 50V 10% Ceramic MLCC C49 100uF 25V electro

    C10 10nF 50V 10% Ceramic MLCC C50 100nF 50V 10% Ceramic MLCC

    C11 10nF 50V 10% Ceramic MLCC C51 10nF 50V 10% Ceramic MLCC

    C12 1uF 50V 10% Ceramic MLCC C52 100uF 25V electro

    C13 1uF 50V 10% Ceramic MLCC C53 10uF 25V electro

    C14 100nF 50V 10% Ceramic MLCC C54 10nF 50V 10% Ceramic MLCC

    C15 100nF 50V 10% Ceramic MLCC C55 4.7nF 50V 10% Ceramic MLCC

    C16 1uF 50V 10% Ceramic MLCC C56 100nF 50V 10% Ceramic MLCC

    C17 100nF 50V 10% Ceramic MLCC C57 1uF 50V electro

    C20 1nF 50V 10% Ceramic MLCC C58 100nF 50V 10% Ceramic MLCC

    C21 100nF 50V 10% Ceramic MLCC C59 100nF 50V 10% Ceramic MLCC

    C22 1nF 50V 10% Ceramic MLCC C60 10nF 50V 10% Ceramic MLCC

    C23 10uF 25V electro C61 100nF 50V 10% Ceramic MLCC

    C24 100nF 50V 10% Ceramic MLCC C62 100nF 50V 10% Ceramic MLCC

    C25 47nF 50V 10% Ceramic MLCC C63 100nF 50V 10% Ceramic MLCC

    C26 100uF 25V electro C67 100nF 50V 10% Ceramic MLCC

    C27 47nF 50V 10% Ceramic MLCC C68 100nF 50V 10% Ceramic MLCC

    C28 47nF 50V 10% Ceramic MLCC C69 100nF 50V 10% Ceramic MLCC

    C27 470pF 50V 10% Ceramic MLCC C70 100nF 50V 10% Ceramic MLCC

    C30 100uF 25V electro C71 1uF 50V electro

    C31 470pF 50V 10% Ceramic MLCC C72 100nF 50V 10% Ceramic MLCC

    C32 1uF 50V electro C73 100nF 50V 10% Ceramic MLCC

    C33 2.2nF 50V 10% Ceramic MLCC C74 100nF 50V 10% Ceramic MLCC

    C34 10nF 50V 10% Ceramic MLCC C75 100nF 50V 10% Ceramic MLCC

    C35 100uF 25V electro C76 100uF 25V electro

    C36 4.7nF 50V 10% Ceramic MLCC C77 100uF 25V electro

    C37 100nF 50V 10% Ceramic MLCC C78 100nF 50V 10% Ceramic MLCC

    C38 1uF 50V electro C79 100nF 50V 10% Ceramic MLCC

    C39 10nF 50V 10% Ceramic MLCC C88 22pF C0G 50V 10% Ceramic MLCC

    C40 10nF 50V 10% Ceramic MLCC C89 100nF 50V 10% Ceramic MLCC

  • DCT Construction Manual – Issue 1 Page 27

    Resistors

    Des Value Des Value

    R1 10K 1/8W 1% 0805 R45 1K 1/8W 1% 0805

    R2 100Ω 1/8W 1% 0805 R46 100K 1/8W 1% 0805

    R3 4.7K 1/8W 1% 0805 R47 1K 1/8W 1% 0805

    R4 1K 1/8W 1% 0805 R48 100K 1/8W 1% 0805

    R5 1K 1/8W 1% 0805 R49 4.7K 1/8W 1% 0805

    R6 1K 1/8W 1% 0805 R50 4.7K 1/8W 1% 0805

    R7 30K 1/8W 1% 0805 R51 100K 1/8W 1% 0805

    R8 10K 1/8W 1% 0805 R52 100K 1/8W 1% 0805

    R9 4.7K 1/8W 1% 0805 R53 1K 1/8W 1% 0805

    R10 4.7K 1/8W 1% 0805 R54 100K 1/8W 1% 0805

    R11 22K 1/8W 1% 0805 R55 470K 1/8W 1% 0805

    R12 4.7K 1/8W 1% 0805 R56 4.7K 1/8W 1% 0805

    R13 2.2K 1/8W 1% 0805 R57 470Ω 1/8W 1% 0805

    R14 2.2K 1/8W 1% 0805 R58 4.7K 1/8W 1% 0805

    R15 4.7K 1/8W 1% 0805 R59 100K 1/8W 1% 0805

    R16 470Ω 1/8W 1% 0805 R60 4.7K 1/8W 1% 0805

    R17 100Ω 1/8W 1% 0805 R61 100Ω 1/8W 1% 0805

    R18 470Ω 1/8W 1% 0805 R62 470Ω 1/8W 1% 0805

    R19 4.7K 1/8W 1% 0805 R63 10K 1/8W 1% 0805

    R20 4.7K 1/8W 1% 0805 R64 56K 1/8W 1% 0805

    R21 10K 1/8W 1% 0805 R65 560Ω 1/8W 1% 0805

    R22 10K 1/8W 1% 0805 R66 100Ω 1/8W 1% 0805

    R23 100K 1/8W 1% 0805 R67 1K 1/8W 1% 0805

    R24 470Ω 1/8W 1% 0805 R68 10K 1/8W 1% 0805

    R25 100K 1/8W 1% 0805 R69 2.2K 1/8W 1% 0805

    R26 1K 1/8W 1% 0805 R70 100Ω 1/8W 1% 0805

    R27 100K 1/8W 1% 0805 R71 470Ω 1/8W 1% 0805

    R28 3.9K 1/8W 1% 0805 R72 2.2K 1/8W 1% 0805

    R29 10K 1/8W 1% 0805 R73 100Ω 1/8W 1% 0805

    R30 4.7K 1/8W 1% 0805 R74 100Ω 1/8W 1% 0805

    R31 1K 1/8W 1% 0805 R75 47Ω 1/8W 1% 0805

    R32 10K 1/8W 1% 0805 R76 470Ω 1/8W 1% 0805

    R33 10K 1/8W 1% 0805 R77 2.2K 1/8W 1% 0805

    R34 10K 1/8W 1% 0805 R78 10Ω 1/8W 1% 0805

    R35 100K 1/8W 1% 0805 R79 27Ω 1/8W 1% 0805

    R36 10K 1/8W 1% 0805 R80 10Ω 1/8W 1% 0805

    R37 4.7K 1/8W 1% 0805 R81 560Ω 1/8W 1% 0805

    R38 470Ω 1/8W 1% 0805 R82 47Ω 1/8W 1% 0805

    R39 62K 1/8W 1% 0805 R83 1K 1/8W 1% 0805

