Intermediate Frequency - Reasons For Using IF

Reasons For Using IF

Intermediate frequencies are used for three general reasons. At very high (gigahertz) frequencies, signal processing circuitry performs poorly. Active devices such as transistors cannot deliver much amplification (gain). Ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. So a high frequency signal is converted to a lower IF for more convenient processing. For example, in satellite television receivers, converting the microwave downlink signal to a much lower intermediate frequency at the dish allows a relatively inexpensive coaxial cable to carry the signal to the rest of the receiver inside the building.

A second reason, in receivers that can be tuned to different frequencies, is to convert the various different frequencies of the stations to a common frequency for processing. It is difficult to build amplifiers, filters, and detectors that can be tuned to different frequencies, but easy to build tunable oscillators. Superheterodyne receivers tune in different frequencies by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same fixed frequency, the IF. Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers.

The main reason for using an intermediate frequency is to improve frequency selectivity. In communication circuits, a very common task is to separate out or extract signals or components of a signal that are close together in frequency. This is called filtering. Some examples are, picking up a radio station among several that are close in frequency, or extracting the chrominance subcarrier from a TV signal. With all known filtering techniques the filter's bandwidth increases proportionately with the frequency. So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency.