Syllabus Sections:-

Demodulation

4o.1 Understand the operation of AM, CW, SSB and FM demodulators.

AM demodulation

In the Intermediate course notes your were shown how a diode detector recovers an AM signal (check back here).

The diode allows all the positive signal to pass but rejects all of the negative part of the signal.

The circuit diagram above shows a parallel tuned circuit, which can be tuned to the desired radio station (frequency) by the variable capacitor.

At the resonant frequency, the parallel tuned circuit has a high impedance (or opposition to AC / RF currents), the effect of this is to generate a maximum AC / RF potential difference across the tuned circuit.

The potential difference is rectified by the diode and DC is smoothed out by the capacitor C. The end result is that the recovered audio is only the overall shape of the signal which is applied to the ear piece / headphones which would have been the same shaped audio signal used to modulate the transmitter hence you hear what was transmitted.

CW demodulator

Now it is possible for the AM simple detector to demodulate a CW signal but you will need a Beat Frequency Oscillator (BFO). This BFO (which is about 700Hz offset from the the IF) mixes with the IF signal which results an audible tone which will be at about 700Hz each time the RF is present which represents the morse key pressed down by the transmitting operator.

SSB demodulator

Also the AM simple detector can demodulate a SSB signal but to do so a Carrier Insertion Oscillator (CIO) is used to replace the carrier removed during the modulation process at the transmitter.

FM demodulator

For the AM simple detector to demodulate a FM signal, a discriminator is used.


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