Intermediate Frequency - Uses

Uses

Perhaps the most commonly used intermediate frequencies for broadcast receivers are around 455 kHz for AM receivers and 10.7 MHz for FM receivers. In special purpose receivers other frequencies can be used. A dual-conversion receiver may have two intermediate frequencies, one higher one to improve image rejection and a second, lower one, for desired selectivity. A first intermediate frequency may even be higher than the input signal, so that all undesired responses can be easily filtered out by a fixed-tuned RF stage.

In a digital receiver, the analog to digital converter (ADC) operates at 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 dependent 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.

Modern satellite television receivers use several intermediate frequencies. The 500 television channels of a typical system are transmitted from the satellite to subscribers in the Ku microwave band, in two subbands of 10.7 - 11.7 and 11.7 - 12.75 GHz. The downlink signal is received by a satellite dish. In the box at the focus of the dish, called a low-noise block downconverter (LNB), each block of frequencies is converted to the IF range of 950 - 2150 MHz by two fixed frequency local oscillators at 9.75 and 10.6 GHz. One of the two blocks is selected by a control signal from the set top box inside, which switches on one of the local oscillators. This IF is carried into the building to the television receiver on a coaxial cable. At the cable company's set top box, the signal is converted to a lower IF of 480 MHz for filtering, by a variable frequency oscillator. This is sent through a 30 MHz bandpass filter, which selects the signal from one of the transponders on the satellite, which carries several channels. Further processing selects the channel desired, demodulates it and sends the signal to the television.