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Old 10th Oct 2010, 10:50 pm   #8
Alistair D
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Join Date: Jan 2010
Location: Edinburgh, UK.
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Default Re: Product detector circuit

Quote:
Originally Posted by Sideband View Post
Product detectors are mainly found in Communications receivers. They are particularly useful when listening to SSB transmissions. Very simply, they are a mixer type demodulator where one input is fed directly with the I.F signal and the second input from the BFO (or the carrier insertion oscillator). The valve (or transistor) then mixes the two inputs to provide an AF output. It won't reproduce AM because, as the name implies, it requires two inputs to provide the output. The circuit is quite simple and just requires a small amount of RF filtering on the output.

This is a very simplistic description and no doubt someone else can give a more detailed description if necessary.


SB
Your description is fine.
A few added points.

The level of the BFO (Beat Frequency Oscillator) injection into a normal diode AM detector has to be high for this method to work. This upsets the operation of the sets AGC circuit. The AGC sees the BFO as a very strong signal and turns the sets gain down accordingly. This is why most sets of this type tell you turn the AGC off and use the RF gain control. A product detector, which is usually tapped off from the same place as the diode detector,does not cause this effect.

A set fitted with a product detector is easier to use. There is usually a switch marked LSB(below 7 Mhz), USB(above 7Mhz) and AM. Select the correct one and use the main tuning until the signal is intelligible.

A product detector is just a variation of the sets mixer circuit.

Sorry if this is a bit wordy

Al
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