![Start via Java Web Start Start](/labalive/img/jws-launch-button.png)
![AM transmission - synchronous detector](am-transmission-synchronous-detector.png)
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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.](received-signal-multiplied-with-local-carrier.png)
Start
![Listen to the audio speech signal that is synchronous demodulated after AM transmission. The audio playback starts automatically when the app is launched.](speech-audio-signal-sink.png)
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 to |
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Increase the modulation index |
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Change the carrier frequency to |
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Change phase of the local carrier to |
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Change the carrier frequency to |
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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 to . Do you still hear the music? |
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Change the carrier frequency to . What effect do you hear? |
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Go back to 50 kHz and set the phase to . How does the signal sound now? |
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