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KL-900ABasic Communication Trainer
MODULE EXPERIMENT MANUAL
K&H MFG CO., LTD.5F, No. 8, Sec. 4 Tzu-Chiang Rd., San Chung City 241, Taipei Hsien, Taiwan R.O.C.TEL 886-2-2286-0700 FAX 886-2-2287-3066E-Mail [email protected] WEB http://www.kandh.com.tw
KL-900A'c Communication TrainerBa
KL-900A CONTENTS
CONTENTSUnit 1 FR Oscillators
1.1 Objectives 1 -1
1,2 Discussion Of Fundamentals ................................ 1 -1
8.4 Experiments And Records...... ...... ............ ............... ......
........ 8-10
8-10Experiment 8-1 LM565 PLL Characteristic MeasurementsExperiment 8-2 LM565 V-F Characteristic MeasurementsExperiment 8-3 PLL Frequency DemodulatorExperiment 8-4 FM to AM Frequency Demodulator
Understanding the operation and characteristics of radio-frequency (RF)
oscillators.
Designing and implementing oscillators.
1.2 DISCUSSION OF FUNDAMENTALS
An oscillator is simply a signal generator converting its dc supply voltage into a
continuously repeating ac output signal without any input signal. Oscillators play
very important roles in communication systems. An oscillator generates the
carrier or local oscillation signal used in any communication system.
Fig.1-1 shows the basic block diagram of oscillator. It includes an amplifier and
a feedback network constructed by the resonator. When dc power is first
applied to the circuit, noise will appear in the circuit and is amplified by the
amplifier and then fed to the input through the feedback network that is a
resonant circuit with filter function. The feedback network permits the signal
frequency equaling the resonant frequency to pass and rejects other
frequencies. The feedback signal will be amplified and fed back again. If the
feedback signal is in phase with the signal at input and voltage gain is enough,
the oscillator will be operation.
For proper operation, an oscillator must meet Barkhausen criterion.
Barkhausen criterion is the relationship between the amplifier's gain A and the
oscillator's feedback factor 3(s) and should be equal to 1. That is
A fl(s) 1 ( 1 -1)
where
A : amplifier's gain
16' (s) : oscillator's feedback factor
Unit 1 RF Oscillators
If the frequency is not very high, the internal capacitances of transistor can be
neglected and the oscillating frequency of Colpitts oscillator can be calculated
by the formula
1 ( Hz )
27r1 + C2
L( Cl C2 )
(1-2)
Output
Fig.1-2 AC equivalent of Colpitts oscillator
In Colpitts oscillator circuit, the feedback factor 13 is C,/C2 and the voltage gain A
is g R. By Eq. (1-1)
A 13(S) = 1
we obtain
,= 1
C2
or
C2gm R = —
ci
For starting oscillation, the loop gain should be at least 1 so that the oscillation
condition can be expressed by
1 -3
Unit 1 RF Oscillators
If operating frequency is not very high, the spray capacitance of transistor canbe neglected and the oscillating frequency is determined by the componentvalues of parallel-resonant circuit and can be calculated by the formula
1( Hz )
277-AL, + L2 )C (1-4)
Output
Fig..1-4 AC equivalent of Hartley oscillator
In Hartley oscillator circuit, the feedback factor # is L2 /Li and the voltage gain A
Understanding the principle of amplitude modulation (AM).Understanding the waveform and frequency spectrum of AM signal andcalculating the percent of modulation.
