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Home Electrical and Electronics Analog & Digital Communication Study of Super-heterodyne Receiver & Characteristics of Radio Receiver
Analog & Digital Communication Lab Experiments

Study of Super-heterodyne Receiver & Characteristics of Radio Receiver



Aim

Study of Super-heterodyne Receiver and Characteristics of Radio Receiver.

Theory

In super heterodyne radio receivers, the incoming radio signals arc intercepted by the antenna arid converted into the corresponding currents and voltages. In the receiver, the incoming signal frequency is mixed with a locally generated frequency. The output of the mixer consists of the sum and difference of the two frequencies. The mixing of the two frequencies is termed heterodyning. Out of the two resultant components of the mixer, the sum component is rejected and the difference component is selected. The value of the difference frequency component varies with the incoming frequencies, if the frequency of the local oscillator is kept constant. It is possible to keep the frequency of the difference components constant by varying the frequency of the local oscillator according to the incoming signal frequency. In this case, the process is called Super heterodyne and the receiver is known as a super heterodyne radio receiver.

block-diagram-of-superheterodyne-receiver

Antenna: As with all radio stations, the antenna will pick up the electromagnetic radio waves from the atmosphere and convert these into very small electrical currents.

Tuned R.F. Amplifier.: This block amplifies the very small currents created in the antenna, to improve the sensitivity of the radio receiver, in the same way that it was used in the TRF radio.

Local Oscillator: A new addition to the superheteradio. This is a sine wave generator which is mechanically linked to the tuning capacitor. This ensures that it always produces a frequency at a fixed amount above the resonant frequency of the tuned amplifier. This is typically in the range 450 kHz to 480 kHz.

Mixer: Again a new addition to the super heterodyne radio, but is the critical addition, as it combines the received modulated radio frequency carrier (fc) from the R.F.Amplifier, and the Local Oscillator (fo). The output of the mixer produces at its output four different frequency signals containing the following frequencies, fc, fo-fc, fo+fc, fo. Three of these frequencies fc, fo-fc, fo+fc are amplitude modulated signals each containing all the information about the original audio signal. The only one that does not contain the original signal is fo, the local oscillator frequency which is a pure sine wave. The most important of these is fo-fc because irrespective of the carrier frequency that is tuned in, this frequency will always be the same, since the output of the local oscillator tracks the carrier frequency tuned in. This modulated frequency is called the intermediate frequency (I.F.) and contains the audio signal from the original radio station no matter what station is tuned in.

IF Filter: The I.F. Filter is a fixed range band pass filter with very high selectivity, specifically designed to pass only the intermediate frequency. It is this which gives the super heterodyne receiver it’s big advantage because no matter what radio station is tuned in, it will be transferred by the mixer to the intermediate frequency and this highly specialised band pass filter will be able to select this single frequency from the four produced by the mixer every time with perfect rejection of the others.

IF Amplifier: This stage provides extra amplification for the signal after the IF filter, and again is carefully designed to provide maximum gain at the IF frequency. The combined effect of the IF filters and IF amplifier gives the super heterodyne receiver its excellent selectivity. Commercial radio receivers may have several pairs of IF filters and amplifiers all tuned to an identical IF Frequency. This is often referred to as the IF Strip.

Detector / demodulator: As in the simple radio receiver, the detector/demodulator block contains the diode and RF filter to produce the non-zero signal, and remove the remaining RF carrier.

AF Amplifier: The recovered audio signal is now amplified so that it can provide a meaning full signal to the loudspeaker.

Loudspeaker: Converts the amplified audio signal into sound.

Result:Study of Super-heterodyne Receiver Completed