ES442 Lab 4 Amplitude Modulation and Demodulation

ES442 Lab#4

I. Objective 1. Build simple AM modulator by using Crystal Oscillator. 2. Build simple envelope detector for AM demodulation.

II. Introduction For AM signals, the information is carried by the amplitude (or envelope) of the signal. There are many ways to detect the envelope of AM signals. Here we will consider the simplest, used by most portable radios, the envelope detector, as shown in Fig. 1.

Fig. 1 Envelope Detector

This is essentially just a halfwave rectifier which charges a capacitor to a voltage to the peak voltage of the incoming AM waveform. When the input wave's amplitude increases, the capacitor voltage is increased via the rectifying diode. When the input's amplitude falls, the capacitor voltage is reduced by being discharged by a ‘bleed’ resistor, R. The main advantage of this form of AM Demodulator is that it is very simple and cheap! Just one diode, one capacitor, and one resistor. That's why it is used so often. However, it does suffer from some practical problems, as shown in Fig. 2.

Tc Tm

Fig. 2 Operations of Envelope Detector

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ES442 Lab#4

Whenever the amplitude of the AM signal starts to drop, the capacitor will discharge through the resistor. Denote the maximum amplitude of the AM signal

as V p , then the capacitor discharge will follow the following equation

Vp’ = Vp . exp(-t/τ)

where t=RC is the time constant of the RC circuits.

It can be seen from Fig. 2 that, in order to minimize the ripple effect at high voltage, we should have τ>> Tc, where Tc is the period of the carrier. On the other hand, in order to minimize the negative peak clipping, we wish the voltage drops as fast as possible, thus τ <

IV. Procedures

A. Amplitude Demodulation with Envelope Detector

1. Use the function generator generate an AM signal s(t)=3[1 + 0.5cos(2 πfmt)]cos(2 πfct) with fm= 10 and fc= 1KHz. Capture the time domain signal in the oscilloscope. (Label the value of Amax and Amin in your report. 2. Construct the circuit shown in Fig. 1 with the prototype board. Use IN4001 as the diode. The time constant of the circuit is chosen to be 10ms (why?). The capacitor is 10uF. Choose the resistor based on your pre-lab calculation. 3. Apply the AM signal to the input of the envelope detector. Use channel 1 of the oscilloscope to observe the AM signal. 4. Use channel 2 of the oscilloscope to measure the output of the envelope detector. To get a stable display, set the trigger signal to channel 2. Capture the display. 5. Change the modulating signal to square wave. Capture the display 6. Set fm= 10 and fc= 1KHz. What do you observe and why? Capture the display. 7. Set fm= 10 and fc= 100Hz. What do you observe and why? Capture the display.

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B. Amplitude Demodulation with AM Radio In this section we are going to demodulate the AM signal with a portable AM radio, and listen to the demodulated signal.

1. Use the function generator to generate an AM waveform

s(t)=3[1 + 0.5cos(2 πfmt)]cos(2 πfct) with fm= 500 and fc= 1MHz. Attach a long wire at the output of the function generator as antenna. Theoretically, the length of the antenna should be ¼ of the carrier wavelength. What is the theoretical length of the antenna? 2. Tune your radio to 1MHz. Adjust the volume until you hear the tone. 3. Use the knob the freely adjust the value of fm and listen to the change of tone you can hear. 4. Adjust the modulation index. What happens? Explain.

B. Amplitude Modulation with Crystal Oscillator In this section we are going to build a simple AM radio transmitter by using a 1MHz crystal oscillator. The circuit diagram of the AM radio transmitter is shown in the following figure.

Fig. 2 A simple AM radio transmitter

1. Build the circuit as shown in Fig. 2 with the audio transformer and 1MHz crystal oscillator. (IMPORTANT: A voltage higher than 9V will destroy the crystal oscillator). 2. Connect the mono-plug or stero-plug to a music player or computer, and play music. 3. Tune the AM radio to 1MHz. Adjust the volume until you have clear reception of the music.

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