- Until shortly before World War II, most radio receivers were of the TRF type, whose block diagram is shown in figure given below
Fig: The TRF receiver
- The TRF receiver is a simple "logical" receiver. The virtues of this type, which is now not used except as a fixed-frequency receiver in special applications,
- Its advantages are its simplicity and high sensitivity.
- Two or perhaps three RF amplifiers, all tuning together, were employed to select and amplify the incoming frequency and simultaneously to reject all others.
- After the signal was amplified to a suitable level, it was demodulated (detected) and fed to the loudspeaker after being passed through the appropriate audio amplifying stages.
- TRF receivers were simple to design.
- They were easily aligned at broadcast frequencies (535 to 1640 kHz).
- TRF receivers presented difficulties at higher frequencies. This was mainly because of the instability associated with high gain being achieved at one frequency by a multistage amplifier.
- In addition, the TRF receiver suffered from a variation in bandwidth over the tuning range. It was unable to achieve sufficient selectivity at high frequencies, partly as a result of the enforced use of single-tuned circuits.
- It had problems of instability, insufficient adjacent-frequency rejection, and bandwidth variation.
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