Tuned Radio Frequency (TRF) Receiver Design - Attributes and Limitations

By David Willcocks

A Tuned Radio Frequency (TRF) Receiver is one of the simpler Radio types in existence. It is characterized by a low component count, reasonable sensitivity, and easy operation (it has only Tuning and Volume controls). The receiver consists of one (or several) stages of RF Amplification, a Detector, and Audio Amplifier.

Most of the Receivers Sensitivity is due to RF amplification within the pass band of a Tank Circuit (LC Tuned). The Tank Circuit usually consists of an Inductor and Capacitor connected in parallel. The Inductor (L) is a fixed component and the capacitor is usually a ganged Variable Capacitor. To calculate the Resonant Frequency of the Tank circuit the following formula can be applied: Resonant Frequency = 1 / (2 pi root LC). The RF Amplifier also amplifies the noise produced in this stage. The noise produced is a result of the Bandwidth of the Resonant Circuit (Higher Bandwidth means more noise) and the circuit design characteristics. Typically the active component in this stage would be a Transistor, FET (Field Effect Transistor) or IC (Integrated Circuit). A low noise Amplifier should always be specified.

The Signal to Noise Ratio (SNR) of the Receiver, which is measured in Decibels (dB), provides the performance Specifications for this type of Receiver.To calculate the SNR the Amplitude of the Signal and Noise will need to be known, and then the following formula can be applied: SNR = 20 log 10 (Signal Amplitude / Noise Amplitude).

Receiver Selectivity is also governed by the Tuned Circuit in the RF Amplifier Stage. This is limited by the Quality Factor (Q) of the Tank Circuit. Q = Inductive Reactance of the Tank Coil (XL) / the coil resistance (R). Unfortunately, the receiver Bandwidth changes with the resonant frequency of the LC tuned circuit, as the Bandwidth = Resonant frequency / Q. The Quality Factor (Q) is a constant. As the Resonant frequency of the Tank Circuit changes, so must the bandwidth. What this means is that the Receiver Selectivity changes with the Tuned Frequency of the Receiver. This is an undesirable characteristic! Selectivity is a measurement of the Receivers ability to reject unwanted signals.

Having the Receivers Selectivity change with frequency is stopped by providing fixed Bandpass characteristics in the RF Amplifier circuit of more advanced Receiver designs (such as in a Superheterodyne Receiver).

David Willcocks is an Electronics professional, Trainer, and Technical writer. For more information on Radio Communications please visit http://www.outbackelectronics.com

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