    R40 3.3K 1/8W 1% 0805 R84 100K 1/8W 1% 0805

    R41 4.7K 1/8W 1% 0805 R85 10K 1/8W 1% 0805

    R42 10K 1/8W 1% 0805 R86 27K 1/8W 1% 0805

    R43 130K 1/8W 1% 0805 R87 10K 1/8W 1% 0805

    R44 4.7K 1/8W 1% 0805 R88 2.2K 1/8W 1% 0805

  • DCT Construction Manual – Issue 1 Page 28

    Semiconductors

    Des Value Des Value

    D1 1N4148W signal diode SOD-123 Q10 MMBT3904 NPN transistor SOT23-3

    D2 1N4148W signal diode SOD-123 Q11 MMBT3906 NPN transistor SOT23-3

    D3 1N4148W signal diode SOD-123 Q12 MMBT3904 NPN transistor SOT23-3

    D4 1N4148W signal diode SOD-123 Q13 MMBT3904 NPN transistor SOT23-3

    D5 1N4148W signal diode SOD-123 Q14 MMBT3904 NPN transistor SOT23-3

    D6 1N4148W signal diode SOD-123 Q18 BSS138 N-ch MOSFET SOT23-3

    ZD1 6.8V 500mW Zener SOD-123 Q19 NDT452AP P-ch MOSFET SOT223-4

    ZD2 6.8V 500mW Zener SOD-123 REG1 78L05 regulator SOT89-3

    ZD3 6.8V 500mW Zener SOD-123 REG2 MCP7100-3.3 regulator SOT23-3

    Q1 BSS138 N-ch MOSFET SOT23-3 REG3 78L05 regulator SOT89-3

    Q2 BSS138 N-ch MOSFET SOT23-3 U1 74HC595 Ser to Par SOIC-16

    Q3 MMBT3904 NPN transistor SOT23-3 U2 ATmega328PA Micro 32TQFP

    Q4 MMBT3904 NPN transistor SOT23-3 U5 S15351 Oscillator 10MSOP

    Q5 BSS138 N-ch MOSFET SOT23-3 U6 SA612 Mixer SOIC-8

    Q6 BSS138 N-ch MOSFET SOT23-3 U7 LM833 Dual op-amp SOIC-8

    Q7 MMBT3906 PNP transistor SOT23-3 U8 LM833 Dual op-amp SOIC-8

    Q8 MMBT3906 PNP transistor SOT23-3 U9 LM4875 headphone amp SOIC-8

    Q9 BSS138 N-ch MOSFET SOT23-3 X1 25MHz crystal 5x3.2

    Coils

    Des Value Des Value

    FB1 Ferrite bead 0805 L2 100uH 1812

  • DCT Construction Manual – Issue 1 Page 29

    8.2 MAIN BOARD THROUGH HOLE PARTS

    Resistors

    Des Value Des Value

    VR1 20K vertical trimpot VR4 10K LIN 9mm vertical potentiometer

    VR2 10K LOG 9mm vertical potentiometer VR5 500Ω multi-turn vertical trimpot

    VR3 10K LOG 9mm vertical potentiometer VR6 20K vertical trimpot

    Semiconductors

    Des Value Des Value

    Q15 BD139 transistor TO-126 Q17 IRF510 N-ch Power MOSFET TO-220

    Q16 BD139 transistor TO-126

    Connectors

    Des Value Des Value

    J1 BNC connector RA LCD1 16 pin male header

    SK3 2.1mm DC connector RA

    Switches

    Des Value Des Value

    SW1 SPDT slide switch RA K1 DPDT relay

    RE1 Rotary encoder with switch vertical

    Coils

    Des Value Des Value

    T1 FT37-43 5:3+3 T3 FT37-43 8T:3T

    T2 FT37-43 8T:3T T4 FT50-43 5T+5T

  • DCT Construction Manual – Issue 1 Page 30

    8.3 CONNECTOR BOARD PARTS

    Resistors

    Des Value Des Value

    J1 2.5mm stereo socket R1 2.2K 1/4W 5% resistor

    J2 3.5mm stereo socket R2 22 Ω 1/4W 5% resistor

    J3 3.5mm stereo socket SK1 8 pin male header RA

    J4 3.5mm stereo socket SK2 2 x 6 pin male header

    PB1 6mm tactile switch RA

    8.4 OTHER PARTS

    QTY Value

    1 16 x 2 blue backlight LCD display

    4 Knobs with screw

    1 Silkscreen printed lid

    1 plastic case

    4 metal 20mm spacers

    4 nylon 12mm spacers

    12 3mm x 6mm screw - zinc

    4 3mm x 6mm screw - black

    4 No. 4 self tapping screw - black

    4 3mm star washers

    1 3mm x 10mm screw - zinc

    1 3mm nut

    1 TO-220 insulating washer

    1 3mm insulating bush

    4 shunts

    4 self-adhesive feet

    8.5 BAND SPECIFIC PARTS

    See section 9 for component details.

  • DCT Construction Manual – Issue 1 Page 31

    9 CONSTRUCTION

    9.1 GENERAL

    The DCT is built on a high quality fiberglass PCB. The PCB is doubled sided with the majority of

    the tracks on the bottom side with the top side forming a ground plane.

    To assist construction the component overlay is screen printed on the top side and a solder

    mask is included to help guard against solder bridges.

