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FM receiver


FM receiver





The FM receiver is more complicated and, thus, more precious than the normal AM receiver.

 

FM receiver

 






As we shall see, an FM receiver also uses superheterodyne principle.





The FM broadcast signals lie in the frequence range between 88 MHz and 108 MHz.





The IF ( intermediate frequence) of an FM
receiver is 10.7 MHz — much * advanced than the IF value of 455 kHz in AM receivers.





Fig-1 shows the block illustration of an FM receiver.





FM receiver
Fig-1




In the interest of understanding, we shall bandy the colorful sections of the FM receiver.





1.R.F. Tuner





The FM signals are in the frequence range of 88 to 108 MHz.





The weak FM signal ( say 2 μV) is pick up by the antenna and is fed to the R.F. tuner.





The R.F. tuner consists of (i) R.F. amplifier (ii) Mixer and (iii) original oscillator.





The R.F. amplifier amplifies the named FM signal (to 200 μV in the present case).





The affair from the RF amplifier is fed to the mixer stage where it's combine with the affair signal from a original oscillator.





The two frequentness beat together and produce an intermediate frequency ( IF).





The intermediate Frequency( IF) is equal to the difference between oscillator frequence and the RF frequence.






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It's because the RF carrier frequentness for FM radio broadcasting are in the 88 to 108 MHz band.






The IF is always10.7 MHz ( Recall IF in AM receiver is 455 kHz) anyhow of the frequence to which the FM receiver is tune.





2.IF Amplifier Stage






The affair signal from the mixer always has a frequence of10.7 MHz
and is fed to the IF amplifiers. Since IF amplifiers are tune to IF ( = 10.7 MHz), they render nice modification.





Note that bandwidth of IF amplifiers is about 200 kHz or 0.2 MHz. The
IF gain is veritably large ( assumed in this case) so that affair is 2V.





3.Limiter Stage






The affair from IF stage is fed to the limiter. This circuit is an IF amplifier tuned to10.7 MHz but its main function is to remove AM hindrance from the FM signal.





Fig-2





Fig-2
shows how the limiter removes AM hindrance from the FM signal.





The input is an FM signal, but it has different breadth situations because of AM hindrance has been added.





Still, the limiter circuit keeps the affair position constant for different input situations.





4.FM Sensor






After the junking of breadth modulation from the FM signal by the limiter,
the IF signal drives the input of the FM sensor.





An FM sensor is a circuit that converts
frequence variations to breadth variations.





The FM sensor is also called a discriminator
because it can distinguish between different frequentness in the input to give different





5.Affair voltages.





The attendant breadth modulated signal is also remedied and amplified for
feeding to speaker for sound reduplication.






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