3. Designing an amplitude modulator using MC1496.Measuring and adjusting an amplitude modulator circuit
3.2 DISCUSSION OF FUNDAMENTALS
Modulation is the process of impressing a low-frequency intelligence signal ontoa high-frequency carrier signal. Amplitude Modulation (AM) is a process that ahigh-frequency carrier signal is modulated by a low-frequency modulatingsignal (usually an audio). In amplitude modulation the carrier amplitude varieswith the modulating amplitude, as shown in Fig. 3-1. If the audio signal isA mcos(2 nfmt) and the carrier signal is A sos(2 nf,t), the amplitude-modulated signalcan be expressed by
X 4m(t) = [ Amcos(2#,M] A, cos(2nf, t)
= p +mcos(27if.t)}A,cos(271f,t)
= Apc A, [1+ mcos(27ifmt)]cos(27-tfet)( 3-1 )
whereApc = dc levelA m = audio amplitudeAc = carrier amplitude
fm = audio frequency
= carrier frequency
m = modulation Index or depth of modulation = A m /A DC
3-1
Aix. A c
0.5mA 1„ 0.5mA DC A
Unit 3 AM Modulators
fc fm fc f fm f(Hz)
Fig.3-2 Spectrum of AM signal
The m in Eq.(3-1), called modulation index or depth of modulation, is animportant parameter. When m is a percentage, it is usually called percentagemodulation. It is defined as
Modulating Amplitude x 100% =
Am x 100%
m= DC Level A Dc
( 3-3 )
It is difficult to measure the A DC in a practical circuit so that the modulation indexis generally calculated by
E.— E„, x100%Ems +
where Emax=i4c+A„, and Emin =A c-A,, , as indicated in Fig. 3-1.
( 3-4 )
As mentioned above, audio signal is contained in the side bands so that thegreater the sideband signals the better the transmitting efficiency. FromEq.(3-2), we can also find that the greater the modulation index, the greater thesideband signals and the better the transmitting efficiency. In practice, themodulation index is usually less or equal to 1; if m > 1, it is called overmodulation.
X( 1 )
( V )
3-3
4)Modulatinginput
o Gain° adjust
Bias adjust(5)
(12)O_
0+ Output(6)
(10)Carrier oinput +0
(8)
R3500(14)
-V 0
C2. 0Audio _ ,
input O--II I
C0.1uF
Carrierinput o— II
R451
luF
14 5RI R2 R R610K 10K 51 51
R9
6.8K
VR 150K-5V
R3IK
R71K
RII
3.9K
0+12VC3
0.1uFR8 1K
EAM—39K
2 38
Coutput
100.1 uF
MC1496
1 124
Unit 3 AM Modulators
Fig.3-3 MC1496 internal circuit
Fig. 3-4 shows an AM modulator circuit whose carrier and audio signals are
single-ended inputs, carrier to pin 10 and audio to pin 1. The gain of entire
circuit is determined by the R8 value. The R9 determines the amount of bias
current. Adjusting the amount of VR1 or the audio amplitude can change the
percentage modulation.
Fig.3-4 Amplitude modulator using MC1496
3-5
Unit 3 AM Modulators
7. Repeat steps 4 and 5.
8. Connect a 150mVp-p, 1 kHz sine wave to the input (I/P2), and a 100
mVp-p, 100kHz sine wave to the carrier input (I/P1).
9. Using the oscilloscope, observe the AM signal at output terminal
(0/P) and record the result in Table 3-3.
10. Using the spectrum analyzer, observe and record output spectrum
in Table 3-3.
11. Using the results above and Eq. (3-4), calculate the percentage
modulation of output signal and record the results in Table 3-3.
1E12. Repeat steps 9 to 11 for carrier amplitudes of 200mVp-p and
300mVp-p.
13. Connect a 150mVp-p, 3kHz sine wave to the audio input (I/P2),
and a 250mVp-p, 100kHz sine wave to the carrier input (I/P1).
14. Using the oscilloscope, observe the modulated signal at output
terminal (0/P) and record the result in Table 3-4.
1 5 . Using the spectrum analyzer, observe and record the output signal
spectrum in Table 3-4.
16. Using the results above and Eq. (3-4), calculate and record the
percentage modulation of output signal in Table 3-4.
E 17. Repeat steps 14 to 16 for the audio frequencies of 2kHz and 1kHz.
