Ramsey Electronics HR40 manual About Direct Conversion Receivers

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 HR40 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 HR40 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 HR40 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.

Advantages of any Direct Conversion receiver include:

•It is simple, and therefore economical and easy to build

•It is quite sensitive even with a simple antenna

• Its tuning oscillator could even be set up to serve directly	as
a transmitter VFO in a simple transceiver setup.

A common problem with DC receivers is that they are easily overwhelmed by strong AM broadcast stations from almost anywhere, whether it’s your local rock and roll broadcaster, Radio Moscow or the Voice of America. Even popular classics like Heath Kit’s HW-8 QRP transceiver exhibit this characteristic to a frustrating degree. Another problem with DC receivers is

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