Tuned Radio Frequency Receiver 52333n

A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier. This type of receiver was popular in the 1920s. Early examples could be tedious to operate because when tuning in a station each stage had to be individually adjusted to the station's frequency, but later models had ganged tuning, the tuning mechanisms of all stages being linked together, and operated by just one control knob. By the mid1930s, it was replaced by the superheterodyne receiver patented by Edwin Armstrong. Block diagram

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TRF receiver includes an o RF stage o A detector stage o And an audio stage Two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage RF section o Used to detect the signal o Bandlimit the received RF signal o And amplifying the received RF signal AM detector o Demodulates the AM wave and converts it to the original information signal Audio section o Used to amplify the recovered signal ADVANTAGES: o TRF receivers are simple to design and allow the broadcast frequency 535KHZ to 1640KHz o High sensitivity DISADVANTAGES:

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At the higher frequency, it produces difficulty in design It has poor audio quality Drawbacks  Instability  Due to high frequency, multi stage amplifiers are susceptible to breaking into oscillation  As gain of RF amplifier is very high, a small from output to input with correct phase can lead to oscillations  Correct phase means a positive and it takes place due through stray capacitances  As reactance of stray capacitances decreases at higher frequencies resulting in increased  Forcing the device to work as an oscillator instead of an amplifier  Variation in BW  The bandwidth is consistent and varies with the center frequency when tuned over a wide range of input frequencies  As frequency increases, the bandwidth (f/Q) increases. Thus, the selectivity of the input filter changes over any appreciable range of input frequencies  Poor selectivity  The gains are not uniform over a very wide frequency range  Due to higher frequencies ability to select desired signal is affected

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Due to these drawbacks, TRF is rarely used

Modern usage Although the TRF design has been largely superseded by the superheterodyne receiver, with the advent of semiconductor electronics in the 1960s the design was "resurrected" and used in some simple integrated radio receivers for hobbyist radio projects, kits, and low-end consumer products. One example is the ZN414 TRF radio integrated circuit from Ferranti in 1972 shown below