3-7
Unit 3 AM Modulators
Table 3-2
(Vc=250mVp-p, fc=100kHz, fm=1 kHz)
AudioAmplitude
Output Waveform Output Signal SpectrumPercentageModulation
250 mVp-p
Emax=
Em,n
200 mVp-p
E = min
150 mVp-p
E„ax=
Emit,
Unit 3 AM Modulators
Table 3-4
(V,=250mVp-p, Vm = 150mVp-p, fa =100 kHz)
AudioFrequency Output Waveform Output Signal Spectrum
PercentageModulation
3 kHz
Ems
Emin
2 kHz
F=-nun(
Emirs
1 kHz
Emax-Emit,
Unit 3 AM Modulators
3.5 QUESTIONS
In Fig. 3-4, if we change the value of R8 from 1 kS2 to 2 IcC2, what is the
variation of the AM output signal?
In Fig. 3-4, if we change the value of R9 from 6.8 k0 to 10 1(0, what is
the variation in the dc bias current of the MC1496?
Understanding the principle of amplitude demodulation.
Implementing an amplitude demodulator with diode.
3. Implementing an amplitude demodulator with a product detector.
4.2 DISCUSSION OF FUNDAMENTALS
A demodulation process is just the opposition of a modulation process. As
noticed in Chapter 3, an AM signal is a modulated signal that is high-frequency
carrier amplitude varied with low-frequency audio amplitude for transmission.
To recover the audio signal in receiver, it is necessary to extract the audio
signal from an AM signal. The process of extracting a modulating signal from a
modulated signal is called demodulation or detection. It is shown in Fig. 4-1. In
general, detectors can be categorized into two types: synchronous and
asynchronous detectors. We will discuss these two types of AM detectors in the
rest of this chapter.
AmplitudeDemodulator
AM Signal Audio Signal
Fig.4-1 Illustration of an amplitude demodulation
Diode Detector
Since an AM modulated signal is the signal that the carrier amplitude varies with
the modulating amplitude, a demodulator is used to extract the original
modulating signal from the AM signal.
4-1
Unit 4 AM Demodulators
Product Detector
Demodulation for AM signal can be also accomplished with the balancedmodulator discussed before. Such demodulator is called synchronous detectoror product detector. Fig. 4-4 provides the internal circuit of MC1496 balanced
modulator. See the discussion in Chapter 3 for details. If xAM(t) represents the
AM signal and x,(t) is the carrier, and are expressed by
X Aivi (i) = V De [1+ mcos(27if,„01[ cos(27-tf, t)] ( 4-1 )
x, = V, cos(27tfct) ( 4-2 )
If these two signals are connected to the inputs of balance demodulator, thenthe output of balance demodulator will be
x ou,(1) = kx ,(t) x xAm(t)
k170( V, 2 [1 +mcos(27zfm t)]cos2 (271f, t)
kV Dc V, 2 + kV 1V2 mcos(27zycnt)2 2
kV .V 2 r+
2 [1+ mcos(2gf„,t)]cos[2(27zict)] (4-3)
where k is the gain of balanced modulator. The first term on the right side ofEq.(4-3) represents dc level, the second term is the modulating signal, and thethird term is the second-order harmonic signal. To recover the modulating signal,the intelligence must be extracted from the AM signal xout(t).
Unit 4 AM Demodulators
Cl
R2 1 k R,1 2k
• 0Cu +12V0 luk
Ri1 C4 - R5 270
0.1u 0.1uR2k
—WV—s— 8 2 31k
Carrierinput
VR,100k
C2
10
0 0.1u
0 II 1UI
MC 1496
AMinput
C3O. lu
R9 1 k C lap 2.2uVR200k 14 12
A A,0
Demodulatedoutput
5Ic, — C9
0.1u T 2.2uT i'°461T
1000T
Fig. 4-5 Product detector circuit
Unit 4 AM Demodulators
8. Adjust the VR1 of AM modulator to get maximum amplitude of AM
signal output.
