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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 HR-series are by far the easiest to build of all
the Ramsey kit receivers and therefore very nice for radio newcomers.
Consequently, our manuals for the 40 and 80 meter receivers are written with
beginners in mind, since both of these bands offer Novice and Technician
privileges. The manuals for the 30 and 20 meter versions assume the general
Ham radio know-how that should accompany a General Class or higher
license.
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 mixer-oscillator stages.
HR20• 5
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