Intermediate frequency (IF) is a replica of a received signal with a shifted frequency, used to simplify tuning across a wide range of frequencies. It replaces most RF stages with IF amplifiers and fixed frequency tuned transformers, resulting in less noise and complicated design. An IF amplifier is tuned only once at the factory, while an IF filter is already tuned to the specific IF. It is used in almost all heterodyne or superheterodyne radio receivers.
Intermediate frequency (IF) is a replica of a received signal but with a frequency that is usually shifted below the frequency of the radio receiver. The use of intermediate frequency is the result of efforts to build a receiver capable of a wide range of receiver frequencies. IF is often used to augment signal processing or convert frequencies to a common one for processing.
Radio receivers are a series of progressive amplification of the signal by a series of radio frequency (RF) amplifiers in cascade. Amplifiers increase the signal level, which means that the antenna signal has to be sent 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 all tuned to the receiver’s frequency. TRF receivers are best suited for receiving only one frequency. For tuning over a wide range of frequencies, TRF receivers may not be practical except in special applications.
A common Transmit Amplitude Modulation (AM) radio is capable of receiving 580 to 1,600 kiloherz (kHz). In tuning, a resonant circuit, usually the shunt capacitance of a tuned frequency transformer, is reduced to tune to higher frequencies. At some point, the circuit ends up with a complicated tunable multicapacitor with four separate capacitors. The next concern is unwanted signal coupling on the fourth stage output, making its way into the first stage input. All of this results in a noisy speaker output.
Other limitations of TRF receivers include the complicated design of inductance and capacitances needed in tuned circuits. The solution is to replace most 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 consists of shifting the frequency of the received RF so that it is fixed at the IF value. For example, for 580 kHz reception, the receiver generates a local frequency of 1.035 kHz. The difference between the local frequency and the receiver frequency is 455kHz, which is the intermediate frequency. This same process is duplicated at various local frequencies, meaning that tuning across a wide range of receive frequencies is simplified when using IF.
An intermediate frequency amplifier is tuned only once at 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 classified as heterodyne or superheterodyne receivers.
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