The tuned radio frequency receiver is one in which the tuning or selectivity is provided at the radio frequency stages.
The tuned radio frequency receiver was used in the early days of wire-less technology but it is rarely used today as other techniques offering much better performance are available.
Earliest tuned radio frequency receivers
It could be argued that the very earliest tuned radio frequency receivers were crystal sets. These sets used a single tuned network, sometimes consisting of a number of coils. The output from this was fed directly into a crystal or "Cat’s Whisker” " detector and then into headphones.
Although crystal radios are seldom used these days because their levels of performance can easily be exceeded by other forms of radio, they are ideal for showing some of the basic principles of radio.
. . . . . . Read more about the How does a crystal radio work
Tuned radio frequency receiver basics
The definition of the tuned radio frequency, TRF receiver is a receiver where the tuning, i.e. selectivity is provided by the radio frequency stages.
In essence the simplest tuned radio frequency receiver is a simple crystal set. Tuning is provided by a tuned coil / capacitor combination, and then the signal is presented to a simple crystal or diode detector where the amplitude modulated signal, in this case, is recovered. This is then passed straight to the headphones.
As vacuum tube / thermionic vale technology developed, these devices were added to provide more gain.
Typically a TRF receiver would consist of three main sections:
- Tuned radio frequency stages: This consisted of one of more amplifying and tuning stages. Early sets often had several stages, each proving some gain and selectivity.
- Signal detector: The detector enabled the audio from the amplitude modulation signal to be extracted. It used a form of detection called envelope detection and used a diode to rectify the signal.
- Audio amplifier: Audio stages to provide audio amplification were normally, but not always included.
The tuned radio frequency receiver was popular in the 1920s as it provided sufficient gain and selectivity for the receiving the broadcast stations of the day. However tuning took a little while as each stage in the early radios needed to be adjusted separately. Later ganged tuning capacitors were introduced, but by this time the superheterodyne receiver was becoming more widespread.
Modern tuned radio frequency receivers
The TRF receiver has largely been disregarded in recent years. Other receiver topologies offer far better levels of performance, and with integrated circuit technology, the additional circuitry of other types of receiver is not an issue.
There was one attempt at making a sufficiently selective tuned radio frequency receiver integrated circuit.
The Ferranti ZN414 integrated circuit was introduced in 1972 and was successfully used in a number of designs. Later versions, the ZN415 and ZN416 included audio amplifiers.
Performance of the chips was intended to allow operation on the medium wave band, up to frequencies of around 1.6 MHz. Generally the limit of operation of these chips was under 5 MHz.
However with the demise of the Ferranti company, the design stopped production. However there have been a few replacement ICs on the market. These include: MK484, YS414, TA7642, UTC7642, LMF501T, LA1050. These can sometimes be purchased on the open market and used in small radios to fit in matchboxes, etc..
By Ian Poole
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