Evolution of Superheterodyne Receivers
Superheterodyne receivers have been in mass production since approximately 1924. However, due to cost constraints, their widespread success did not materialize until the 1930s, marking a significant evolution in radio technology.
Pioneering Era: Alternatives to Superheterodyne Receivers
Before the outbreak of the Second World War, simpler receiver technologies like the Tuned Radio Frequency (TRF) receiver and the regenerative receiver were prevalent. Despite the emergence of superheterodyne technology, these alternatives remained widely used, showcasing the diverse landscape of radio reception methods.
Updating Old Technology: Enhanced Sensitivity and Selectivity
This circuit, rooted in traditional technology, has been modernized for contemporary use. Its crucial component is the input stage, where positive feedback is utilized to achieve exceptional sensitivity and selectivity. The initial stage is finely tuned to the brink of oscillation, enhancing gain and selectivity, resulting in a narrow bandwidth. Achieving this precision demands careful adjustment of the potentiometer connected to the FET drain; the receiver’s optimal performance hinges on this setting. Under ideal conditions, a 50 cm antenna should capture several strong stations during the day, and this number should significantly increase at night. Covering a frequency range from 6 MHz to 8 MHz, this circuit encompasses the 49 m and 41 m shortwave bands, which are home to many European stations. Remarkable performance considering its simplicity!
Six-Transistor Design for Efficient Reception
This circuit incorporates six transistors to deliver exceptional performance. The first stage functions as a selective amplifier, followed by a transistor detector. Two low-frequency amplifier stages complete the setup. The final stage adopts a push-pull arrangement to ensure optimal drive for the low-impedance loudspeaker. This configuration is often referred to as a ‘1V2 receiver’ (one preamplifier, one detector, and two audio-frequency stages). Setting up the receiver is straightforward: adjust P1 until the circuit starts oscillating, producing a whistle from the loudspeaker. Fine-tune the potentiometer to eliminate the whistle, and the receiver can then be tuned to a desired broadcaster. Occasional adjustments might be necessary after tuning to ensure optimal performance. Operating on a supply voltage between 5 V and 12 V, the circuit consumes minimal current. A 9 V PP3 (6F22) battery offers long-lasting functionality.