Electrical Communications Systems

Course No. 0909-331-01

Spring 2005

Laboratory Project 2

AM and DSB-SC Modulation and Detection

Objectives

This project has 2 parts. In Part 1, you will investigate the performance of the envelope detector. In Part 2, you will generate and detect AM and DSB-SC bandpass signals using the LM1596/LM1496 Balanced Modulator-Demodulator.
In both parts, you will test the system with single-tone AM ( w and w/o added Gaussian noise) and multi-tone AM signals (w/o added noise).

Equipment and Software

HP 33120A Function Generator/Arbitrary Waveform Generator
HP 54645A Oscilloscope
HP8591EM EMC Analyzer/ HP Infinium Oscilloscope /HP 54657A FFT Module
1N4148 Fast Switching Diode
LM 613 Dual Operational Amplifier. Download data-sheet.
LM1596/LM1496 Balanced Modulator-Demodulator ICs. Download data-sheet.
Assorted Resistors and Capacitors (based on design).
PC speakers
MATLAB
Agilent IntuiLink Connectivity Software
Audio Player Software on PC
CD with your favorite music!

Project Requirements

Part 1

Design the envelope detector circuit shown in Figure 1. The design equation is f_co = 1/(2pR₁C₁) where f_co is the cut-off frequency of the low-pass filter R₁C₁ . C_c is a coupling capacitor, chosen such that C_c>> C₁ . As initial design parameters, you can choose = 10 k-ohms and = 0.001 microfarads, which provides a cut-off frequency of 15.9 kHz. (As part of the theory section in your report, you are expected to derive this design equation).

Figure 1: Envelope detector circuit.

Single-Tone AM Detection: No added Gaussian Noise

Using the HP Arb. Function Generator, generate an AM wave with carrier frequency of 600 kHz (in the AM band) and modulation frequency 2.5 kHz (single tone).

Vary the modulation depth from 10% to 120%.
For each modulation depth, observe and digitally capture the input and output waveforms.
Perform a spectral analysis of the input and output waveforms. Observe the spectral components due to ripple and harmonic distortion.
Listen to the output signal using the PC speakers. Compare with the pure tone of the same frequency.

Set the modulation index to an optimal value (max value for minimum distortion) and vary the modulating signal frequency to determine the 3-dB bandwidth of the detector (value at which the output signal falls to 0.707 of its maximum value).
Experiment with varying the carrier frequency and/or cut-off frequency of the LPF.

Multi-Tone AM Detection: No added Gaussian Noise

Figure 2 shows the overall block diagram for this part of the lab project.

Figure 2: Multi-tone AM generation and detection.

Obtain a multi-tone baseband signal by playing a CD on your PC and tapping into D/A converter output of the sound card.
Feed this baseband signal into the external AM modulating source input at the back of the HP 33120A Function Generator.
Choose a suitable carrier frequency (in the AM broadcast band) and generate an AM signal by choosing external modulating source in the HP 33120A menu.
Observe the modulated signal on the oscilloscope. Vary the modulation depth by varying the gain of the operational amplifier.
Apply the multi-tone AM signal to the input of the envelope detector. For each modulation depth:

Observe, calculate the modulation depth and digitally capture the input and output waveforms.
Perform a spectral analysis of the input and output waveforms.

Single-Tone AM Detection: With added Gaussian Noise

Repeat the single-tone AM detection experiment by digitally synthesizing single-tone AM signals with varying SNR. Figure 3 shows the overall block diagram for this part of the lab project.

Figure 3: Block diagram of the approach for single-tone AM detection with added Gaussian noise.

At each stage, note your obervations and conclusions.

Part 2

The obective is to observe (in time and spectral domains) & listen as audio tones progresses along a modulation-demodulation system, both in the presence and absence of added Gaussian noise.

Prelab: Download and print out the data-sheet for the LM1596/LM1496 balanced modulator-demodulator.
Implement the DSB-SC Modulator circuit shown in the data sheet (reproduced in Figure 4 below). Test the system response by feeding two audio-frequency tones to the input pins. Observe the waveforms on the oscilloscope; listen using the speakers.

Figure 4: Application circuit from the LM1596/LM1496 data-sheet.

Increase the carrier frequency input to the RF range (AM broadcast band) and observe the output waveforms. Vary the carrier null potentiometer setting to obtain standard AM and DSB-SC modulation (you may need to replace the two 10 k resistors feeding the potentiometer with a pair of 750 ohm resistors to get appreciable carrier feedthrough). Observe the system response both in the time and spectral domains (you need to perform a signal capture followed by an FFT operation to do this).
Experiment with various carrier and modulation frequency settings.
Vary the modulating signal amplitude - calculate the corresponding changes in modulation index. Determine the spectral response.
Repeat the experiment by mixing in a multi-tone modulating signal (generated as in Part 1).
Implement the product detection circuit shown in the data-sheet (reproduced in Figure 5 below). Test the circuit by feeding in an externally modulation multi-tone AM signal from the arb (the block diagram for this part is similar to Figure 2; except that the envelope detector is replaced by the LM1496 demodulator circuit). Observe and listen to the audio output.

Figure 5: Application circuit from the LM1596/LM1496 data-sheet.

Test the product detector circuit by feeding in digitally synthesized single-tone AM and DSB-SC signals with varying SNR. When does the modulator fail to detect the message signal?
Link the modulator-demodulator circuits and observe signal progression from input to output.

Required Reading

Sections 4.11 - 4.13, 5.1 - 5.4 of textbook.
User's Guides for HP 33120A Function Generator/Arbitrary Waveform Generator, HP 54645A Oscilloscope and HP 54657A FFT Module

Click here for required lab project report format.
Click here for suggestions for a good lab report.

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