Lab 4

Table of Contents1. Abstract.........................................................................................................................................2

2. Introduction...................................................................................................................................2

3. Method and material.....................................................................................................................2

4. Experimental Procedure................................................................................................................2

4.1. Practical Exercise: A Single Sideband Transmitter..................................................................2

4.2. Practical Exercise: Receiving the SSB Signal...........................................................................6

5. Results...........................................................................................................................................9

5.1. Practical Exercise: A Single Sideband Transmitter..................................................................9

5.2. Practical Exercise: Receiving the SSB Signal.........................................................................10

6. Discussion....................................................................................................................................11

6.1. Practical Exercise: A Single Sideband Transmitter................................................................11

6.2. Practical Exercise: Receiving the SSB Signal.........................................................................12

7. Assessment..................................................................................................................................14

8. Conclusion...................................................................................................................................14

9. References...................................................................................................................................14

1. AbstractThe aim of this experiment is to:

a. Describe the characteristics of single sideband and double sideband AM

transmissions.

b. Explain how the information signal is prepared for single sideband transmission.

c. Describe how a receiver can recover the information signal from a single sideband

transmission.

2. IntroductionSingle Sideband Systems

Single-sideband modulation (SSB) or Single-sideband suppressed-carrier

(SSB-SC) is a refinement of amplitude modulation that more efficiently uses

electrical power and bandwidth.

Amplitude modulation produces a modulated output signal that has twice the

bandwidth of the original baseband signal. Single-sideband modulation avoids this

bandwidth doubling, and the power wasted on a carrier, at the cost of somewhat

increased device complexity and more difficult tuning at the receiver.

3. Method and materiala. Anacom 1/1 board

b. Anacom ½ board

c. Power supply

d. Connector

e. Digital oscilloscope

4. Experimental Procedure

4.1. Practical Exercise: A Single Sideband Transmittera. Connect the ANACOM 1/1 board to the power supply as shown in Figure 60

below.

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b. On ANACOM 1/1 the following inital conditions were set:

i. AUDIO INPUT SELECT switch to INT position.

ii. MODE switch to SSB.

iii. OUTPUT AMPLIFIER GAIN preset set to maximum (fully clockwise).

iv. SPEAKER switch to OFF.

v. In the AUDIO OSCILLATOR both the AMPLITUDE PRESET and the

FREQUENCY PRESETS should be set to maximum (fully clockwise).

vi. In the BALANCED MODULATOR the BALANCE PRESET should be

set to maximum (fully clockwise).

vii. In the BALANCED MODULATOR & BANDPASS FILTER CIRCUIT 2

the BALANCE PRESET should be set to maximum (fully clockwise).

c. The power supply switched on.

d. Oscilloscope was used to monitor the input to the BALANCED MODULATOR at

tpl5. The oscilloscope controls were adjusted to display 3 - 5 complete cycles with

a peak-to-peak amplitude of 1 division. Use this signal to trigger the oscilloscope.

e. This waveform in recorded using the copy of Figure 61.

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f. Using Channel 2 of your oscilloscope, the output from the BALANCED

MODULATOR was monitored at tp17 and the oscilloscope was adjusted as

necessary to provide a waveform with a peak-to-peak amplitude of 4 divisions.

The output waveform at this time will be a DSB AM envelope.

g. On the AUDIO OSCILLATOR, the AMPLITUDE preset control was turned fully

counter-clockwise to reduce the information amplitude to zero and, without

adjusting the oscilloscope settings, record the carrier input waveform in Figure 61

of your workbook. This signal is now the carrier which is at a frequency slightly

less than 455kHz as we saw in Section 4.10.

h. The BALANCE preset on the BALANCED MODULATOR was turned slowly

counter-clockwise to reduce or 'balance out' the carrier. Near to the midpoint of its

travel, the amplitude of the carrier will be reduced to zero and the carrier will be

completely suppressed. If the carrier amplitude cannot be reduced to zero, or very

close to zero, then the frequency of the 455kHz OSCILLATOR needs to be

adjusted slightly. To do this, follow the procedure described in Appendix 1.

i. The AUDIO OSCILLATOR AMPLITUDE preset control was increased to its

maximum (fully clockwise) position will re-introduce the information signal and

the output will now be a DSBSC signal.

j. This signal was recorded in Figure 61.

k. The output of the CERAMIC BANDPASS FILTER was monitored at tp20

together with the audio information signal at tp 15. Note that the envelope of the

signal at tp20 now has a fairly constant amplitude as shown in Figure 62.