    The ground plane is substantial and can sink quite a bit of heat from low wattage soldering irons

    so ensure you use a good quality iron that can sustain the power required. You may find that

    sometimes solder doesn’t appear to flow through to the top side. This is not necessarily a

    problem because the plated through holes make a connection to the top side automatically.

    Another point to consider is that plated through holes consume more solder than non-plated

    holes and makes it more difficult to remove components. So check the value and orientation of

    components before soldering!

    To make assembly much simpler the PCB comes with all SMD parts already soldered. The

    construction only requires the installation of the through hole parts and mechanical assembly.

    9.1.1 RECOMMENDED TOOL LIST Below is a minimum list of tools required to build the DCT:

    soldering iron

    solder

    side cutters

    drill

    reamer

    small file

    small knife or scalpel

    metric rule

    Phillips head screwdriver

    Small flat blade screwdriver

    9.1.2 WINDING TOROIDS Winding toroids is a task that many first time builders find difficult. However with patience and

    attention to detail it’s not that hard. Here are a few tips:

    Cut the wire to length before starting.

    Whenever a wire passes through the centre of the toroid, it is counted as a turn, so count

    inside not outside.

    For windings with many turns it is easier to pass the wire though the hole to half way.

    Wind on half the number of turns with one end and then repeat for the other end.

    Use a knife, scalpel or sandpaper to remove the enamel from the ends of the wires and

    then tin with solder before inserting in the PCB.

    To twist a bifilar winding, a battery drill with a cup hook in the chuck makes it easy.

    Hold one end firm and loop the other end over the cup hook. Run the drill until the

    desired number of twists per centimetre is achieved.

  • DCT Construction Manual – Issue 1 Page 32

    9.2 BALANCED MODULATOR

    9.2.1 TRIMPOT Insert the 500 ohm multi-turn trimpot into the PCB and solder in place. The adjustment screw faces out from the edge of the PCB.

    9.2.2 TRANSFORMER The transformer is wound on a FT37-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.

    Designator Winding Length Turns

    T1 Primary 100mm 5

    Secondary 100mm x 2 3+3

    The secondary is a bifilar winding. Take the two lengths and place alongside each other. Then, holding one pair of ends firm, rotate the other pair of ends until there are about 3 twists per centimetre. The combined secondary wires are then wound on the toroid as one. Scrape enamel off the end of each wire and spread them apart. Determine the start and end of each of the secondary windings with a multimeter set to continuity. Take the start of one winding and the end of the other and twist them together to form a centre tap. Tin the wire ends with solder before inserting into the PCB.

    Primary

    Start 1 Start 2

    End 1

    End 2

  • DCT Construction Manual – Issue 1 Page 33

    9.3 TRANSMIT LOW PASS FILTER

    9.3.1 CAPACITORS The PCB has provision for two capacitors at each location, however only one is required and

    installed as shown below.

    Designator 80M 40M

    Marking Value Marking Value

    C80 821 820pF 471 470pF

    C81 - - - -

    C82 152 1500pF 821 820pF

    C83 - - - -

    C84 152 1500pF 821 820pF

    C85 - - - -

    C86 821 820pF 471 470pF

    C87 - - - -

    9.3.2 COILS All coils are wound on red T37-2 powdered iron toroids using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling. Spread the turns to cover about 80% of the circumference. Scrape the enamel off the ends of the wires and tin with solder before inserting into the PCB.

