Here's an 80m direct conversion receiver that uses only one active device. A 2N3904 transistor serves as both a high-gain AF amplifier and the local oscillator; and yet both the receiver sensitivity and audio volume are surprisingly good. For example, on the first night of listening I copied OZ4UN's 10W CW signal from Denmark. In fact, it's possible to lay the headphones on the bench and copy stronger signals quite easily.
Here is a short audio recording made just before local noon; the SSB weather station heard at the beginning of the recording operates just below the 80m amateur band. Here's what OK2RZ sounded like on a late February afternoon. The evening band is awash with signals as I tune around in this last recording.
A sub-harmonic (Polyakov) mixer follows the two-resonator 80m input bandpass filter. The AF amplifier is a standard common-emitter configuration. Transformer T4 steps-up the headphone impedance to provide a fairly high collector load impedance. This, together with the decoupled emitter resistor and just under two milliamperes of quiescent emitter current helps to produce a single-stage voltage gain of just over two-hundred.
This same transistor operates simultaneously as a common-base HF oscillator. The parallel LC tank at the collector is resonant at one-half the receiver working frequency. The sub-harmonic mixer requires an oscillator signal amplitude in the range of 300 to 400mVpp.
The oscillator level amplitude is clamped to 1.2Vpp at the collector using Silicon diodes D3 and D4. While it's entirely possible to substitute different inductors for the devices that I've used, it's important to preserve the low oscillator L/C ratio and degree of feedback coupling indicated on the schematic diagram. This oscillator is a bit different than most, inasmuch as it must remain at all times within the bounds of Class-A operation. The overall loop-gain ought to be high enough to provide reliable starting, but not so large as to "slam" the oscillator waveform into the diode clamps; a situation that would create excessive harmonic energy. Ideally, we want a "soft" oscillation to gently rise until it begins to feel a tickle of nonlinearity from the diode clamps. The values shown in the schematic produced a clean and steady sinusoidal waveform at the transistor's collector. If the oscillator does not immediately start, please insure the phasing is correct by swapping the emitter feedback winding connections on T3.
The frequency vs. temperature stability improved considerably by replacing the ferrite-core oscillator inductor with an air-core device.
22 March 2008
In the past month I've used this receiver to make more than a dozen contacts on 80m. Most of the contacts were made using a one-transistor (2N697) transmitter with 250mW output. One afternoon I continued to have a solid copy (S4-5) of Jim, W1PID's, signal after he'd reduced his output power to ~100mW.
Of their MicroR1, three transistor, direct conversion receiver design; Hayward, Campbell and Larkin comment (EMRFD, page 8.4)
"The challenges inherent in simple equipment are not necessarily disadvantages - it takes more skill to cross a harbor in a sailing dinghy than a motor boat. Copying signals from across the oceans with a three transistor circuit is similarly rewarding."