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l. The oscilloscope was triggered with the signal at tp20 and, the trace was expanded

to observe the waveform. Note that the signal is a good, clean sinewave,

indicating that only a single frequency is present and therefore only the upper

sideband has been allowed to pass through the filter.

m. Next decrease the AUDIO OSCILLATOR'S FREQUENCY preset whilst

continuing to observe this waveform. It may happen that some distortion is

present at low frequencies. This is because the upper and lower sidebands are now

very close together and the filter is having difficulty in removing the whole of the

lower sideband. Nevertheless, the amplitude of the lower sideband is still very

much less than the upper sideband.

n. Make sure that, in the AUDIO OSCILLATOR, both the amplitude and the

frequency presets are set to maximum (fully clockwise). Use your oscilloscope to

measure the frequency of the signal at the output of the CERAMIC BANDPASS

FILTER at tp20.

o. The output of the BALANCED MODULATOR & BANDPASS FILTER

CIRCUIT 2 was monitored at tp22 (triggering on this signal), the timebase was set

to display 3 to 5 complete cycles and adjust its BALANCE preset until you

observe a clean sinewave.

p. Oscilloscope was used to measure the frequency of the signal at tp22.

q. In the BALANCED MODULATOR AND BANDPASS FILTER CIRCUIT 2 turn

the balance preset to maximum (fully clockwise).

r. The timebase control of oscilloscope was set to 0. 1 ms/division and monitor the

transmitter output signal at tp 13 (trigger the oscilloscope on the signal at tpl5).

Observe the effect of reducing the preset control in the AUDIO OSCILLATOR to

minimum (fully counter-clockwise).

s. With the amplitude preset still at its minimum setting, observe the effect of

switching between SSB and DSB transmissions.

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t. The power supply was switched off.

4.2. Practical Exercise: Receiving the SSB Signala. Connect the ANACOM 1/1 and ANACOM 1/2 boards to the power supply as

shown in Figure 63 below:

b. On ANACOM 1/1 set the following initial conditions:

i. AUDIO INPUT SELECT switch to INT position.

ii. MODE switch to SSB.

iii. OUTPUT AMPLIFIER GAIN preset set to maximum (fully clockwise).

iv. SPEAKER switch to OFF.

v. In the AUDIO OSCILLATOR both the AMPLITUDE PRESET and the

FREQUENCY PRESET should be set to maximum (fully clockwise).

vi. The TX OUTPUT SELECT should be set to ANT. (antenna) and the

antenna should be in a vertical position and fully extended.

c. On ANACOM 1/2 set the following initial conditions:

i. In the AUDIO AMPLIFIER switch the SPEAKER to ON and decrease the

VOLUME preset to its minimum value (fully counter-clockwise).

ii. RX INPUT SELECT switch to the ANT. (antenna).

iii. In the RF amplifier switch the TUNED CIRCUIT SELECT to INT

(internal) position and increase the RF AMPLIFIER GAIN CONTROL to

maximum (fully clockwise).

iv. Set the AGC switch to the OUT position.

v. Set the detector switch to the PRODUCT position.

vi. Switch the BEAT FREQUENCY OSCILLATOR to the ON position.

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vii. Fully extend the antenna and set in a vertical position.

d. The power supply was switched on.

e. The ANACOM 1/1 and ANACOM 1/2 were placed approximately 6 inches

(150mm) apart.

f. The first step was to tune the receiver to the incoming SSB signal. To do this, your

oscilloscope was used to monitor the output of IF Amplifier 2 at tp28 and turn the

tuning dial until the amplitude of the received signals reaches a maximum value,

this should occur at about 85 - 95 on the TUNING dial.

g. The signal was checked that it was actually the SSB signal by decreasing

ANACOM 1/1's AMPLITUDE preset (in the AUDIO OSCILLATOR block) to its

minimum position and checking that the monitored signal also falls to zero.

Return the AMPLITUDE preset to its maximum setting (fully clockwise).

h. To ensure that overloading does not occur, take the following steps:

i. Turn the GAIN preset in ANACOM 1/1's OUTPUT AMPLIFIER block so

that the preset's arrow head is pointing horizontally to the left.

ii. On the ANACOM 1/2 receiver, carefully adjust the tuning control until the

signal at tp28 is at its maximum value.

iii. Adjust the GAIN preset in ANACOM 1/2's RF AMPLIFIER block until

the amplitude of the monitored signal is about 2 volts peak-to-peak.

iv. Now repeat the last two steps to make sure the receiver is accurately tuned.

i. Oscilloscope was used to measure the input frequencies applied to the PRODUCT

DETECTOR at tp28 and tp46.

j. Double-beam oscilloscope was used to monitor the audio information signal on

the transmitter at tp15. Use this as the trigger input.

k. The VOLUME preset in the receiver AUDIO AMPLIFIER was increased to

enable the tone to be audible at a comfortable level. If you still cannot hear the

tone, try a slight adjustment to the receiver TUNING control. If the tone becomes

irritating to those around you the loudspeaker can be switched off and a pair of

headphones (provided) can be plugged into the AUDIO AMPLIFIER.

l. Similarly, the loudspeaker on the transmitter AUDIO AMPLIFIER was switched

on and the VOLUME preset tone was increased to be audible at a comfortable

level. If desired the loudspeaker can be switched off and a pair of headphones

(provided) can be plugged into the AUDIO AMPLIFIER as on the receiver board.

