This receiver circuit is ideal for discovering just how simple a true “Communications Receiver” can be . That’s right- there is a BIG difference between the Ramsey HR20 and other simple short wave radio circuits we can try to build.
Yet it is very easy to build. Our
ABOUT DIRECT CONVERSION RECEIVERS:
The HR20 is a “Direct Conversion” receiving circuit. There is no need for IF (intermediate frequency) circuitry. The receiver “processes” the incoming signal right at its own frequency, with no need to mix or transform it with additional internal oscillators and amplifiers running at intermediate frequencies such as 455 KHz.
The advantage of this type of receiver is that it permits tuning of CW, AM and SSB signals with no need for a separate BFO. (A BFO or “beat frequency signal oscillator” is an entirely separate oscillator circuit used in the IF “intermediate frequency” section of a superhetrodyne receiving circuit). Since this frequency is designed specifically for SSB and CW, you will also hear the carrier signal of any AM shortwave broadcast station.
The most elementary “DC” receiver consists of just an oscillator and an antenna connected to the inputs of a “Product Detector” whose tiny audio output is then amplified for listening. A product detector can be made from simple diodes, or a pair of transistors, or a dual gate FET transistor, or various IC’s. The Ramsey HR20 efficiently utilizes the Signetics SA602 IC for both the tunable oscillator and the product detector circuits, giving the equivalent of seven transistors in the
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