intermediate-frequency amplifier

intermediate-frequency amplifier

Intermediate-frequency amplifier

An amplifying circuit in a radio-frequency (RF) receiver that processes and enhances a downconverted or modulated signal. Signal frequency spectrum downconversion is achieved by multiplying the radio-frequency signal by a local oscillator signal in a circuit known as a mixer. This multiplication produces two signals whose frequency content lies about the sum and difference frequencies of the center frequency of the original signal and the oscillator frequency. A variable local oscillator is used in the receiver to hold the difference-signal center frequency constant as the receiver is tuned. The constant frequency of the downconverted signal is called the intermediate frequency (IF), and it is this signal that is processed by the intermediate-frequency amplifier.

Unfortunately, radio-frequency signals both higher and lower than the local oscillator frequency by a difference equal to the intermediate frequency will produce the intermediate frequency. One of these is the desired signal; the undesired signal is called an image. See Mixer, Oscillator

Aside from demodulation and conversion, the purpose of each stage of a radio receiver is to improve the signal-to-noise ratio (SNR) through a combination of signal amplification and noise/interference suppression. Unlike the broadband tunable radio-frequency amplifier, the intermediate-frequency amplifier is designed to operate over a narrow band of frequencies centered about a dedicated fixed frequency (the intermediate frequency); therefore, the intermediate-frequency amplifier can be an extremely efficient stage. If the intermediate frequency is on the order of a few megahertz, the undesirable images may be efficiently rejected, but narrow-band filtering for noise and adjacent-channel-signal rejection is difficult and expensive because of the high ratio of the intermediate frequency to the bandwidth of the intermediate-frequency amplifier. If the intermediate frequency is much smaller, say, on the order of a few hundred kilohertz, then inexpensive and more selective filters are possible that can separate the desired signal from closely packed adjacent signals, but they do not reject images very well. A high-quality double-conversion receiver combines the best of both approaches by cascading both high- and low-frequency intermediate-frequency stages that are separated by a second fixed-frequency mixer.

The superheterodyne structure is common for television, ground-based and satellite communications, cell phones, ground-based and airborne radar, navigation, and many other receivers. The intermediate-frequency amplifier function is ubiquitous. See Amplifier, Radio-frequency amplifier