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m. The TUNING dial was slowly turned from about 100 to 80 on the dial and noticed

that how the tone at the receiver output changes. This is because the local

oscillator frequency is being adjusted. This has the effect of changing the

frequency of the IF frequency. Now, if the IF frequency is changed and the BFO

frequency remains constant, the resultant 'difference' frequency is changed. This

'difference' frequency is the audio output signal that we are listening to.

n. With the receiver's tuning dial on the counter-clockwise side of the minimum

frequency position, the TUNING DIAL was adjusted carefully counterclockwise

until the transmitter audio tone and the receiver tones are the same. Now adjust

the FREQUENCY preset in the transmitter AUDIO OSCILLATOR across its

range of tones. We will hear the range of tones being repeated at the receiver

output.

o. The receiver TUNING control was turned clockwise until the tone is again

matched on the other side of the minimum frequency position.

p. The receiver was re-tuned to correctly receive the SSB signal.

q. The other beam of the oscilloscope was used to monitor the received audio output

from the PRODUCT DETECTOR at tp38.

r. The oscilloscope was used to monitor the final output at tp39 and compare it with

the original information at tpl5 in the transmitter. Apart from the instability, the

output signal is a very good reproduction of the input. In practice, tuning signal is

achieved by listening to a speech transmission and simply tuning the receiver to

provide good quality intelligible speech.

s. The power supply was switched off.

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5. Results

5.1. Practical Exercise: A Single Sideband Transmitter

Scale of X: 100s/div

Scale of Y: 0.5V/div

Figure 3.1: Information Signal at tp15.

• Peak-to-Peak Voltage = 2.5V• Time = 330s

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Scale of X: 100s/div

Scale of Y: 0.5V/div

Figure 3.2: Carrier Input at tp17.

• Peak-to-Peak Voltage = 2V

Scale of X: 100s/div

Scale of Y: 0.2V/div

Figure 3.3: DSBSC AM wave at tp17.

• Peak-to-Peak Voltage = 0.6 V

• The DSBSC signal recorded in Figure 3.5 contains only two frequencies, both of

which are at RF frequencies.

• The change in amplitude of the signal at tp20 as the input frequency is changed is

most likely due to the frequency response of the CERAMIC BANDPASS FILTER.

• The measured frequency at tp20 (in kHz) is 450 kHz.

• The measured frequency at tp22 (in MHz) is 1MHz.

5.2. Practical Exercise: Receiving the SSB Signal• The frequency of the signal at tp28 is 454 kHz which is close to 455 kHz.

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• The measured frequency (in kHz) for the BFO output at tp46 is 455 kHz.

6. Discussion

6.1. Practical Exercise: A Single Sideband TransmitterFigure 3.1 shows the information signal at tp15 obtained is a sinewave with Vpp

value = 2.5V. The frequency of the information signal obtained is very low with value of

3.3 kHz and insufficient to transmit at this frequency. This signal is the original DSB

signal as it is detected at a point just before entering the balanced modulator. The signal

here is only the information signal.

The waveform obtained at tp17 as shown in Figure 3.2 is a DSB AM envelope

with Vpp value = 0.64V. It is the waveform after being modulated in the balanced

modulator. The frequency of the carrier has to be high to modulate the information signal

so that the information signal can be transmitted to the receiver. The carrier signal

frequency is generated by a 455 kHz oscillator. This final DSB waveform is generated

when the information signal from the Audio Oscillator have been ‘combined’ with the

signal generated by the carrier generator.

The information signal and the carrier signal are modulated by the Balanced

Modulator to generate a signal with upper side band, lower side band and carrier. At the

DSBSC AM wave at tp17 as shown in Figure 3.3 with Vpp value = 0.8V, the amplitude

preset control in the Audio Oscillator is being turned fully counter-clockwise .This is to

reduce the information signal to zero and reducing the carrier signal to slightly less than

455 kHz. After that, the balance preset on the balanced modulator is slowly turned

counter-clockwise in order to ‘balance out’ or finally reduce the carrier in the signal.

Lastly, the carrier is fully suppressed and eliminated.

The resulting signal is the DSBSC signal consists of two frequencies which are

lower side band and upper side band, both at RF. When the amplitude of the information

signal is decreased, the DSBSC amplitude is also decreased. When the frequency of the

information signal is decreased, the DSBSC frequency is also decreased. The DSBSC

AM wave shows the signal carrier have been removed to decrease the power dissipated

during the transmission process.

