The Stages of Super Heterodyne Receivers. Posted By : Tymon Hytem-5496
The Stages Of Super Heterodyne Receivers.
There are different stages of a super heterodyne receiver namely antenna or aerial, radio frequency amplifier, frequency converter stage, intermediate amplifier stage, second detector, audio frequency amplifier and loud speaker.
An aerial intercepts the electromagnetic waves. Voltage induced in the antenna re communicated to the receiver input circuit by means of a feeder
wire or lead in wire. A parallel tuned circuit at the input of the receiver responds only to voltage at the desired carrier frequency and rejects voltage at all other frequencies. The voltage so picked up is fed to the input of radio frequency amplifier stage.
Radio frequency amplifier stage is generally a tuned voltage small signal amplifier tuned to the desired carrier frequency. The chief functions of radio amplifier stage are to amplify the input signal voltage to a suitably high level before feeding it to the frequency mixer which contributes large noise. Thus signal to noise ratio is improved and the other function is to provide discrimination of selectivity against image frequency signal and intermediate frequency signal.
Frequency converter stage consists of a local oscillator and frequency mixer. To the frequency mixer are fed both the local oscillator voltage as well as signal v0oltage. The mixer, being a non linear device, produces at its output the various inter modulation terms. The difference frequency voltage is picked up by the tuned circuit in the output circuit of the mixer. The difference frequency is called the intermediate frequency, the value of which is constant of a receiver. For all wave receivers, typical value of intermediate frequency is four hundred sixty five kilo hertz or four hundred fifty six kilo hertz. Some times two separate transistors are used as local oscillator and frequency mixer but more often only one transistor functions both as local oscillator and frequency mixer. Such a transistor is then referred to as a frequency converter transistor. Thus with the help of frequency converter stage, radio frequency signal of any carrier frequency is converted into similarly modulated fixed frequency intermediate frequency signal.
Intermediate frequency amplifier stage consists of two or more stages of fixed frequency voltage amplifier having a three decibel bandwidth of ten kilo hertz for a amplitude modulation broadcast. This intermediate frequency amplifier provides most of the receiver amplification and selectivity.
Output of the last intermediate frequency amplifier stage is fed to this second detector which is generally a linear diode detector. Output of this detector is the original modulation frequency voltage. For satisfactory operation of this detector that is for linear detection, it is necessary that the carrier voltage fed to it be at least one volt. Audio frequency output from second detector is fed to the audio frequency amplifier which provides original amplification. Then amplified voltage is fed to the loud speaker.