    Designator 80M 40M

    Wire length Turns Wire length Turns

    L4 360mm 25 280mm 18

    L5 360mm 26 280mm 19

    L6 360mm 25 280mm 18

    Figure 14 Transmit Low pass Filter components

  • DCT Construction Manual – Issue 1 Page 34

    9.4 RECEIVE BANDPASS FILTER

    9.4.1 CAPACITORS Select and install the capacitors as shown below. The kit may be supplied with disc ceramic,

    MLCC ceramic or a mix of types.

    Designator 80M 40M

    Marking Value Marking Value

    C18 561 560pF 471 470pF

    C19 222 2200pF 152 1500pF

    9.4.2 COIL The coil is wound on a red T37-2 powdered iron toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling. Spread the turns to evenly over the circumference. Scrape the enamel off the ends of the wires and tin with solder before inserting into the PCB.

    Designator

    80M (3.5 – 3.7) 80M (3.7- 3.9) 40M

    Wire length

    Turns Wire

    length Turns

    Wire length

    Turns

    L1 440mm 33 420mm 31 250mm 18

  • DCT Construction Manual – Issue 1 Page 35

    9.5 PRE-DRIVER

    9.5.1 BD139 TRANSISTOR Figure 15 shows how to identify the BD139 pins. With the transistor placed on a flat surface,

    and the writing facing up, the Base lead is on the right hand side.

    Figure 15 BD139 Lead identification

    Insert the transistor into the PCB with the Base lead towards capacitor C61. Push down until it

    rests against the PCB and solder in place.

    9.5.2 TRANSFORMER The transformer is wound on a FT37-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.

    Designator Winding Length Turns

    T2 Primary 140mm 8

    Secondary 80mm 3

    Scrape enamel off the end of each wire and tin the wire ends with solder before inserting into the PCB.

  • DCT Construction Manual – Issue 1 Page 36

    9.5.3 FILTER CAPACITORS Select and install the capacitors as shown below. The kit may be supplied with disc ceramic,

    MLCC ceramic or a mix of types.

    Designator 80M 40M

    Marking Value Marking Value

    C64 681 680pF 391 390pF

    C65 221 220pF 151 150pF

    C66 681 680pF 391 390pF

    9.5.4 FILTER COIL The coil is wound on a red T37-2 powdered iron toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling. Spread the turns to cover about 80% of the circumference. Scrape the enamel off the ends of the wires and tin with solder before inserting into the PCB.

    Designator 80M 40M

    Wire length Turns Wire length Turns

    L3 290mm 21 200mm 14

  • DCT Construction Manual – Issue 1 Page 37

    9.6 DRIVER

    9.6.1 BD139 TRANSISTOR Insert the transistor into the PCB with the Base lead towards capacitor C67. Push down until it

    rests against the PCB and solder in place. See Figure 15 for lead identification.

    9.6.2 TRANSFORMER The transformer is wound on a FT37-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.

    Designator Winding Length Turns

    T3 Primary 140mm 8

    Secondary 80mm 3

    Scrape enamel off the end of each wire and tin the wire ends with solder before inserting into the PCB.

  • DCT Construction Manual – Issue 1 Page 38

    9.7 LCD

    9.7.1 TRIMPOT Insert a 20K trimpot into position VR1 and solder.

    9.7.2 MALE HEADER Insert a 16 pin male header into the PCB with the long pins facing upwards. Solder a single pin

    first and check the strip is sitting flat and at right angles to the PCB before soldering the

    remaining pins. If the kit is supplied with a strip longer than 16 pins, simply snap off the

    unwanted pins.

    9.7.3 STANDOFFS Mount four 12mm nylon spacers into the LCD mounting holes using 3mm x 6mm long screws.

    9.7.4 FEMALE HEADER Insert the 16 pin female header into the LCD from the rear. Solder a single pin first and check it

    is sitting flat and at right angles to the PCB before soldering the remaining pins.

    Don’t install the LCD at this stage. This will be done once the main PCB is finished.

  • DCT Construction Manual – Issue 1 Page 39

    9.8 CONNECTORS AND SWITCHES

    9.8.1 RELAY One end of the relay has a line printed on top of the case for identification. This side faces

    towards the centre of the PCB. Once inserted hold against the PCB and solder the pins.

    9.8.2 BNC Insert the BNC connector into the PCB and solder. Make sure it sits flat against the board. The

    large mounting posts will need a reasonable wattage iron for the solder to flow properly.

    9.8.3 DC Insert the 2.1mm DC connector into the PCB and solder. Make sure it sits flat against the board.

    9.8.4 SLIDE SWITCH Insert the slide switch into the PCB and solder. Make sure it sits flat against the board.

  • DCT Construction Manual – Issue 1 Page 40

    9.9 RF POWER AMPLIFIER

    9.9.1 TRIMPOT Insert a 20K trimpot into position VR6 and solder.

    9.9.2 OUTPUT TRANSFORMER The output transformer is wound on a FT50-43 ferrite toroid using 0.4mm diameter enamelled covered wire. The wire lengths shown in the table allow for the required turns and a bit extra at each end for handling.

