LAB 4 – AM PEAK DETECTOR
1. Introduction
The simplest circuit available for demodulating AM DSBFC waveforms is a diode peak
detector. A diode peak detector is a noncoherent AM demodulator. It detects the peaks of
the AM envelope and thus produces an output Voltage that is proportional to the shape of
either the positive or negative half of the envelope. The output Voltage from a peak
detector depends on both the amplitude and frequency of the modulating signal.
Essentially, the carrier component of the AM envelope captures the diode and forces it to
turn on and off (rectify) synchronously. A lowpass filter, placed after the diode, separates
the demodulated audio signal from the rectified AM envelope. The block diagram for the
test circuit used in this experiment is shown in Figure 1.
Vout
AM DSBFC
Modulator
Carrier
Signal
Generator
AM Peak
Detector
Audio
Signal Generator
(Modulating Signal)
Figure 1. AM Peak Detector.
2. Materials Required
Equipment
1
1
1
1
1
- protoboard
- dual dc power supply (+12 V dc and 0 to + 8 V dc)
- audio signal generator (0 to 20 kHz)
- standard oscilloscope (10 MHz)
- assortment of test leads and hookup wire
Parts List
1
1
3
2
1
1
- XR-2206 function generator
- LM148 operational amplifier
- 4.7 k resistors
- 10 k resistors
- 47 k resistor
- 100 k resistor
1
1
2
1
2
1
- 1N914 diode
- 1 k variable resistor
- 0.001 F capacitors
- 0.01 F capacitors
- 1 F capacitor
- 10 F capacitor
3. Pre-lab
Bring with you to the lab session the XR-2206 data sheet.
LAB 4 – AM PEAK DETECTOR
4. Circuit 1 – Peak Detector
In this section, we examine the rectifying action of a peak detector. The schematic
diagram for the peak detector used in this section is shown in Figure 2. During the
positive half-cycle of the signal generator output signal, D1 conducts, charging C1.
During the negative half-cycle, D1 is off and C1 discharges slightly through resistor R1.
Therefore, the output waveform is simply a filtered half-wave rectified signal with a
positive average value.
D1 1N4001
Vout
Signal
Generator
R1
100 K
C2
0.01 F
Figure 2. Peak Detector Schematic Diagram.
Procedure
1.
2.
3.
4.
5.
Construct the peak detector circuit shown in Figure 2.
Set the signal generator output to a 4-VP-P sinusoid with a frequency of 100 kHz.
Measure the DC and peak-to-peak Voltages at Vout and sketch the waveform.
Repeat step 3 for 10 kHz, 1 kHz, and 100 Hz.
Set the signal generator frequency to 100 kHz and vary the amplitude of the signal
generator output Voltage while observing the waveform at Vout.
6. Describe what effect varying the amplitude of the signal generator output Voltage has
on the output waveform.
7. Reverse the polarity of Diode D1 and set the signal generator output to a 4-VP-P
sinusoid with a frequency of 100 kHz. Sketch the waveform.
Note: Do not disassemble this circuit, as it is needed later.
5. Circuit 2 – Linear Integrated-Circuit AM DSBFC Modulator
The XR-2206 function generator generates an AM DSBFC waveform. The schematic
diagram for the linear integrated-circuit AM DSBFC modulator circuit used in this
section is shown in Figure 3. (This is the same circuit that we used in Lab 4.)
Procedure
1.
2.
3.
4.
Construct the function generator circuit shown in Figure 3.
Adjust the amplitude of the control Voltage to 0 V.
Set the amplitude of the audio signal generator output Voltage to 0 V.
Calculate the function generator free-running frequency and check that the frequency
of Vout is close to this.
5. Adjust R2 until a sine wave with minimum distortion is observed at pin 2.
6. Increase the control Voltage to +4 V dc.
2
LAB 4 – AM PEAK DETECTOR
7. Set the frequency of the audio signal generator to 1 kHz and adjust its output Voltage
amplitude until pin 2 shows an AM envelope with 50% modulation.
R2 1 K
+12 V dc
4
1 F
VC
10 F
16 15
14 13
VCC
2
+1
1
Multiplier/
Sine Shaper
0.001 F
to Demod
4.7 K
Mult.
5
3
10 K
Signal
Gen.
VCO
6
C1
0.001
F
9
DC
Voltage
11
Bypass
8
7
47 K
XR-2206
Current
Switches
10
R1
10 K
1 F
+12 V dc
4.7 K
GND
12
10 F
4.7 K
Figure 3. AM DSBFC modulator.
6. Circuit 3 – AM Peak Detector
In this section, we examine the operation of an AM peak detector. The input waveform to
an AM peak detector comprises a carrier frequency and its upper and lower side
frequencies (i.e., an AM envelope). A diode is a nonlinear device. Therefore, nonlinear
mixing (heterodyning) occurs between the carrier and its associated side frequencies. The
difference between the upper side frequency and the carrier and the lower side frequency
and the carrier is the modulating signal frequency (audio). The audio is separated from
the composite waveform with a simple lowpass filter. The amplitude-modulated signal is
generated by the circuit of Figure 3. The schematic diagram for the peak detector circuit
used in this section is shown in Figure 4. This is simply the peak detector examined in
Section B preceded by a follower amplifier to avoid overloading the modulator circuit.
During the positive half-cycles of the AM envelope, D1 is forward biased and on and
thus, C1 charges to a Voltage equal to the peak envelope minus the barrier potential of the
diode. During the negative half cycles, D1 is reverse biased and off and C1 discharges
slightly through R1. Each successive peak of the AM envelope has a different peak
amplitude. Therefore, C1 charges and discharges in proportion to the shape of the
envelope (i.e., proportional to the modulating signal). Thus, the waveform on the output
3
LAB 4 – AM PEAK DETECTOR
side of the diode is the detected audio signal riding on top of a DC Voltage. The DC
Voltage is approximately equal to the amplitude of the unmodulated carrier minus the
barrier potential of the diode. The minimum Vout is zero, which means that if Vmin of the
modulated signal is less than the barrier potential of the diode, the Vout waveform will be
distorted.
+12
from
Mod
2
4
3
11
D1 1N4001
Vout
1
R2
100 K
-12
LM148 or
equivalent
C2
0.01 F
Figure 4. AM DSBFC Demodulator-Peak Detector.
Procedure
1. Construct the peak detector circuits shown in Figure 4 and connect it to the AM
DSBFC modulator. Turn on the power to the modulator and set the frequency of the
audio signal generator to 100 Hz. Check that the modulator output is an AM
waveform with about 50 % modulation.
2. Sketch the waveform at Vout of Figure 4, and record Vdc and Vpeak-to-peak.
3. Increase the audio signal generator frequency to 1 kHz. Sketch the waveform at Vout.,
and record Vdc and Vpeak-to-peak. Name the distortion present.
4. Calculate fm(max) for m = 0.5.
5. Set the audio signal frequency to 100 Hz and slowly increase the frequency until you
can just discern diagonal clipping. Compare this with the calculated fm(max).
6. With the signal generator frequency at 100 Hz, adjust its output Voltage so that the
modulator output shows 100 % modulation. Sketch the Vout waveform. (At 100 %
modulation, Vmin is zero. Because Vout can’t become negative, its waveform will be
distorted.)
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