AM transmission - envelope detector

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AM transmission - envelope detector
AM transmission and envelope detector - audio demo. The carrier is modulated so that its envelope carries the audio signal.

Problems with audio playback?

Transmitted signal with envelope - Double-sideband amplitude modulation DSB-AM
AM modulated signal spectrum
Standard AM: Envelope detection can be deployed because the transmitted signal's envelope doesn't change sign. Transmit spectrum of DSB-AM.

An easy way to recover the message signal of an AM-modulated signal is the envelope detection. This non-coherent detection doesn't require a carrier recovery circuit. In its simplest form it consists of a rectifier diode and a low-pass filter.

envelopedemodulator
Simple hardware envelope demodulator. The direct voltage is filtered by a subsequent loud speaker.

The discovery of crystal detectors in the early 20th century enabled the development of simple radio receivers. The crystals capability to rectify signals caused amplitude modulation (AM) to become the prevalent method to send and receive messages. The amplitude of a high frequency carrier is modified by a low frequency information signal, which is detected afterwards by rectification. AM requires only a minimum effort of hardware.

This setup implements the transmission of a speech audio waveform by amplitude modulation.

Start

A speech audio signal is AM modulated, transmitted and demodulated using a simple envelope detector.

Look at the transmission signal in the frequency domain and see the carrier frequency and two adjacent sidebands. Note that each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. This standard AM is also called "double-sideband amplitude modulation" (DSB-AM) to distinguish it from further AM modulation methods.

rectifier-oscilloscope
Sink signal, AM demodulated
The diode ('Rectifier') rectifies the AM modulated signal and is the first step to detect the envelope. After this the high frequency carrier and the direct voltage are filtered out by the band pass filter.

Listen to the audio speech signal that is envelope detected after AM transmission. The audio playback starts automatically when the app is launched.


Experiment

Now that you are familiar with the envelope detection analyze what happens when one of the following parameters is changed:

Change Effect on demodulated signal
Increase the modulation index to m=4
Distorted, louder
Switch off the carrier s^
Distorted, quieter
Change the carrier frequency to 60kHz
No effect
Change phase of the carrier to 60°
No effect

Next

AM is inefficient in power usage as most of the power is consumed by the carrier signal. But it conveys none of the original information. Measure the power of the total transmission signal and the shares taken by the carrier and the two sidebands.

Take a look at the alternative AM demodulation technique Synchronous detector.

Check out Wikipedia for more information: Envelope detector, Crystal radio

quant diagram
Modulating audio signal spectrum.
quant diagram
AM modulated transmission signal spectrum. Lower sideband (LSB) and upper sideband (USB) - each sideband is a mirror image of the other.