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 called “microphonics” which is a phenomenon where almost anything in the physical circuitry of the receiver can act as a sort of microphone or audio sound pickup. Touch or bump such radios and you will hear a thump or ring in the speaker or earphones. Still another problem is that of AC line hum whenever an unregulated power supply is used rather than batteries.
The Ramsey HR20’s use of the SA602 integrated circuit chip offers a circuit configuration that is as immune as any simple superhetrodyne to the classic problems with Direct Conversion receivers. The receiver is not as easily overloaded by the VOA or Radio Moscow broadcasts, and there are few annoying ”microphonics” or incurable AC hum.
Circuit Description:
U1 combines a double-balanced active mixer and oscillator in a single 8-pin IC chip. L1 peaks the RF input to pins 1 and 2 of U1. L2, with varactor diode D1 and R2, C1, C2, and C4, control the resonant frequency of U1’s internal oscillator. Rotating R2 gives about a 250 KHz tuning range. The audio output is fed from pins 4 and 5 through R3 (volume control) directly to the LM 386 audio amplifier. R1 controls RF gain. C9 boosts the gain figure of the LM 386 from 20 to 50.