    Designator Length Turns

    T4 150mm+150mm 5

    The winding is a single bifilar winding. Take the two lengths and place alongside each other. Then, holding one pair of ends firm, rotate the other pair of ends until there are about 3 twists per centimetre. The combined secondary wires are then wound on the toroid as one. Scrape enamel off the end of each wire and spread them apart. Determine the start and end of each of the windings with a multimeter set to continuity. It does not matter which one is labeled winding one or two. Take the start of one winding and the end of the other and twist them together to form a centre tap. Tin the wire ends with solder before inserting into the PCB.

    The centre tap goes into the pad closest to C75.

    Start 1

    Start 2 End 1

    End 2

  • DCT Construction Manual – Issue 1 Page 41

    9.9.3 IRF510

    The IRF510 MOSFET is a static sensitive device. While it is unlikely it will be damaged, it is best to use anti-static precautions such as not directly touching the pins and using a grounded wrist strap.

    Bend the leads up at right angles 6mm from the edge of the plastic housing.

    Place a bush on the 3mm x 10mm screw and insert into the IRF510 tab hole. Place the insulating

    washer over the screw onto the heatsink tab.

    Insert the assembly from the bottom side of the PCB, so that the 3 bent leads come up though

    the holes in the PCB and the screw protrudes through the PCB mounting hole.

    Check the insulating washer is aligned correctly, then place a nut on the screw and tighten.

    Solder the leads.

    Using a multimeter set to continuity, check that there are no short circuits between the IRF510 tab and the PCB ground.

    6mm

    3mm nut

    Insulating

    washer

    3mm x 10mm screw

    Bush

  • DCT Construction Manual – Issue 1 Page 42

  • DCT Construction Manual – Issue 1 Page 43

    9.10 CONNECTOR BOARD

    Note that the connector board has parts inserted from both top and bottom side. Follow the

    steps below closely and check the parts are positioned correctly before soldering. Removing

    parts later will be difficult without proper de-soldering equipment.

    9.10.1 BOARD ASSEMBLY

    1 Insert the 2 resistors into

    the PCB from the top side. R1 is 2.2K and R2 is 22 ohms. Splay the leads slightly underneath, solder and trim the excess.

    2 Mount the 2.5mm jack

    socket (J1) into the PCB. Ensure it sits flat before soldering in place. Do not use too much heat as the small plastic body can easily melt and distort.

    3 Mount the three 3.5mm

    jack sockets (J2, J3 and J4) into the PCB. Ensure they sit flat and not rotated before soldering in place. Do not use too much heat as the small plastic body can easily melt and distort.

    4 Mount the right angle

    tactile pushbutton switch (PB1) into the PCB. Ensure it sits flat and at right angle to the PCB before soldering in place. Do not use too much heat as the small plastic body can easily melt and distort.

  • DCT Construction Manual – Issue 1 Page 44

    5 Insert the two 6 pin header

    strips into the bottom side of the PCB. The short pins go into the PCB pads. Ensure they sit flat and at right angle to the PCB before soldering in place.

    6 Mount the right angle 8 pin

    header strip in the top side of the PCB. The short pins go into the PCB pads. Ensure it sits flat and at right angle to the PCB before soldering in place.

    7 Fit the cap to the

    pushbutton and the shunts to the dual 6 pin header. See section 10 on selecting the shunt positions.

    9.10.2 STANDOFFS Install the four 20mm metal standoffs in the corners of the main board using 3mm x 6mm long

    screws. Insert a 3mm star washer between the standoff and the PCB.

    9.10.3 CONNECTOR BOARD INSTALLATION Insert the Connector board into the 8 pin header pads labeled SK1.

    Place the lid on top of the four standoffs and maneuver the connector board so that the

    connectors and pushbutton pass through the holes in the lid. Temporarily screw the lid down in

    two places to prevent the connector board moving. Turn the assembly over and solder the

    header pins. Remove the lid.

  • DCT Construction Manual – Issue 1 Page 45

    9.11 CONTROLS

    9.11.1 ROTARY ENCODER The rotary encoder is the tune control and is installed on the right hand side of the LCD.

    Insert it into the PCB and ensure it sits flat and vertical and solder one of the small pins. To

    check it’s positioned correctly, place the lid onto the 4 corner standoffs. Check the encoder shaft

    is on the centre of the hole in the lid. If not, gently push the encoder in the required direction.

    Then carefully take off the lid without moving the encoder and solder the remaining pins.

    9.11.2 POTENTIOMETERS There are 3 pots to install. VR2 and VR3 are logarithmic taper while VR4 is a linear taper type.

    Use the table below to identify them. The code can be seen on the underside of the pot body.

    Designator Function Value Code

    VR2 AF Gain 10K LOG A103

    VR3 Mic Gain 10K LOG A103

    VR4 CW Drive 10K LIN B103

    Install the potentiometers in the same fashion as the rotary encoder, using the lid placed on top

    as an alignment guide.

  • DCT Construction Manual – Issue 1 Page 46

    10 CONNECTOR BOARD JUMPER SETTINGS The DCT connector board has a jumper block that configures the DCT for three types of

    microphones.

