# AM transmission - synchronous detector

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AM transmission - synchronous detector
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

• Double-sideband suppressed-carrier, DSB-SC or
• Double-sideband reduced carrier (DSB-RC) or
• Single-sideband suppressed-carrier, SSB-SC

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. 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.

## 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.

 Set the carrier $s^$ to $1V$. Do you still hear the music? Change the carrier frequency to $60kHz$. What effect do you hear? Go back to 50 kHz and set the phase to $60°$. How does the signal sound now?