Tuned radio frequency receiver
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A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is usually composed of several tuned radio frequency amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and amplify it. Popular in the 1920s, it could be difficult to operate because each stage must be individually tuned to the station's frequency. By the 1930s it was replaced by the superheterodyne receiver invented by Edwin Armstrong.
The TRF receiver was patented in 1916 by Ernst Alexanderson. His concept was that each stage would amplify the desired signal while reducing the interfering ones. All tuned stages of the radio must track and tune to the desired reception frequency. This is in contrast to the modern superheterodyne receiver that must only tune the receiver's RF front end and local oscillator to the desired frequencies; all the following stages work at a fixed frequency and do not depend on the desired reception frequency.
Antique TRF receivers can often be identified by their cabinets. They typically have a long, low appearance, with a flip-up lid for access to the vacuum tubes and tuned circuits. On their front panels there are typically two or three large dials, each controlling the tuning for one stage. Inside, along with several vacuum tubes, there will be a series of large coils. These will sometimes be tilted slightly to reduce interaction between their magnetic fields.
A problem with the TRF receiver in the time of triode vacuum tubes was that interelectrode capacitance (the so-called Miller capacitance) can cause instability and oscillation. In 1922, Louis Alan Hazeltine invented the technique of neutralization which uses additional circuitry to stabilize the amplifier. Neutralization was used in the popular Neutrodyne series of TRF receivers. The later development of the tetrode vacuum tube reduced the Miller capacitance and consequently improved stability.
How it works
A TRF receiver consists of three sections:
- One or more tuned RF amplifier stages. These amplify the signal of the desired station while rejecting all other signals picked up by the antenna
- a detector, which extracts the audio (modulation) signal from the radio carrier signal.
- optionally, one or more audio amplifier stages which increase the power of the audio signal.
Each tuned RF stage consists of an amplifying device (tube or transistor) and a tuned circuit which performs the filtering function. In the classic TRF receivers of the 1920s, the amplifier was a triode vacuum tube. Between each pair of stages was an air-core RF coupling transformer which served to couple the signal from the plate circuit of one tube to the input grid circuit of the next tube. One of the windings of the transformer had a capacitor across it to make a tuned circuit. A variable capacitor was used, with a knob on the front panel to tune the receiver. Each RF stage had to be tuned to the same frequency, so the capacitors had to be tuned in tandem when bringing in a new station. In later sets the capacitors were mounted on the same shaft, or had a mechanical linkage so that the radio could be tuned with a single knob. The RF stages usually had identical circuits to simplify design.
The detector was usually a grid-leak detector, consisting of a triode tube biased near cutoff so it would only conduct on the positive half of the RF cycles. Some sets used a crystal detector (semiconductor diode) instead. Occasionally, a regenerative detector was used, to increase selectivity.
Some TRF sets that were listened to with earphones didn't need an audio amplifier, but most sets had one to three transformer-coupled or RC-coupled audio amplifier stages to provide enough power to drive a loudspeaker.
This schematic diagram shows a typical TRF receiver. This particular example uses six triodes. It has two radio frequency amplifier stages, one grid-leak detector/amplifier and three class ‘A’ audio amplifier stages. Generally, two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage. The detector recovers the audio frequency signal from the modulated RF signal, and the audio stage amplifies the information signal to a usable level. The final stage was often simply a grid-leak detector.
Advantages and disadvantages
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, but the bandwidth of a filter with a given Q factor increases with frequency. So to achieve a narrow bandwidth at a high radio frequency implies high Q or many filter sections. In contrast, a superheterodyne receiver translates 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.
During the 1920s, an advantage of the TRF receiver over the regenerative receiver was that, when properly adjusted, it did not radiate interference. The popular regenerative receiver, in particular, used a tube with positive feedback operated very close to its oscillation point, so it often acted as a transmitter, emitting a signal at a frequency near the frequency of the station it was tuned to. This produced audible heterodynes, shrieks and howls, in other nearby receivers tuned to the same frequency, bringing criticism from neighbors. In an urban setting, when several regenerative sets in the same block or apartment house were tuned to a popular station, it could be virtually impossible to hear. Britain, and eventually the US, passed regulations that prohibited receivers from radiating spurious signals, which favored the TRF.
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.
Although the TRF design has been largely superseded by superheterodyne and other circuits, it was "resurrected" in 1972 in silicon as the ZN414 TRF radio integrated circuit from Ferranti, thus affording the design a new lease of life in hobbyist radio projects, kits and some commercial products.
- Lee, Thomas H. (2004). The Design of CMOS Radio-Frequency Integrated Circuits, 2nd Ed.. UK: Cambridge University Press. p. 16. ISBN 0521835399.
- Glasgow, R. S. (June 1924). "Radiating Receivers". Radio In The Home (Philadelphia, PA: Henry M. Neely Publishing Co.) 3 (1): 16, 28. Retrieved March 14, 2014. "But the interference due to regenerative receivers when in the oscillating condition cannot be eliminated by anything the receiving operator can do. ... All types of regenerative sets will cause the connected aerial to radiate energy if allowed to oscillate."
- Terman, Frederick E. (1943), Radio Engineers' Handbook, McGraw-Hill
- Tomasi, Wayne (2004), Electronic Communications Systems: Fundamentals Through Advanced (5th ed.), Pearson Education
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