Intermediate frequency (IF) is a replica of a received signal but with frequency that is shifted usually below radio receiver frequency. The use of intermediate frequency is a result of the efforts to build a receiver capable of a wide range of receiver frequencies. IF is often used to increase signal processing or convert frequencies to a common one for processing.
Radio receivers are a series of progressive signal amplification by a set of cascaded radio frequency (RF) amplifiers. Amplifiers increase the signal level, which means the antenna signal has to be fed to a very sensitive first stage, amplified, and sent to the second stage and so on for further amplification. The first radio receivers built were tuned radio frequency (TRF) receivers, which had RF stages that were all tuned to the receiver frequency. TRF receivers are best suited for receiving only one frequency. For tuning to a wide range of frequencies, TRF receivers may not be practical except in special applications.
A common broadcast amplitude modulation (AM) radio is capable of receiving about 580 to 1,600 kiloherz (kHz). In tuning, a resonant circuit, usually a tuned frequency transformer’s shunt capacitance, is decreased to tune to higher frequencies. At some point, the circuit ends up with a complicated ganged-tunable multi-capacitor with four separate capacitors. The next concern is undesired coupling of the signal on the fourth stage output, finding its way into the input of the first stage. All these result in noisy speaker output.
Other limitations of TRF receivers include complicated design of the inductance and capacitances needed in the tuned circuits. The solution is to replace most of the RF stages with IF amplifiers with fixed frequency tuned transformers. These intermediate frequency transformers have a primary winding and a secondary winding wound on an adjustable core.
The IF strategy is to shift the frequency of the received RF so that it is fixed at the IF value. For reception of 580 kHz, for example, the receiver generates a local frequency of 1,035 kHz. The difference of the local frequency and receiver frequency is 455 kHz, which is the intermediate frequency. This same process is duplicated at various local frequencies, which means tuning to a wide range of receive frequencies is simplified when using IF.
An intermediate frequency amplifier is tuned only once in the factory. A device that is already tuned to the specific IF is called an intermediate frequency filter. Most of these filters are quartz-controlled or pre-cut crystals tuned to the IF. The intermediate frequency is used in almost all radio receivers categorized as heterodyne or super-heterodyne receivers.