Next, the DSBSC signal is passed into the Ceramic Bandpass Filter to remove the

lower side band and the unwanted signal. The resulting signal is SSB signal that consist

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only upper side band. The process goes like this: the inputs to the balance modulator

compromise audio inputs which extend from 300Hz to 3.4 kHz and the carrier input. In

Anacom 1/1, this carrier oscillator actually operates around 453 kHz although being

marked 455 kHz. This is to ensure that only the upper sideband can pass through the

ceramic bandpass filter.

The frequency of the signal after passing the Ceramic Bandpass Filter is measured

at test point 20 and the value obtained is 450 kHz which is very close to the 455 kHz (The

frequency value mark on the ANACOM 1/1 board).This is the frequency for single side

band signal. Some variation of the amplitude at tp20 may be acknowledged as the

frequency of the information signal is altered. This is most likely due to the frequency

response of the Ceramic Bandpass Filter.

The SSB signal is at an inconvenient frequency so the BALANCED

MODULATOR & BANDPASS FILTER CIRCUIT 2 is used to shift the output

frequency into the mf (medium frequency) broadcast band so that the receiver can receive

it. Once again, the upper sideband is selected. Monitoring the output of the BALANCED

MODULATOR & BANDPASS FILTER CIRCUIT 2 at tp22, the oscillator shows a clean

sine wave. The unwanted signal is removed from the SSB signal.

Then, a carrier signal is combined with the SSB signal to shift the SSB signal

frequency. The DSBSC signal will then have two sidebands, upper and lower. The lower

sideband can be filtered out from the signal as it differs largely from the upper sideband.

The resultant signal frequency is the sum of the SSB signal frequency and the carrier

frequency. Hence, the frequency measured at test point 22 is 1MHz.

6.2. Practical Exercise: Receiving the SSB SignalThe receiver is of the normal superhet design. It is first tuned to receive the

incoming SSB signal and this is done by using the oscilloscope to monitor the output of

IF Amplifier 2 at tp28 and turn the turning dial until the amplitude of the received signal

reaches a maximum value, about 80-100 of the turning value. Since the incoming SSB

signal does not contain any carrier information, the receiver’s AGC circuit cannot make

use of the carrier amplitude to control the receiver gain.

The RF signal received from the antenna is amplified by the RF Amplifier. A

Mixer is used to shift the frequencies of the RF signal in order to extract the audio signal.

The Mixer mixed the RF signal with the Local Oscillator Signal. The resulting signal is

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the IF signal. Then the IF signal is further amplified by the IF Amplifier 1 and IF

Amplifier 2 before it is send to Product Detector.

The frequency of the signal measured at test point 28 is 454 kHz. This is close to

the original SSB signal frequency (455 kHz) generated at the transmitter. The SSB signal

has been fully recovered from the RF signal.

Insertion of right carrier is needed to extract information from sideband. Beat

Frequency Oscillator (BFO) is adjusted to produce acceptable audible beat frequency.

The frequency keeps on decreasing and increasing until the tone reaches a minimum

which is called ‘minimum frequency position’. In other words, any increase in tone at the

transmitter will make the tone receiver decrease. The Product Detector mixed the IF

signal (455 kHz + audio frequencies) with another input of 455 kHz from the BFO. The

Beat Frequency Oscillator output frequency measured at test point 46 is 455 kHz. This

frequency value is same with the BFO frequency mark on the ANACOM 1/2 board. As

the result, it produced ‘sum’ and ‘difference’ frequencies. The resultant frequency is

passed to a Low-Pass Filter to remove the ‘sum’ signal.

However, this signal frequency is very unstable. After adjustment, it is only a

second or two before the frequency begins to drift. This is due to small changes in the

frequency of the oscillators in the transmitter and in the receiver. Such changes have no

noticeable effect when using a double sideband system but become very critical for SSB

transmissions. Oscillator drift in an SSB system is a serious problem since it shifts all the

frequency components by the same amount.

The output signals at tp38 (Output of Product Detector) still contains some high

frequency components that will be filtered out with a low-pass filter at the input to the

AUDIO AMPLIFIER. The final output at tp39 is the ‘difference’ frequency which consist

only the original audio signal. The audio signal is passed to loud speaker.

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7. Assessmenti. 4.2a) b

4.3a) d4.6a) c4.9a) c4.10a) a4.13a) a4.14a) c4.14b) a4.14c) 454.5kHz4.14d) 1.45MHz4.15a) c

4.15b) 452kHz

ii. 1. b2. d3. d4. a5. c6. b7. b8. a9. c10. c

8. ConclusionThe objectives are achieved.

9. References1. J. Crisp. LJ Technical Curriculum Manual AT02: An Introduction to Analog

Communications, LJ Technical Systems Ltd., Norwich.

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