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 VoltVoltage that is proportional to the shape of either the positive or negative half of the envelope. The output VoltVoltage 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.

Figure 1. AM Peak Detector.

2.  Materials Required

Equipment

1 / - protoboard
1 / - dual dc power supply (+12 V dc and 0 to + 8 V dc)
1 / - audio signal generator (0 to 20 kHz)
1 / - standard oscilloscope (10 MHz)
1 / - assortment of test leads and hookup wire

Parts List

1 / - XR-2206 function generator / 1 / - 1N914 diode
1 / - LM148 operational amplifier / 1 / - 1 kWOhm variable resistor
3 / - 4.7 kWOhm resistors / 2 / - 0.001 mF capacitors
2 / - 10 kWOhm resistors / 1 / - 0.01 mF capacitors
1 / - 47 kWOhm resistor / 2 / - 1 mF capacitor
1 / - 100 kWOhm resistor / 1 / - 10 mF capacitor

3.  Pre-lab

Bring with you to the lab session the XR-2206 data sheet.

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.

Figure 2. Peak Detector Schematic Diagram.

Procedure

  1. Construct the peak detector circuit shown in Figure 2.
  1. Set the signal generator output to a 4-VP-P sinusoid with a frequency of 100 kHz.
  2. Measure the DC and peak-to-peak Voltages at Vout and sketch the waveform.
  3. Repeat step 3 for 10 kHz, 1 kHz, and 100 Hz.
  4. Set the signal generator frequency to 100 kHz and vary the amplitude of the signal generator output VoltVoltage while observing the waveform at Vout.
  5. Describe what effect varying the amplitude of the signal generator output VoltVoltage has on the output waveform.
  6. 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. Construct the function generator circuit shown in Figure 3.
  1. Adjust the amplitude of the control VoltageVoltVoltage to 0 V.
  2. Set the amplitude of the audio signal generator output VoltVoltage to 0 V.
  3. Calculate the function generator free-running frequency and check that the frequency of Vout is close to this.
  4. Adjust R2 until a sine wave with minimum distortion is observed at pin 2.
  5. Increase the control VoltageVoltVoltage to +4 V dc.
  6. Set the frequency of the audio signal generator to 1 kHz and adjust its output VoltVoltage amplitude until pin 2 shows an AM envelope with 50% modulation.

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 VoltVoltage equal to the peak envelopeVoltVoltage 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 side of the diode is the detected audio signal riding on top of a DC VoltageVoltVoltage. The DC VoltageVoltVoltage 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.

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.
  1. Sketch the waveform at Vout of Figure 4, and record Vdc and Vpeak-to-peak.
  2. 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.
  3. Calculate fm(max) for m = 0.5.
  4. 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).
  5. 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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