    1. Handheld speaker/microphone (Electret).

    2. Dynamic microphone.

    3. Individual Electret microphone

    Electret microphones differ from dynamic types in that they contain an amplifier which requires

    a bias power supply. This is the main reason for the jumpers - to enable the bias power line for

    Electret types. A secondary reason is to reconfigure the microphone connector between the

    non-standard speaker/mic configuration and a more common configuration when an individual

    microphone is used.

    Microphone Type Jumper Settings Notes

    Handheld speaker/microphone

    Tip – not used Ring – mic + bias

    Sleeve – PTT (GND is via speaker connector)

    Dynamic

    Tip – mic Ring – PTT

    Sleeve - GND

    Individual Electret

    Tip – mic + bias Ring – PTT

    Sleeve - GND

  • DCT Construction Manual – Issue 1 Page 47

    11 MAKING CONNECTIONS

    11.1 POWER SUPPLY

    11.1.1 DC POWER SUPPLY The DCT can be powered from an external power supply between 11 and 17V.

    The 2.1mm DC connector centre pin is positive and the outer barrel is negative or ground.

    11.1.2 INTERNAL BATTERY For portable operation the DCT can be powered from an internal battery pack. Connection is

    made by soldering wires onto the two pads on the PCB marked 12V.

    The power switch on and off positions are reversed when changing power sources.

    11.2 ANTENNA

    The DCT requires a matched 50 ohm impedance antenna to be connected to the BNC connector

    for proper operation.

    11.3 MORSE KEY

    A straight Morse key is connected to the Key connector. The Key connector is a stereo 3.5mm

    jack, however the key cable can have a stereo or mono plug. If using a stereo plug the ring

    circuit is not used.

    Sleeve - Ground Ring – not used Tip - key

    11.4 HEADPHONES

    The DCT headphone socket is a stereo 3.5mm jack intended to connect a set of 32 ohm stereo

    headphones. The Connector board has a 22 ohm resistor in series to limit the power level.

    Sleeve - Ground Ring – Right Tip - Left

  • DCT Construction Manual – Issue 1 Page 48

    11.5 SPEAKER/MICROPHONE

    The DCT is compatible with a commonly available BAOFENG handheld speaker/mic. This uses

    two connectors, a 2.5mm stereo jack for the speaker and ground connections plus a 3.5mm

    stereo jack for the microphone signal and PTT line.

    3.5mm Sleeve - PTT Ring – mic signal + bias voltage Tip – not used

    2.5mm Sleeve - Ground Ring – not used Tip - speaker

    11.6 MICROPHONE

    If a separate microphone is used it can be either an Electret type or a dynamic type.

    The DCT connector is a 3.5mm stereo jack with the ring not used.

    Sleeve - Ground Ring – not used Tip – mic signal

    For proper operation of the microphone the jumper settings on the Connector board must be set to the correct positions. See section 10.

  • DCT Construction Manual – Issue 1 Page 49

    12 FIRST TIME POWER UP With the DCT main board finished it’s time to power up, but first make a thorough visual

    inspection of the PCB. Pay particular attention to part placement, solder bridges across pads and

    dry solder joints around coil wires.

    The kit comes with a pre-programmed microcontroller so once power is applied it should come

    to life.

    Plug in the LCD and secure with four 3mm x 6mm long screws.

    Connect a 50 ohm dummy load to the BNC antenna connector. This is to terminate the

    transmitter if something goes wrong.

    If a bench top power supply is used, set it to 13.8V and current limit to around 200mA.

    If the power supply does not have a current meter use a multimeter to measure supply

    current.

    Plug a speaker/microphone or headphones into the Connector board.

    Rotate the PA bias trimpot fully counter clockwise.

    Rotate all pots fully anticlockwise.

    Plug the power into the DC connector and move the slide switch to the ON position.

    The current should be around 80mA. If far from this turn off immediately and look for

    errors.

    The LCD backlight should come on and 2 beeps should be heard. Adjust the LCD contrast

    control VR1 until the characters are visible and clear. At power on an opening screen is

    displayed, followed a few seconds later by the operating screen.

    Referring to Figure 16 check the voltages around the circuit for receive mode.

  • DCT Construction Manual – Issue 1 Page 50

    13 CONFIGURATION Before testing and operation can commence the DCT must be configured. Configuration holds

    settings that only need to be set once or changed infrequently.

    The minimum items to set at this stage are the Frequency and Freq Steps. The other items can

    be set later as needed.

    13.1 TO ACCESS CONFIGURATION:

    Turn off power to the DCT.

    Hold down the Menu button and turn on power.

    Wait until the LCD displays Configuration and 4 beeps are heard.

    Release the Menu button.

    13.2 NAVIGATING CONFIGURATION

    Pressing the Menu button steps though the configuration items one by one. As each new

    item is displayed a beep is heard.

    Settings are displayed for each item if they have already been programmed.

    Options within a configuration item are selected and set with the Tune knob, either

    rotating or pressing as described below.

    By simply stepping through the items with the Menu button, previously set values will

    not be altered.

    At the completion of all items the settings are saved in non-volatile memory and

    operation returns to normal.

