Tuned Radio Frequency Receiver - Disadvantages of TRF Receiver

Disadvantages of TRF Receiver

Terman (1943, p. 658) characterizes the TRF's disadvantages as "poor selectivity and low sensitivity in proportion to the number of tubes employed. They are accordingly practically obsolete." Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections. In contrast a superheterodyne receiver can translate the incoming high radio frequency to a lower intermediate frequency where selectivity is easier to achieve.

An additional problem for the TRF receiver is tuning different frequencies. All the tuned circuits need to track to keep the narrow bandwidth tuning. Keeping several tuned circuits aligned is difficult. A superheterodyne receiver only needs to track the RF and LO stages; the onerous selectivity requirements are confined to the IF amplifier which is fixed-tuned.

Although a TRF receiver can not be engineered for a high degree of selectivity relative to its carrier frequency, there is no reason it cannot reach the same level of sensitivity as other designs. The 1930s era BC-AN-229/429 military receiver was a six-valve design covering 201 to 398 kHz and 2.5 to 7.7 MHz (requiring several sets of plug-in coils to cover those ranges). This equipment probably exemplifies the limit of T.R.F. performance. Although the receiver bandwidth does vary, as noted above, the sensitivity of the set was around 8 microvolts for 10 milliwatts of audio output, comparable to that of the famous AN/ARC-5 superhet receiver that superseded it.