9. Set the vertical input of scope to DC coupling and observe the output
waveforms of the amplifier and the diode detector, and record the
results in Table 4-2.
D 1 O. Change the audio frequencies for 2kHz and 1kHz, and repeat step 9.
Unit 4 AM Demodulators
Table 4-1
(Vc=250mVp-p, Vm=150mVp-p, fc=200kHz)
AudioFrequency
Input Waveform Detector Output Waveform
3 kHz
2 kHz
1 kHz
4-9
Unit 4 AM Demodulators
Table 4-3
(V,=250mVp-p, Vm = 150mVp-p, fc=500kHz, m=50%)
AudioFrequency
Input Waveform Detector Output Waveform
3 kHz
2 kHz
1 kHz
4-11
Unit 4 AM Demodulators
4.5 QUESTIONS
1. In the diode detector circuit of Fig. 4-3, if the operational amplifier pA741
is neglected, what is the output signal?
2. In the product detector circuit of Fig. 4-5, if the carrier signal and the AM
ignal are asynchronous, what is the output signal?
What is the function of R9, C 7 or Cg in Fig. 4-5?
What is the function of VR, or VR 2 in Fig. 4-5?
5. What is the function of R5 or R6 in Fig. 4-5?
DSB-SC AND SSB MODULATORS
5.1 Objectives 5-1
5.2 Discussion Of Fundamentals ......... ..... ...... 5-1
Learning how to generate double-sideband suppressed carrier and
single-sideband modulated signals.
Learning how to test and adjust double-sideband suppressed carrier and
single-sideband balanced modulators.
5.2 DISCUSSION OF FUNDAMENTALS
The principle of circuit operations of this chapter is similar to that of Chapter 3
mentioned before. The circuit of Fig. 5-1 is a double-sideband
suppressed-carrier (DSB-SC) modulator. The balance circuit consisted by the
VR i is used to control the LM1496 operating in balance state. By adjusting the
VR, properly, this will ensure that the modulator operates in balance state. In
short, the major difference between DSB-SC and AM modulated signals is the
DSB-SC modulated signal containing no carrier. To achieve the requirement of
suppressing carrier, we should first connect the audio input to ground, and then
observe the LM1496 output to ensure no carrier presented by carefully
adjusting the VR i . If this is made and then reconnects the audio signal, the
DSB-SC modulated signal containing the upper- and lower-sideband signals will
be presented at LM1496 output.
Unit 5 DSB-SC and SSB Modulators
Since the amplitude-modulated signal contains these two sideband signals, it is
sometimes called as double-sideband AM. In double sideband suppressed
carrier modulation, the carrier signal is removed or suppressed by the balanced
modulator, and the modulated signal containing no carrier as shown in Fig. 5-2c.
Notice that these two sidebands contain the same audio signal when the
modulated signal is transmitted, while receivers may recover the audio signal
from each sideband signals by demodulation technique. This means that only
one of two sidebands is need in transmitting process. Thus an amplitude
modulation called single-sideband (SSB) is shown in Fig. 5-2d.
Suppose the audio input signal (pins 1 and 4) of LM1496 is Amcos2nfmt and the
carrier input signal (pins 8 and 10) is A ccos24t, then its output signal at pin 6
should be
V0 (t) = k(A„, cos27Cf„,t)(A, cos27Lfet)
kA. A, [cos271-(fn, + fc )t +cos27-1-(fm — L)t] ( 5-1 )
where k is the modulator gain, and ( fc-f-fm ) and ( fc-fm ) are the upper and lower
sideband modulated signals, respectively.
In Fig. 5-1, the source follower consisted of Q 1 and Q2 acts as a buffer due to
the characteristics of high input impedance and low output impedance. The
coupling capacitors C1, C2, C4, C5 and C8 are used for blocking dc signal while
coupling ac signal. The R 11 is for adjusting the gain of the balanced modulator
and the R12 is for bias current adjustment. Resistors R 1 , R2 , R13 and R14 provide
dc bias for operating requirement. Resistors R5 and R 10 are for AGC control.