    13.3 CONFIGURATION ITEMS

    13.3.1 FIRMWARE This screen displays the current installed firmware version and is not editable.

    13.3.2 CALLSIGN This item allows the entering of a callsign of up to 10 characters. This is used for two purposes.

    Firstly to personalize the DCT by displaying the callsign in the opening screen, and secondly,

    sent as part of the automatic CQ caller.

    Press the Tune knob to step through the character positions.

    Rotate the Tune knob to select and set the required character.

    To clear and start again, press the Tune knob down for more than 1 second.

    The available characters are digits 0-9, letters A-Z, and a slash (‘/’).

    13.3.3 FREQUENCY This sets the initial frequency and band of operation.

    Press the Tune knob to step through the digit positions.

    Rotate the Tune knob to select the required digit.

    The range is 100KHz to 100MHz.

  • DCT Construction Manual – Issue 1 Page 51

    13.3.4 FREQ STEPS As the tuning knob is rotated the frequency changes value by a fixed step.

    This item allows the selection of the number of available steps and the value of each step.

    Rotate the Tune knob to select the desired step as shown in the LCD top row.

    Press the Tune knob to add the step to the list of selected steps. These are shown in the

    bottom row.

    To clear the selections and start again press and hold the Tune knob down for more than

    1 second.

    A minimum of 1 step and a maximum of 4 steps can be programmed.

    The steps may have any combination of 1Hz, 10Hz, 50Hz, 100Hz, 500Hz, 1KHz, 5KHz or 10KHz.

    13.3.5 30S FREQ SAVE This item enables automatic saving of the current frequency in EEPROM so that it is recalled the

    next time the DCT is powered on. This is performed only once per frequency change and only

    after a 30 second idle period has elapsed. If the encoder is operated so that there is less than 30

    seconds between changes no saves will be performed. Pressing the encoder button toggles the

    selection.

    Options are Yes and No.

    13.3.6 FREQ CALIBRATE This item allows trimming of the VFO output frequency in firmware so that the operating

    frequency matches the displayed frequency.

    Rotating the Tune knob changes the value.

    Pressing the Tune knob switches between plus and minus.

    Range is -990Hz to +990Hz in 10Hz steps.

    13.3.7 VOLTS CALIBRATE The microcontroller measures the battery voltage using an analog to digital converter through a

    resistive divider. Due to the tolerance in components the displayed value may not be the same

    as the battery voltage. This option allows the displayed value to be trimmed so it is spot on.

    Rotating the Tune knob changes the value.

    Pressing the Tune knob switches between plus and minus.

    The range is -0.9V to +0.9V in 0.1V steps.

    13.3.8 PWR MTR CAL This option adjusts the LCD transmit bar graph to display the correct power. After calibration

    each bar is approximately equal to 1W of output power.

    Connect a 50 ohm load and power meter to the antenna connector.

    Plug in a Morse key and turn the CW drive control to minimum.

    Press the Tune knob briefly to place the DCT in CW test mode.

    Operate the Morse key and adjust the CW drive level to show 5W on the external power

    meter.

    Rotate the Tune knob until the bar graph shows exactly 5 full bars. Do not hold the key

    down for long periods otherwise the PA may get too hot.

    The number in the top row of the LCD is a calibration number used by the firmware and

    is displayed for guidance only.

    The range is -50 to +50.

  • DCT Construction Manual – Issue 1 Page 52

    14 TESTING AND ALIGNMENT

    14.1 GENERAL

    The minimum tools and accessories needed to get the DCT tested and aligned are listed below:

    Digital multimeter.

    50 ohm dummy load capable of dissipating at least 5 Watts.

    Power supply capable of 13.8V DC regulated at 1 Amp or a suitable battery.

    Small flat blade adjustment screwdriver.

    These additional instruments are recommended to fully test and align the DCT.

    QRP wattmeter or oscilloscope.

    An audio signal generator.

    An RF signal generator.

    Figure 16 lists typical voltages at various points around the board and can be used to verify operation.

    14.2 RECEIVE

    Plug in a speaker/microphone or headphones.

    Set the Tune control to around the middle of the range.

    Turn the AF gain control to halfway. You should hear some low level hiss indicating that

    the audio stages are working.

    Apply an RF signal of about 100uV to the antenna connector and rotate the Tune control

    until a clear tone is heard in the headphones. If no signal is heard check the receive

    circuit by tracing the signal from the product detector back through the audio chain.

    Check the AF gain control operates.

    Check that the LCD bar graph moves up and down as the signal is tuned.

    Using the Menu button select the various attenuator settings and verify operation.

    Check the BPF and LPF can be selected via the Menu button.

    If you don’t have an RF signal generator simply plug in an antenna and listen. You will get a good idea if the receiver is working correctly by comparing to another receiver.

  • DCT Construction Manual – Issue 1 Page 53

    14.3 TRANSMIT

    During the transmitter testing phase operate the PTT for only short periods until fully set up. If the PTT is operated for long periods with high PA current the output MOSFET will get very hot.

    Remove the RF signal source and reconnect the dummy load. It will be an advantage to

    have either a QRP power meter or oscilloscope to measure the power output.

    Turn the Mic gain pot fully counter-clockwise.

