Activity: AM Modulation and the Envelope Detector

Objective:

In this lab activity we will use the ADALM1000 to introduce amplitude modulation (AM) and envelope detection demodulation. A signal's envelope is equivalent to its outline, and an envelope detector connects the amplidude peaks of the signal. Envelope detection has numerous applications in the fields of signal processing and communications, one of which is amplitude modulation (AM) detection or demodulation.

Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio frequency 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:

where:

- message signal
- carrier signal
k - modulation index (typically varies between 0 and 1)
ω_m - message frequency
A - carrier amplitude
ω_c - 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. (ω_c » ω_m)

It consists of two main elements:

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

Materials:

ADALM1000 Active Learning Module
Solder-less breadboard, and jumper wire kit
1 - 10 KΩ resistor (brown black orange)
2 - 0.1uF capacitors (104)
1 - 1N914 diode

Envelope Detector

Consider the circuit presented in Figure 1. Two 0.1 uF capacitors are used in parallel to form a 0.2 uF total capacitance which along with the 10 KΩ resistor for a simple low pass filter.

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 circuit from figure 1 on your solderless breadboard for the envelope detector. Leave the CH A and CH B connections to the ADALM1000 disconnected for now while we generate an amplitude modulated test signal.

Procedure:

We will use the channel A waveform generator as source to provide the AM signal, with the following parameters:

Min = 1.7
Max = 3.3
Freq = 100Hz

We will use the channel B waveform generator as source to provide the carrier signal, with the following parameters:

Min = 1.5
Max = 3.5
Freq = 10KHz

With both CH A and CH B set to SVMI mode and Shape Sine you should see waveforms something like those shown in figure 2. Set the Horizontal time scale to 2.0 mSec/Div to display 2 cycles of the 100 Hz waveform.

Figure 2. Generated modulating and carrier signals

To generate the modulated signal we will use the Math function from the ALICE signal generator CH A. With the program paused, select the Math option under the Shape drop down menu. Enter the following equation which multiplies the captured modulation waveform from channel A with the captured carrier waveform from channel B. Because the signals are centered on 2.5 V that DC portion of the waveforms much be subtracted out. The 2.5 volt offset is then added back to after the multiplication to center the modulated signal in the 0 to 5 V range of the ALM1000.

(VBuffA-0.6)*(VBuffB-2.5)+2.5

The generated waveform is presented in Figure 3 as the green trace from CH A. Disconnect the capacitors from the circuit and observe the output signal. With CH B in the Hi Z mode connect it both CH-A and CH-B to your circuit.

Figure 3. Positive Half of the generated AM signal

Use the vertical range and position controls to shift the traces so that they don not overlap each other. This make it easier to see the two waveforms. Without the capacitor connected, the circuit works like a positive half-wave rectifier that keeps the part of the signal that is above 2.5 V.

Now connect the capacitors back to the circuit. A plot example is presented in Figure 4.

Figure 4. Filtered demodulated signal

The resulting demodulated signal is the envelope of the positive half wave obtained previously. It is actually the 100 Hz message signal with some 10 KHz ripple on it.

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Activity: AM Modulation and the Envelope Detector