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ELECTRONICS 2 LAB
AUTOMATIC GAIN CONTROL (AGC)
AUTOMATIC
PART 1
GAIN
CONTROL
AGC with passive peak detector
Circuit Diagram
AMPL IFIE R
Var iable
amplitude
si gna l
+1 5 V
12k
0,33 uF
V in
Vw
X1
X2
12k
47 pF
ANALO G
MULTIPL IE R
Y1
Y2
2 ,2k
A +V s
D
W
6
Z
3
3 -V s
22k
L F34 7
Co nsta nt
amplitud e
si gna l
V o1
A1
15 V
AG C
VOLTAG E
1N414 8
INTEG RATOR
0 ,33 u F
12k
12k
PASS IVE
PEAK
DE TE CTOR
L F34 7
A2
+1 5 V
1 00k
100k
0,33 uF
+1 5 V
1N523 4 B
AD633 Analog Multiplier
47 0
LF347 Quad Op Amp
Rev. 9/26/2002
AGC LAB
Page 1
ELECTRONICS 2 LAB
AUTOMATIC GAIN CONTROL (AGC)
Pre-lab
1.
Determine the regulated amplitude of Vo1 – look up Zener data at http://www.fairchildsemi.com/
2.
Determine VY1 min and max assuming typical saturation voltages for LF347 with Vsup = ±15V
3.
Determine the range of input voltages (main input Vin) for which the AGC is operational.
Why does not the AGC work outside this above-calculated range?
4.
What is the peak AC voltage of Vw?
5.
Determine the peak to peak ripple at the peak detector output and at VY1 for frequencies of 1 kHz
and 10 kHz. Draw the expected ripple waveforms.
Lab Procedure
NOTE:
1.
The HP signal generator displays half the actual O/P amplitude if load >> 50Ω
If load = 50Ω, it displays the correct amplitude.
Assemble circuit as per breadboard layout provided at the end.
2.
Using a 1 kHz input sinewave, adjust the amplitude of Vin in order to obtain the DC voltages
for VY1 listed below. For each case measure the corresponding values of Vo2 (DC), Vo1 (rms), Vw(rms) and
Vin (rms) with the DMM. Monitor Vin and Vo1 on the oscilloscope throughout. Calculate Amult from
measurements.
3.
Repeat the above for VY1(DC) slightly above minimum and slightly below maximum when Vo1 is
still constant.
VY1 (DC)
min
1
2
4
6
8
10
max
Vo2 (DC)
Vo1 (rms)
Vin (rms)
Vw(rms)
A mult =
4.
Vw
Vin
Measure the exact value of the Zener reference voltage – record value.
5.
Measure the ripple voltage (with the scope) at the peak detector output and at VY1 at frequencies
of 100 Hz, 1 kHz and 10 kHz – ripple may be too small to be measured in some cases. Ensure that Vin
is within operating range.
Rev. 9/26/2002
AGC LAB
Page 2
ELECTRONICS 2 LAB
AUTOMATIC GAIN CONTROL (AGC)
6.
With Vin set such as to obtain VY1 around 4V DC, measure Vo1 (rms) at 1 kHz, 10 kHz, 100 kHz
and measure the minimum and maximum frequencies at which Vo1 is no longer regulated.
7.
Measure THD of Vin, Vw and Vo1 at 1 kHz when AGC loop functions at VY1 = 1V and VY1=10V.
%THD of Vw
%THD of Vin
%THD of Vo1
VY1 = 1V
VY1 = 10V
Post lab
1.
Compare measured values to pre-lab values everywhere applicable.
2.
Did Vo1 vary at all? What was the % variation of Vo1 over the entire range of Vin at 1 kHz?
Is this % variation acceptable? Explain.
3.
Which device introduced the most distortion? Explain. Was distortion different for different
input levels?
4.
Using the results of step 3 of the procedure, explain how the AGC circuit works.
PART 2
AGC with active peak detector
Circuit Diagram
Va ria ble
am plitu de
sig na l
+1 5V
12k
0,33 uF
V in
ANALO G
MUL TIPLIER
X1
X2
12k
AMPLIFIER
Vw
Y1
Y2
2 ,2k
A +Vs
D
W
6
Z
3
3 -V s
22k
V o1
LF3 47
C on sta nt
am p litude
sig na l
A1
1 5V
AGC
VO LTAGE
12 k
AC TIVE
PEAK DETEC TO R
INT EGR ATO R
0,3 3 uF
12k
100k
1 2K
LF3 47
1N4 148
A4
A2
+1 5V
A3
+15 V
1 N 52 34B
100K
0,33 uF
47 0
Pre-lab
Rev. 9/26/2002
AGC LAB
Page 3
ELECTRONICS 2 LAB
AUTOMATIC GAIN CONTROL (AGC)
1.
Determine the regulated amplitude of Vo1 – look up Zener data at http://www.fairchildsemi.com/
2.
Determine VY1 min and max assuming typical saturation voltages for LF347 with Vsup = ±15V
3.
Determine the range of input voltages (main input Vin) for which the AGC is operational.
Why does not the AGC work outside this above-calculated range?
4.
What is the peak AC voltage of Vw?
5.
Determine the peak to peak ripple at the peak detector output and at VY1 for frequencies of 1 kHz
and 10 kHz. Draw the expected ripple waveforms.
Lab Procedure
1.
Assemble circuit as per breadboard layout provided at the end – don’t forget to remove the 47 pF
capacitor in the inverting amplifier.
2.
Using a 1 kHz input sinewave, adjust the amplitude of Vin in order to obtain the DC voltages
for VY1 listed below. For each case measure the corresponding values of Vo2 (DC), Vo1 (rms), Vw(rms) and
Vin (rms) with the DMM. Monitor Vin and Vo1 on the oscilloscope throughout. Calculate Amult from
measurements.
3.
Repeat the above for VY1(DC) slightly above minimum and slightly below maximum when Vo1m
is still constant.
VY1 (DC)
min
1
5
10
max
Vo2 (DC)
Vo1 (rms)
Vin (rms)
Vw(rms)
A mult =
4.
Vw
Vin
Measure the exact value of the Zener reference voltage – record value.
5.
Measure the ripple voltage (with the scope) at the peak detector output and at VY1 at frequencies
of 100 Hz, 1 kHz and 10 kHz – ripple may be too small to be measured in some cases. Ensure that Vin
is within operating range.
6.
With Vin set such as to obtain VY1 around 4V DC, measure Vo1 (rms) at 1 kHz, 10 kHz, 100 kHz
and measure the minimum and maximum frequencies at which Vo1 is no longer regulated.
Rev. 9/26/2002
AGC LAB
Page 4
ELECTRONICS 2 LAB
Post lab
AUTOMATIC GAIN CONTROL (AGC)
1.
Compare measured values to pre-lab values everywhere applicable.
2.
Did Vo1 vary at all? What was the % variation of Vo1? Is it good? Explain.
3.
Using the results of step 3 of the procedure, explain how the AGC circuit works.
4.
Compare the performance of the two AGC circuits. Which one performed better? Explain.
Which circuit has a more predictable and accurate output? Explain.
Which one operated over a wider frequency range?
Rev. 9/26/2002
AGC LAB
Page 5
ELECTRONICS 2 LAB
AUTOMATIC GAIN CONTROL (AGC)
BREADBOARD LAYOUT
Rev. 9/26/2002
AGC LAB
Page 6
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