Intermediate frequency

The intermediate frequency (IF) is the frequency given in radio devices that use the superheterodyne principle. This is obtained by mixing the tuned signal on the antenna with a variable frequency generated locally in the device itself by means of a local oscillator (OL) and which has a constant difference with it. This difference between the two frequencies is precisely the intermediate frequency.

In conventional radio receivers the value of the intermediate frequency is normally 455 or 470 kilohertz, in amplitude modulation (AM) receivers and 10.7 megahertz in frequency modulation (FM), although in In more sophisticated devices, the so-called double conversion devices, a second, smaller IF value is used. In the television receivers of the PAL system used in Germany, Spain and other countries, the IF is selected at 38.9 megahertz.

The usefulness of using an intermediate frequency lies in the fact that all the existing tuned circuits from the stage in which the mixing is carried out work at a fixed frequency (the IF frequency) and therefore are more easy to adjust. In this way the selectivity is improved and the design of the amplifier stages is facilitated. If the intermediate frequency were not used, it would be necessary to design tuning circuits that had at the same time a high selectivity and a wide range of selection of actuation frequencies, something difficult and expensive to achieve.

History

Intermediate frequency was first used in a superheterodyne radio receiver, invented by American scientist Major Edwin Armstrong in 1918, during World War I. Member of the U.S. Army Signal Corps, Armstrong was building radio directional search equipment to track German military signals at very high frequencies, from 500 to 3500 kHz. Triode vacuum tube amplifiers of this era did not amplify stably above 500 kHz, however they were easy to oscillate above that frequency. Armstrong's solution was to install an oscillator tube that would create a frequency close to the frequency of the tuned signal, and mix it with the tuned signal in a mixing tube, creating a 'heterodyne' of the tuned signal. or lower frequency difference signal, which could be easily amplified. For example, to capture a signal at 1500 kHz the local oscillator will be tuned to 1450 kHz. Mixing the two frequencies gives an intermediate frequency of 50 kHz, which is suitable for tubes.

Intermediate frequency in PAL television system

In the PAL system used in Germany, Spain and other countries for modulation and transmission of the TV signal, the intermediate frequency of the receiver is 38.9 MHz. This frequency varies depending on the variant of the system used, but the operation The basic system is the same, even for the American NTSC system. Hereinafter, the values quoted for the frequencies correspond to those of the PAL system.

The local oscillator (LO) is operated at a frequency equal to that of the channel to be selected plus the IF. This signal is mixed with the radio frequency (RF) signal coming from the antenna, giving rise to a displacement and inversion of the signal spectrum: the frequency of the luminance carrier becomes 38.9 MHz, and the frequency of the sound carrier, which in RF is 5.5 MHz higher than the luminance carrier, becomes 5.5 MHz lower in IF than the luminance carrier. That is, in plain language, what was previously "on the left" now it passes "to the right" and vice versa (spectrum inversion).