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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Regenerative shortwave radio receiver has audio limiter

This receiver combines simplicity with good performance and can be configured to cover any or all segments of the MW and SW bands, with AM, SSB/CW and FM modes. Selectivity is very sharp, sensitivity is high and short-term stability is very good.

The RF signal is fed through an input attenuator consisting of D1 & D2 and several 1kΩ resistors. Forward bias for the diodes is obtained from the voltage across LED1.

VR1 provides an adjustable countervailing voltage which varies the dynamic resistance of the two diodes and therefore provides smooth control of the RF signal, preventing overload from strong local stations and also acting as a volume control.

The signal from the attenuator is fed to the emitter of PNP transistor Q1, a grounded base buffer providing little gain but isolating any loading effects from the antenna and input stage. The signal from Q1’s collector is fed via the tuning stage involving coils L1, L2 and the associated tuning gang VC1a & VC1b.

FET Q2 operates as an infinite impedance detector, with the detected audio appearing across the source load resistors. A small amount of residual RF appearing at the source of this FET is returned to tuning coil L2 via diode D3 and the 10nF cap-
acitor. Potentiometer VR2 is the regeneration control and it varies the dynamic resistance of D3.

The resultant positive feedback effectively cancels all losses in coil L1, greatly increasing its "Q", so that the gain and selectivity of this stage is increased accordingly.

IC1 operates as an inverting amplifier and provides most of the audio gain. D4 &D5 in the negative feedback network provide audio limiting to prevent loud signals deafening the operator.

Transistors Q3-Q5 function as a simple "totem pole" amplifier to drive headphones or a speaker. (Editor’s note: the 680kW bias resistor for Q3 may need adjustment to obtain half supply [ie, 6V] at the junction of the emitters of Q4 & Q5).

It is recommended that the circuit be built on a PC board. This will guarantee component rigidity, greatly enhancing stability and performance.

The prototype used a 5-pin DIN socket to facilitate coil changing. VC1 is an AM/FM polycon tuning capacitor from a defunct radio-cassette player which also furnished the tuning pulleys and shaft.

It is recommended to use the largest tuning reduction possible, to ease tuning, especially when attempting to cover a large shortwave segment with one coil. The smaller FM capacitor section is available at the coil socket, as well as the 150pF MW section; use the latter for greater coverage.

A few metres of wire will suffice as the antenna, as the receiver is very sensitive. In use, rotate RF gain control VR1 fully clockwise and rotate regeneration control VR2 clockwise until background or atmospheric noise is heard. By rotating the tuning control, stations will be heard, possibly as heterodynes at this stage. Reduce the regeneration to the point where the station becomes audible and reduce the RF gain for a suitable listening level.

The plug-in coils are wound on a pen case which, after the internals have been removed, is then glued to a 5-pin DIN plug. A full set of coils can be wound to cover all of the HF ranges and a link to the 150pF gang will extend coverage at the lower frequencies. The prototype was capable of resolving SSB on 8867kHz, the aircraft net, for periods of more than 30 minutes without retuning, once stabilised. This level of performance requires good construction practice, with rigid mounting of the PC board into a metal cabinet.

D. S. Edwards,

Taylorville, NZ. ($70)

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