    Ensure the PA Bias trimpot VR6 is fully counter clockwise.

    Operate the PTT. Check the power supply current. This is the idle transmit current and

    should be about 160mA. Anything far from this indicates a problem and should be

    investigated.

    Write the idle transmit current value down so that it can be used to measure and set the

    PA bias current.

    Check the TX mode DC voltages as shown in Figure 16. If there are any off scale readings

    turn off immediately and look for construction errors.

    14.3.1 PA BIAS SETUP Operate the PTT and using a screwdriver slowly rotate the PA Bias trimpot VR6

    clockwise. At about half rotation the power supply current should start to rise gradually

    and smoothly. Any excessive spikes in current indicate instability and need to be

    investigated.

    Keep increasing until the power supply current is around 150mA more than the idle

    transmit current written down in the step above. The increase in current is mainly due

    to the output MOSFET starting to conduct and move into linear operation, but some may

    be because we have not balanced the carrier yet causing some power output.

    14.3.2 CARRIER BALANCE Operate the PTT and adjust carrier balance trimpot VR5 for minimum power output.

    Initially if the power is high enough you can use the LCD bar graph as a monitor, but as

    the mixer reaches balance the output power will decrease significantly and the bar

    graph will not indicate any power. At this point it’s necessary to use some other way to

    monitor the power output to do the final balancing. The following are some suggested

    methods:

    o Using a second receiver placed nearby listen to the transmitted signal. Ensure

    you are not listening directly to the VFO signal as this is on the same frequency.

    o Connect an oscilloscope across the dummy load.

    o Connect a sensitive RF probe across the dummy load.

    o Connect a spectrum analyser across the dummy load. An attenuator may be

    necessary.

    In all cases it’s simply a matter of monitoring the transmitted signal for minimum output

    while carefully adjusting VR5.

    With the carrier balanced rotate the bias trimpot VR6 fully counter-clockwise again to

    turn off the MOSFET.

    Operate the PTT. Slowly rotate the bias trimpot clockwise again until the power supply

    current is 150mA higher than the idle transmit current recorded earlier. Typically this

    will be 320mA. The output stage bias current is now set.

  • DCT Construction Manual – Issue 1 Page 54

    14.3.3 VOICE TEST Configure the connector board for a dynamic microphone.

    Make a cable with 3 wires soldered to the tip, ring and sleeve of a 3.5mm stereo plug.

    Apply an audio signal generator to the tip and sleeve wires set to 1KHz at around 50mV.

    Operate the PTT by shorting the ring wire to ground.

    Slowly rotate the Mic gain trimpot clockwise while monitoring the power output. The

    power output should increase smoothly without any sudden dips or surges and you

    should be able to easily achieve 5 Watts output. At this stage the LCD bar graph may not

    indicate 5 bars. This can be calibrated later.

    Remove the signal generator and check that the RF output goes to zero.

    Remove the temporary cable and re-configure the connector board if using an Electret

    microphone.

    Plug in a microphone and operate the PTT. Speaking into the microphone should result

    in RF output, and the LCD bar graph should move up and down.

    14.3.4 CW TEST Set the mode to CW with the Menu button.

    Plug a key into the Key connector.

    Press the Key down. The DCT should go into transmit. Adjust the CW drive control to

    verify the power can be set between 0 and at least 5W.

    Verify the LCD power meter operates.

  • DCT Construction Manual – Issue 1 Page 55

    Location V DC Receive V DC Transmit

    Q19 Source 13.8 13.8

    REG1 out 5 5

    REG2 out 3.3 3.3

    U6 pin 8 6.8 Note 1 0

    U7 pin 8 11.5 0

    U7 pin 3 5.7 0

    U7 pin 1 5.7 0

    U7 pin 7 5.7 0

    U8 pin 1 5.7 0

    U8 pin 7 5.7 0

    REG3 out 5 5

    U9 pin 5 2.5 2.5

    Q12 collector 0 9

    Q10 collector 0 4

    Q13 collector 0 4.25

    Q15 emitter 0 1.45

    Q16 emitter 0 1.55

    ZD2 cathode 0 6.8 Note 1

    Q17 gate 0 4.3 Note 2

    Notes: 1. Zener voltage can vary by +- 5%. 2. PA bias adjusted for 150mA idle current.

    DC readings taken with a digital multimeter. Power supply voltage set at 13.8V DC.

    Figure 16 Typical DC circuit voltages.

  • DCT Construction Manual – Issue 1 Page 56

    15 FINAL ASSEMBLY

    15.1 CASE HOLES

    The case requires holes to be cut in the side so that the connectors and switch can protrude.

    Using Figure 17 as a guide, mark out the positions of the hole and cutouts with a pencil or

    scriber.

    Drill a central pilot hole for the round hole and open up with a reamer or round file.

    Drill a series of holes around the inside perimeter of the rectangular cutouts, and finish off with

    a small flat or square file.

    Figure 17 Case holes

    15.2 INSTALLING THE LID

    The lid is mounted on top of the main board and held in place by the four 20mm metal spacers.

    Make sure the connector board jacks and pushbutton fit neatly inside the lid holes.

    Screw the l