Capacitors C3, C6 and C7 are used to bypass undesired noise. The VIR, are for
balancing, optimum operating point, minimizing distortion and determining types
of output signal (i.e., AM or DSB-SC).
5-3
R1
Carrier CI •
Input 10u
0-71
2k
Audio c2QiK30Ainput I or,
I0
SSBoutput
o455
_
Unit 5 DSB-SC and SSB Modulators
Amplitude
Lowersideband
Uppersideband
Frequency
fF La' f fctfnil
Fig. 5-2d Spectrum of SSB signal
To generate a SSB modulated signal from DSB-SC, a low-pass or high-passfilter is commonly used to filter one sideband signal. Unfortunately, it is difficultto take out single sideband signal from DSB-SC signal with 1st- or 2nd-orderlow- or high-pass filters because these two sideband spectrums are so close toeach other. A good solution of this problem is the use of ceramic or crystal filters.For example, we use the FFD455 ceramic band-pass filter to take out the uppersideband signal in experiment circuit, as shown in Fig. 5-3.
R7 270
C30 I u
C60 lu
1 k
R„1 K
WV'R,„ 1K
Ru 270
PA/Vt-1C5 8 2 30.1u
10
R41 k
LM1496
C4 VR1
0.1U R6 14
2k 100K100K4 5
RR26.8k
Rlk
Fig. 5-3 SSB modulator circuit.
5-5
Unit 5 DSB-SC and SSB Modulators
6. Using the oscilloscope, measure and record the waveforms listed in
Table 5-1.
7. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-1.
8, Change the audio amplitude to 600mVp-p. Measure and record the
waveforms listed in Table 5-2 using the oscilloscope.
9. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-2.
10. Change the carrier amplitude to 600mVp-p. Measure and record the
waveforms listed in Table 5-3 using the oscilloscope.
11. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-3.
12. Change the audio amplitude to 300 mVp-p and frequency to 2kHz,
and the carrier amplitude to 300mVp-p and frequency to 1 MHz. Using
the oscilloscope, measure and record the waveforms listed in Table
5-4.
13. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-4.
14. Remove the connect plug from J1 and insert it in J2 to change R11
(270Q) to R15 (3300). Change the audio amplitude to 600mVp-p and
frequency to MHz, and the carrier amplitude to 600mVp-p and
frequency to 500kHz. Hold VIR, position. Using the oscilloscope,
measure and record the waveforms listed in Table 5-5.
5-7
Unit 5 DSB-SC and SSB Modulators
Experiment 5-2 SSB Modulator
1. Locate SSB Modulator circuit on Module KL-93003. Insert the connect
plug in J2 to bypass ceramic filters.
2. Check each of source follower circuits for a proper bias. Set the vertical
input of oscilloscope to AC and observe the source output signal and
the input signal. Ensure that these two signals are the same but the
output amplitude is slightly smaller than the input amplitude. If done,
insert connect plugs in J3 and J4.
3. Turn the VIR, to its mid-position.
4. Connect the audio input (I/P2) to ground and connect a 500mVp-p, 457
kHz sine wave to the carrier input (I/P1). Carefully adjust VR, to get a
minimum output or zero. Then remove the connect plug from J2 and
insert it in J1.
5. Connect 'a 300mVp-p, 2kHz sine wave to the audio input and change
the carrier amplitude to 300mVp-p.
6. Using the oscilloscope, measure and record the waveforms listed in
Table 5-7.
7. Using the spectrum analyzer, observe and record the output signal
spectrum in Table 5-7.
8. Change the audio amplitude to 600mVp-p. Measure and record the
waveforms listed in Table 5-8 using the oscilloscope.