TRF Radio Receiver Basics and Circuit Design

This application note covers the basics of Tuned Radio Frequency (TRF) receivers. It explains the TRF radio circuit and how it serves as an improvement over a basic crystal radio receiver.

As shown in the figure below, stages of amplification are added both before and after the demodulator. This significantly improves the sensitivity of the tuned radio frequency receiver.

TRF radio receiver circuit

As illustrated in the figure, three stages of RF amplification are used between the antenna and the detector. This configuration substantially increases the amplitude of the received signal.

Furthermore, two audio amplifiers are employed after the detector. These amplifiers provide the necessary gain to operate a loudspeaker effectively.

A key advantage of the TRF design is improved selectivity. When multiple resonant LC circuits, all tuned to the same frequency, are cascaded, the overall selectivity is enhanced. The more stages included, the narrower the bandwidth becomes, and the steeper the skirts of the frequency response.

TRF receivers have been widely adopted in various applications.

Most TRF circuits incorporate a knob for tuning the capacitance, which in turn adjusts the resonant frequency. In earlier designs, each tuned capacitor in the cascaded stages had its own separate knob. However, later designs integrated a single knob to control all capacitances simultaneously, simplifying the tuning process.

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