Ramsey Electronics QRP-80 Bedienerhandbuch PDF herunterladen (Seite 6)

HR-80• 6

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 HR-80’s use of the NE602 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.

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Ramsey Electronics QRP-80 Bedienerhandbuch Seite 6