Intermediate frequency

Lore & Background

The intermediate frequency was first used in the superheterodyne radio receiver, invented by American scientist Major Edwin Armstrong in 1918 during World War I. While building radio direction finding equipment for the Signal Corps, Armstrong worked with signals at 500 to 3500 kHz. Triode vacuum tube amplifiers of the day could not amplify stably above 500 kHz, but could oscillate above that frequency. Armstrong's solution was to set up an oscillator tube to create a frequency near the incoming signal and mix it in a mixer tube, producing a lower difference frequency that could be amplified easily. For example, to pick up a signal at 1500 kHz, the local oscillator would be tuned to 1450 kHz, creating an intermediate frequency of 50 kHz. The name superheterodyne was a contraction of supersonic heterodyne, to distinguish it from receivers where the heterodyne frequency was low enough to be directly audible.

After the war, in 1920, Armstrong sold the patent for the superheterodyne to Westinghouse, who subsequently sold it to RCA. The increased complexity of the superheterodyne circuit 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 eventually won out; by 1930, most radios sold were 'superhets'. During the development of radar in World War II, the superheterodyne principle was essential for downconversion of very high radar frequencies to intermediate frequencies. Since then, the superheterodyne circuit, with its intermediate frequency, has been used in virtually all radio receivers.

Various intermediate frequencies have been used historically, including 45 kHz (first commercial superheterodyne receiver), 110 kHz (European AM longwave broadcast receivers), 175 kHz (early wide band and communications receivers), 260 kHz (early standard broadcast receivers), and 455 kHz (most common for AM radio receivers). For FM radio receivers, 10.7 MHz is the most common IF, though others such as 262 kHz, 455 kHz, 1.6 MHz, 5.5 MHz, 10.8 MHz, 11.2 MHz, 11.7 MHz, 11.8 MHz, 13.45 MHz, 21.4 MHz, and 75 MHz have been used.

Reader's Guide

The intermediate frequency is a cornerstone of modern radio electronics, enabling three general benefits: improved processing at high frequencies, conversion of multiple station frequencies to a common frequency, and enhanced frequency selectivity. At very high (gigahertz) frequencies, signal processing circuitry performs poorly; active devices such as transistors cannot deliver much amplification, and ordinary circuits using capacitors and inductors must be replaced with cumbersome high frequency techniques such as striplines and waveguides. Converting a high frequency signal to a lower IF allows more convenient processing. For example, in satellite dishes, the microwave downlink signal is converted to a much lower IF at the dish so that a relatively inexpensive coaxial cable can carry the signal to the receiver inside the building.

In receivers that can be tuned to different frequencies, the IF allows conversion of various station frequencies to a common frequency for processing. It is difficult to build multistage amplifiers, filters, and detectors that can track tuning across different frequencies, but it is comparatively easy to build tunable oscillators. Superheterodyne receivers tune in different frequencies by adjusting the local oscillator frequency on the input stage, and all processing after that is done at the same fixed IF. This gives the receiver a constant bandwidth over its tuning range, unlike early tuned radio frequency receivers where bandwidth increased with frequency.

The main reason for using an IF is to improve frequency selectivity. With all known filtering techniques, the filter's bandwidth increases proportionately with frequency. A narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing filtering at that frequency. FM and television broadcasting with their narrow channel widths, as well as modern telecommunications services such as cell phones and cable television, would be impossible without frequency conversion. In digital receivers, the analog-to-digital converter operates at low sampling rates, so input RF must be mixed down to IF to be processed. Modern satellite television receivers use multiple intermediate frequencies, with the downlink signal converted to an IF range of 950–2150 MHz by a low-noise block downconverter, then further converted to a lower IF of 480 MHz for filtering.

Did You Know?

• The intermediate frequency was invented by Major Edwin Armstrong in 1918 while building radio direction finding equipment for the Signal Corps during World War I.
• The first commercial superheterodyne receiver, the RCA Radiola AR-812 of 1923/1924, used an intermediate frequency of 45 kHz.
• The most common intermediate frequency for AM broadcast receivers is 455 kHz, while for FM receivers it is 10.7 MHz.
• Modern satellite television receivers use an intermediate frequency range of 950–2150 MHz, converted from the Ku microwave band by a low-noise block downconverter.