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Activity: Envelope Detector

Objective:

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:

s(t) = [1 + k cos(w_m t)] A cos(w_c t)

where:

  • m(t) = k cos(w_m  t) - message signal
  • c(t) = Acos(w_c t) - carrier signal
  • k - modulation index (typically varies between 0 and 1)
  • ωm - message frequency
  • A - carrier amplitude
  • ωc - carrier frequency

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:

  • Diode / rectifier - serves to enhance one half of the received signal over the other.
  • Low pass filter - required to remove the high frequency elements that remain within the signal after detection / demodulation. The filter usually consists of a very simple RC network but in some cases It can be provided simply by relying on the limited frequency response of the circuitry following the rectifier.

Materials:

ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
1 - 1 KΩ resistor
1 - 1uF capacitor
1 - 1N914 diode

Envelope Detector

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.

Hardware Setup:

Build the following breadboard circuit for the envelope detector circuit.

Figure 2. Envelope Detector breadboard circuit

Procedure:

Use the first waveform generator as source to provide the AM signal, with the following parameters:

  • k = 0.5
  • ωc = 10KHz
  • ωm = 100Hz
  • A = 3

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.

university/courses/electronics/electronics-lab-envelope-detector.1519315267.txt.gz · Last modified: 22 Feb 2018 17:01 by Antoniu Miclaus