Tuned radio frequency (TRF) receiver is the simplest radio receiver.in TRF signal is passed to a RF stage.This stage generally contains two or three RF amplifier. Actually,these RF(radio frequency)amplifiers are tuned RF amplifiers i.e. they have variable tuned circuit at the input and output sides.At the input of the receiver,there is a receiving antenna.this antenna signals from different source are present. However,with the help of input variable tuned circuits of RF amplifier the desired signal is selected .but this selected signal is usually very weak of the order of μV. this selected weak signal is amplified by the RF amplifier.
thus the function of RF stage is to select the desired signal and amplify it.
After this,the amplified incoming modulated signal is applied to the demodulator. the demodulator or detector demodulated 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 audio amplifier.After that,this audio signal is further amplified by a power amplifier upto desired power level to drive the loudspeaker.The last stage of this receiver is the loudspeaker.A loudspeaker is a transducer which changes electrical signal into sound signal.
(i)The TRF receiver suffers from a tendency to oscillate at higher frequencies from the multistage RF amplifiers with high gain and operating at same frequency.If such an amplifier has gain of 20,000 then if a small portion of the output leaked back to the input of the RF stage,then positive feed back 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.
thus the function of RF stage is to select the desired signal and amplify it.
After this,the amplified incoming modulated signal is applied to the demodulator. the demodulator or detector demodulated 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 audio amplifier.After that,this audio signal is further amplified by a power amplifier upto desired power level to drive the loudspeaker.The last stage of this receiver is the loudspeaker.A loudspeaker is a transducer which changes electrical signal into sound signal.
(i)The TRF receiver suffers from a tendency to oscillate at higher frequencies from the multistage RF amplifiers with high gain and operating at same frequency.If such an amplifier has gain of 20,000 then if a small portion of the output leaked back to the input of the RF stage,then positive feed back 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.
(ii)The selectivity of a receiver is its ability to distinguish between a desired signal and an undesired signal.The selectivity of TRF receiver is poor.In fact,it is difficult to achieve sufficient selectivity at high frequencies due to the enforced use of single-tuned circuits.
(iii)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 According to the definition,the Quality factor Q of this tuned circuit must be
Q=resonance frequency/bandwidth
=540/10
now, at the order end of this AM broadcast band(i.e.1640 KHz),the Quality factor Q of the coil, according to above equation,must increase by a factor of 1640/535(i.e.3) to a value of 164. however,in practice due to 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
Δf=fr/Q=(1640/120)=13.8kHz
No comments:
Post a Comment