What is a Tuned Radio Frequency Receiver || How to Construct a Tuned Radio Frequency Receiver

A tuned radio frequency receiver is a type of radio receiver, which utilizes multiple stages of tuned radio frequency amplifiers connected in series, which is followed by a detector to detect the audio from the radio signal. The audio signal is further amplified and it drives an earphone or a loudspeaker.

Working of a TRF Receiver
The input from the antenna is tuned to a specific frequency using a tuned circuit which tunes to an incoming radio signal. The signal goes to the next stage, which is a tuned radio frequency amplifier stage that tunes and amplifies it. There may be two or three stages of tuned RF amplifiers that further tunes and amplifies the radio signal.

When the required amplification is achieved, the signal is detected using an envelope detector or a product detector, which detects or extracts the audio signal from the radio frequency signal. The weak detected audio signal is then amplified using an audio power amplifier and fed through a loudspeaker.

Functioning of the TRF Receiver
The multiple tuned RF amplifier stages will amplify the weak signals and it also improves the selectivity of the receiver. The loosely coupled and narrowly tuned RF amplifier stages reduce the bandwidth and improve the selectivity of the receiver.

 

All tuned stages must be tuned to the same desired incoming signal to keep track of the signal and to improve the overall strength and selectivity. Adjacent radio frequency amplifier stages must be well isolated to prevent coupling of the output signal back to the input producing feedback and oscillations.

TRF Receiver Integrated Circuits
An earlier version of a TRF receiver was available in an IC chip, which could introduce sufficient sensitivity and selectivity to the radio.

The old Ferranti ZN414, ZN415 and ZN416 integrated chips had the TRF receiver circuit built into them. It could operate successfully in the medium wave band and around 1.6 MHz, and the maximum frequency limit was up to 5 MHz. Other similar replacement ICs are MK484, YS414, TA7642, UTC7642, LA1050 etc.

 

The TRF receiver is now largely replaced by the superheterodyne receiver, which helps to overcome many of its disadvantages.

Disadvantages of the TRF Receiver
1. Poor selectivity of the receiver. Selectivity becomes poor due to multiple tuned RF amplifier stages that can get off-tuned from the desired signal frequency.
2. Low sensitivity of the receiver. There is low sensitivity to weak signals despite the addition of multiple RF amplifier stages.
3. Bandwidth of the receiver increases with frequency. This requires high Q filters for tuning to higher frequencies. Maintaining constant bandwidth over the entire band is difficult to achieve.
4. Collectively tuning all the radio frequency tuned circuits to the same frequency is complicated. Aligning all the tuned circuits while tuning through the entire band is difficult.
5. There is chance for feedback if the gain exceeds the threshold limit for feedback.

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