AM transmission - synchronous detector

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AM transmission - synchronous detector
AM transmission - synchronous detector

Problems with audio playback?

Transmitted signal with envelope - Double-sideband suppressed-carrier, DSB-SC
Transmit spectrum - Double-sideband suppressed-carrier, DSB-SC
AM without a carrier: Envelope detection cannot be deployed because the transmitted signal's envelope changes sign. Transmit spectrum of DSB-SC.

In AM transmissions without or with a reduced carrier like

a simple envelope detection or non-coherent detection is not possible because the envelope detection condition is not met for overmodulated signals. A more complex synchronous or coherent detection is required like implemented in this setup. The received AM-signal is multiplied with a local carrier, a signal ideally equal to the carrier in the transmitter and created by a local oscillator within the demodulator. It is exactly synchronized in frequency and phase with the carrier wave.

Spectrum of received signal after being multiplied with local carrier.
Spectrum of received signal after being multiplied with local carrier. A subsequent low-pass filter recovers the modulating signal in the baseband.

Start

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

Experiment

Now that you are familiar with the principle of synchronous demodulation, analyze what happens when one of the following parameters is changed:

Change Effect on demodulated signal
Switch on the carrier s^ to 1V
No effect
Increase the modulation index m
Louder
Change the carrier frequency to 60kHz
No signal
Change phase of the local carrier to 60°
Quieter
Change the carrier frequency to 50.0003kHz
Becomes quieter and louder

Check your answers with the simulation. Change the corresponding parameter and listen to the demodulated signal.

Next

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

Check out Wikipedia for more information: Synchronous detector, Double-sideband suppressed-carrier transmission

Set the carrier s^ to 1V. Do you still hear the music?

carrier-to-0
Change the carrier frequency to 60kHz. What effect do you hear?
carrier-to-60mhz
Go back to 50 kHz and set the phase to 60°. How does the signal sound now?
phase-to-60deg