Intermediate frequency

In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal, resulting in a signal at the difference or beat frequency. Intermediate frequencies are used in superheterodyne radio receivers, in which an incoming signal is shifted to an IF for amplification before final detection is done. There may be several such stages of intermediate frequency in a superheterodyne, which is called "double" (or "triple") "conversion".

Overview

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) without becoming unstable. 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 processing.

A second reason to use an IF, in receivers that can be tuned to different stations, 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 stations simply by adjusting the frequency of the local oscillator on the input stage, and all processing after that is done at the same 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 station was changed, as was necessary in the early tuned radio frequency receivers.

But the main reason for using an intermediate frequency is to improve frequency selectivity. In communications circuits a very common task is to separate 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.

The most commonly used intermediate frequencies are 10–70 MHz in the satellite and radar world. However, the intermediate frequency can range from 10–100 MHz. Intermediate frequency (IF) are generated by mixing the RF and LO frequency together to create a lower frequency called IF. Most of the ADC/DAC operates in low sampling rates, so input RF must be mixed down to IF to be processed. Intermediate frequency tends to be lower frequency range compared to the transmitted RF frequency. However, the choices for the IF are most depending on the available components such as mixer, filters, amplifiers and others that can operate at lower frequency. There are other factors involved in deciding the IF frequency, because lower IF is susceptible to noise and higher IF can cause clock jitters.

History

An intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major Edwin Armstrong in 1918 during World War I. [cite web
last=Redford
first=John
title=Edwin Howard Armstrong
date=February 1996
work=Doomed Engineers
publisher=John Redford's personal website
url=http://world.std.com/~jlr/doom/armstrng.htm
accessdate=2008-05-10] [cite web
last=alisdair
title=Superheterodyne
publisher=everything.com
url=http://everything2.com/index.pl?node_id=1356743
accessdate=2008-05-10] A member of the Signal Corps, Armstrong was building radio direction finding equipment to track German military signals at the then very high frequencies of 500 to 3500 kHz. The existing triode vacuum tube amplifiers wouldn't amplify stably above 500 kHz, however it was easy to get them to oscillate above that frequency. Amstrong's solution was to set up an oscillator tube that would create a frequency near the incoming signal, and mix it with the incoming signal in a 'mixer' tube, creating a 'heterodyne' or signal at the lower difference frequency where it could be amplified easily. For example, to pick up a signal at 1500 kHz the local oscillator would be tuned to 1450 kHz. Mixing them created an intermediate frequency of 50 kHz, which was well within the capability of the tubes.

After the war in 1920, Armstrong sold the patent for the superheterodyne to Westinghouse, who sold it to RCA. The increased complexity of the superheterodyne compared to earlier regenerative or tuned radio frequency receiver designs slowed its use, but the advantages of the intermediate frequency for selectivity and static rejection won out, and by 1930 most radios sold were 'supehets'. During the World War II development of radar, the superheterodyne principle was essential for the down conversion of the very high radar frequencies to intermediate frequencies. Since then the superheterodyne circuit with its intermediate frequency has been used in virtually all radio receivers.

Commonly used intermediate frequencies

*Television receivers: 30 MHz to 900 MHz
*FM radio receivers: 5.5 MHz, 10.7 MHz, 98 MHz. In double conversion superheterodyne receivers, often a first intermediate frequency of 1.6 MHz is used followed by a second intermediate frequency of 470 kHz.
*AM radio receivers: 455 kHz, 460 kHz, 465 kHz, 470 kHz, 475 kHz, 480 kHz
*Satellite uplink downlink equipment 70 MHz, 950-1450 Downink first IF
*Terrestrial microwave equipment 250 MHz, 70 MHz
*Radar 30 MHz
*RF Test Equipment 310.7 MHz, 160 MHz, 21.4 MHz

Footnotes