LAB 3 – LINEAR INTEGRATED-CIRCUIT AM MODULATOR

1.Introduction

In this experiment we examine the operation of the XR-2206 monolithic function generator as an AM DSBFC modulator. See the data sheet for information on the XR-2206.

2.Materials Required

Equipment

1 / - protoboard
1 / - Dual dc power supply(+l2Vdc and 0 to+6 Vdc)
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 / - 1 k-Ohm variable resistor
3 / - 4.7 k-Ohm resistors / 2 / - 0.001 Fcapacitors
2 / - 10 k-Ohm resistor / 2 / - 1 F capacitors
1 / - 47 k-Ohm resistor / 2 / - 10 F capacitor

3.Pre-lab

Print the XR-2206 data sheet and bring it with you to the lab session. In the Principles ofOperation section of the data sheet, find and record the equation for the theoretical frequency of operation, f0.

4.Circuit 1 - Output Amplitudeversus DC Input Control Voltage

In this section,we examine the output amplitude-versus-input Voltage characteristics of the XR-2206 by applying a DC control Voltage to its modulator. The schematic diagram for the function generator circuit used in this section is shown in Figure 1.

Procedure

  1. Construct the function generator circuit shown in Figure 1. Set the DC control Voltage VCto 0 Vdc.
  1. Adjust R2until a sine wave with minimum distortion is observed at Vout;then measure its frequency and amplitude
  2. Calculate the VCO free-running frequency from the formula in the XR-2206 data sheet, and compare this to the frequency you measured in step 2.
  3. Increase the amplitude of the control Voltage in 0.5-Volt steps for values from 0 to +6Vdc, andmeasure the amplitude of the sine wave observed at Voutfor each value of control Voltage.
  4. Using Excel, graph the output amplitude-versus-control Voltage for the control Voltages using the data obtained in step 5. On the same graph, plot the theoretical relation shown in Figure 6 in the XR-2206 data sheet.

Figure 1. Output Amplitude Versus-Bias Voltage.

5.Circuit 2 – Amplitude-modulatedDouble-sideband Full-carrier (AM DSBFC) Modulator

In this section we use the XR-2206 to generate an AM DSBFC waveform byapplying a signal to the XR-2206 modulator that contains both a dc and an ac component. The schematic diagram for the linear integrated-circuit AM DSBFC modulator is shown in Figure 2. The function generator circuit of the XR-2206 provides the carrier, and an external audio signal generator provides the modulating signal.

IMPORTANT – Do not disassemble this circuit when you complete the lab; you will use it again for Lab 5.

Figure 2. AM Modulator Using the XR-2206 Function Generator.

Procedure

  1. Construct the function generator circuit shown in Figure 2. (Note that the only added component is the 10-F capacitor that connects to the function generator.)
  1. Set the amplitude of the control Voltage VCto 0 V.
  2. Set the amplitude of the audio signal generator output Voltage to 0 V.
  3. Check that the function generator free-running frequency has not changed.
  4. Check that the R2is still adjusted to produce a sine wave with minimum distortion is at Vout.
  5. Increase the control Voltage to +4V dc.
  6. Set the output of the signal generator output Voltage to 3 VP-P at 1 kHz.
  7. Adjust the amplitude of the audio signal generator output Voltage until an AM envelope with100% modulation is observed at Vout.
  8. Sketch the waveform envelope observed in step 8, indicating its amplitude and frequency.

  1. Set the amplitude of the audio signal generator output Voltage to 1.5 VP-P and determine the percent modulation of the output envelope using the following formula.

Percent AM modulation

whereVmax =maximum peak-to-peak envelope amplitude, Volts

Vmin =minimum peak-to-peak envelope amplitude, Volts

(This is most easily done by using the oscilloscope cursors to find the peak-to-peak Voltages at the modulation maximum and at the modulation minimum.)

  1. Make the necessary connections between the AM modulator and the oscilloscope to display a trapezoidal pattern, and record this pattern, indicating the Voltages on it that you need to compute the per cent modulation.
  2. Vary the amplitude and frequency of the audio signal generator output Voltage and note what effect varying them has on the trapezoidal pattern.

6. Discussion Questions

  1. Does adjusting the sine wave for minimum distortion affect the VCO frequency?
  2. In Figure 2, for a fixed function generator amplitude, which components affect the per cent modulation?

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