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In this lab activity we will use ADALM2000 to introduce envelope detection and amplitude modulation. The signal's envelope is equivalent to its outline, and an envelope detector connects all the peaks in this signal. Envelope detection has numerous applications in the fields of signal processing and communications, one of which is amplitude modulation (AM) detection.
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels.
A typical amplitude modulated signal has the following equation:
An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the envelope of the original signal. (ωc » ωm)
It consists of two main elements:
ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
1 - 1 KΩ resistor
1 - 1uF capacitor
1 - 1N914 diode
Consider the circuit presented in Figure 1.
Figure 1. Envelope Detector Circuit
The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal.
Build the following breadboard circuit for the envelope detector circuit.
Figure 2. Envelope Detector breadboard circuit
Use the first waveform generator as source to provide the AM signal, with the following parameters:
To generate the AM signal use the math function from the Scopy signal generator. Set the main frequency to 100Hz and apply the following function: (1+0.5*cos(t))*3*cos(100*t). The generated waveform is presented in Figure 3.
Figure 3. Generated AM signal
Configure the scope so that output signal is displayed on channel 2.
Disconnect the capacitor from the circuit and observe the output signal. A plot example is presented in Figure 4.
Figure 4. Positive Half of the generated AM signal
Without the capacitor connected, the circuit works like a positive half-wave rectifier that keeps the part of the signal that is above 0V.
Now connect the capacitor back to the circuit. A plot example is presented in Figure 5.
Figure 6. Generated AM signal
The obtained signal is the envelope of the positive half wave obtained previously. It is actually the 100 Hz message signal with some 10 KHz variations on it.