Question

1. Explain how image frequency signals are received in a superheterodyne receiver? how can these signals be rejected? ( Please explain in detail)

2. Why do some superheterodyne receivers use an RF stage while others do not? (please explain in detail)

Answer 1

The image frequency is an unwanted input frequency which is demodulated by superheterodyne receiver along with the desired incoming signal. This results in two stations being received at the same time, thus producing interference.

Superhetrodyne receiver

A typical superhetrodyne receiver consists of an rf filter, rf amp, local oscillator, mixer, if filter, if amp, a demodulator and audio amp.

Image frequency

when the receiver demodulates the incoming desired signal at f_RF, unfortunately it demodulates down to IF also an unwanted signal at f_{RF +}2f_IF . This frequency is called image frequency.

To reduce teh design complexity of the receivers the IF frequency is chosen in such a way that the signal at fRF+ 2f_IF can be rejected by a simple tunable RF bandpass filter such as a tank circuit with a variable capacitor

2)Broadband tunning is applied to the RFstage. The purpose of this is to reject the signals on the image frequency and accept those on the wanted frequency. It must also be able to track the local oscillator so that as the receiver is tuned, so the RF tuning remains on the required frequency.Typically the selectivity provided at this stage is not high. Its main purpose is to reject signals on the image frequency which is at a frequency equal to twice that of the IF away from the wanted frequency. As the tuning within this block provides all the rejection for the image response, it must be at a sufficient sharp to reduce the image to an acceptable level.However the RF tuning may also help in preventing strong off-channel signals from entering the receiver and overloading elements of the receiver, in particulat the mixer or possibly even the amplifier