Tuned Radio Frequency (TRF) Receiver

receivers have been mass-produced since around 1924, but for reasons of
cost did not become successful until the 1930s. Before the second world
war other, simpler receiver technologies such as the TRF
receiver and the regenerative receiver were still widespread. The
circuit described here is based on the old technology, but brought
up-to-date a The most important part of the circuit is the input stage,
where positive feedback is used to achieve good sensitivity and
selectivity. The first stage is adjusted so that it is not quite at the
point of oscillation. This increases the gain and the selectivity,
giving a narrow bandwidth.

To achieve this, the potentiometer connected to the drain of the FET
must be adjusted very carefully: optimal performance of the receiver
depends on its setting. In ideal conditions several strong stations
should be obtainable during the day using a 50 cm antenna. At night,
several times this number should be obtainable. The frequency range of
the receiver runs from 6 MHz to 8 MHz. This range covers the 49 m and
the 41 m shortwave bands in which many European stations broadcast. Not
bad for such a simple circuit! The circuit employs six transistors. The
first stage is a selective amplifier, followed by a transistor detector.
Two low-frequency amplifier stages complete the circuit.

Tuned Radio Frequency (TRF) Receiver Circuit

Tuned Radio Frequency (TRF) Receiver Circuit Diagram

The final stage is a push-pull arrangement for optimal drive of the
low-impedance loudspeaker. This circuit arrangement is sometimes called a
‘1V2 receiver’ (one preamplifier, one detector and two audio-frequency
stages). Setting-up is straightforward. Adjust P1 until the point is
reached where the circuit starts to oscillate: a whistle will be heard
from the loudspeaker. Now back off the potentiometer until the whistle
stops. The receiver can now be tuned to a broadcaster. Occasional
further adjustment of the potentiometer may be required after the
station is tuned in. The receiver operates from a supply voltage of
between 5 V and 12 V and uses very little current. A 9 V PP3 (6F22)
battery should give a very long life.

Author: Gert Baars – Copyright: Elektor 2004

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