TRF Receiver
- The full form of the TRF receiver is a Tuned Radio Frequency (TRF) Reciever.
- A tuned radio frequency (TRF) receiver is the simplest radio receiver among all which we have discussed above.
TRF Receiver Block Diagram
- The figure shows the block diagram of a tuned radio frequency receiver.
- The very first block of this receiver is an RF stage. This stage generally contains two or three RF amplifiers. Actually, these RF (radio frequency) amplifiers are tuned RF amplifiers i.e. they have variable tuned circuits at the input and output sides.
- At the input of the receiver, there is a receiving antenna as shown in the block diagram in the figure. At this antenna signals from different sources (i.e. stations) are present. However, with the help of the input variable tuned circuit of RF amplifiers the desired signal (i.e. station) is selected. But this selected signal is usually very weak in the order of µV. This selected weak signal is amplified by the RF amplifier (i.e. R.F stage).
- After this, the amplified incoming modulated signal is applied to the demodulator. The demodulator or detector demodulates the modulated signal and thus at the output of the demodulator, we get modulating or baseband signal (i.e. audio signal). This audio signal is amplified by an audio amplifier.
- After that, this audio signal is further amplified by a power amplifier up to desired power level to drive the loudspeaker. The last stage of this receiver is the loudspeaker. A loudspeaker is a transducer that changes the electrical signals into sound signals.
Drawbacks of TRF Receiver
- As discussed above, although the TRF receiver is cheaper and the simplest one, it has certain drawbacks as:
- The TRF receiver suffers from a tendency to oscillate at higher frequencies from the multistage RF amplifiers with high gain and operating at the same frequency. If such an amplifier has a gain of 20,000 then if a small portion of the output leaked back to the input of the RF stage, then positive feedback and oscillation will result. This type of leakage could result from power supply coupling, stray capacitance coupling, radiation coupling, or coupling through any other element common to the input and output stages, Definitely, this type of condition is undesirable for a good receiver. This problem is also termed instability of the receiver.
- The selectivity of a receiver is its ability to distinguish between the desired signal and an undesired signal. The selectivity of the TRF receivers is poor. In fact, it is difficult to achieve sufficient selectivity at high frequencies due to the enforced use of single-tuned circuits.
- Another problem associated with the TRF receiver is the bandwidth variation over the tuning range. For example, in AM broadcast system, let us consider that a tuned circuit is required to have a bandwidth of 10 kHz at a frequency of 540 kHz.
- According to the definition, the Quality factor Q of this tuned circuit must be
Q=\frac{resonance\;frequency}{bandwidth}=\frac{540}{10}=54
- Now, at the other end of this AM broadcast band (i.e. 1640 kHz), the Quality factor of the coil. according to the above equation, must increase by a factor of 1640/535 (i.e. 3) to a value of 164. However, in practice, several losses dependent upon frequency would prevent such a large increase. Thus, practically, the Quality factor Q of this tuned circuit is unlikely to exceed 120 and hence: providing a bandwidth of the tuned circuit equal to
\Delta=\frac{f_r}{Q}=\frac{1640}{120}=13.8kHz
- Therefore, due to this increased bandwidth of 13.8 kHz in place of a fixed bandwidth of 20 kHz, the receiver would pick up or select adjacent frequencies (i.e. stations) with the desired frequency or station. This means that the bandwidth of the TRF receiver varies with the incoming frequency.
Also Read: Radio Receivers in Detailed.
Frequently Asked Questions of TRF Receiver
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What is the use of a TRF receiver?
Ans: The classic TRF receivers of the 1920s and 30s usually consisted of three sections: one or more tuned RF amplifier stages. These amplify the signal of the desired station to a level sufficient to drive the detector while rejecting all other signals picked up by the antenna.
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What is the problem with the TRF receiver?
Ans: Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections. An additional problem for the TRF receiver is tuning different frequencies.
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Which receiver is better TRF or superheterodyne?
Ans: Because the detector and amplifiers of a superheterodyne receiver can be designed to amplify only intermediate frequency (IF), this type of receiver is more selective and offer high fidelity (exact reproduction quality of the transmitted signal). In the TRF receiver, amplification is not constant over the tuning range.
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What are the limitations of TRF receivers how do you overcome them?
Ans:
1. TRF receiver suffers from variations in BW over the tuning range ( s 40 – 1650 kHz)
2. The gain of TRF RX is not uniform over the tuning range.
3. The TRF is unstable at high frequency.
4. Gang tuning of more number of capacitors simultaneously is difficult. -
Which of the following is an advantage of TRF receivers?
Ans: 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 reject all others.
