Intermediate frequency

Intermediate frequency ( IF ) (Engl. Intermediate frequency / IF) is a term used in electronics . In general, it describes a frequency or frequency range in reception technology to which an input signal is converted. It is mostly used in connection with heterodyne receivers and denotes the fixed frequency to which the received signal coming from the antenna is mixed down , amplified and demodulated . The advantage of permanently set oscillating circuits is that the superhet receives constant reception properties over the entire reception range.

Intermediate frequency stage in a historic television receiver from 1950

Only the received signal that is offset by the value of the intermediate frequency from the oscillator frequency of the local oscillator (LO) can pass the intermediate frequency amplifier:

${\displaystyle f_{\mathrm {ZF} }=f_{\mathrm {LO} }-f_{\mathrm {e} }}$  or  . ${\displaystyle f_{\mathrm {ZF} }=f_{\mathrm {LO} }+f_{\mathrm {e} }}$

The intermediate frequency is usually lower than the input frequency because the circuit complexity for amplifiers of low frequencies is lower than for high frequencies. The exact value depends on the receiver technology: AM radio receivers operate in the long , medium and short wave range with 460 kHz, VHF radio receivers use a frequency of 10.7 MHz, and television receivers 38.9 MHz for the video signal . The intermediate frequency for the sound is obtained from the mixture of the frequency-modulated sound signal coming from the transmitter with the amplitude-modulated image signal. The distance between these two signals is z. B. in the analog TV system PAL B set to 5.5 MHz, so that the sound intermediate frequency of 5.5 MHz always arises when mixing.

Internationally it was agreed not to operate powerful transmitters in a sufficiently wide range around the usual intermediate frequencies that would generate interference that is difficult to mask out. These can only be eliminated (at least in the vicinity of these transmitters) with extremely effective suction circuits or precisely matched symmetrical mixers .

In wideband receivers and radio scanners , the first intermediate frequency is often the highest receiving frequency to also have a sufficiently high despite insufficient preselection image frequency spacing to achieve.

With LNBs for satellite reception , the frequency of the local oscillator is fixed. Here the received signal is mixed down in the 11 GHz range to intermediate frequencies of 950 to 2150 MHz. The frequency reduction is only made here in order to be able to transmit the lower IF with less loss via coaxial cable .

In professional radio systems, multi-level signal processing is often used, with unusual intermediate frequencies also being used. The first intermediate frequency can be 10.8 MHz, frequencies from 450 to 470 kHz are used for the second IF.

After amplification, the intermediate frequency is fed to the corresponding demodulators or, in the case of double superhetero receivers, to other mixers.

Intermediate frequencies are standardized, since no strong transmitters should be operated on these frequencies (irradiation would be heard on all received transmitters) and emissions from the receivers in this frequency range can hardly be avoided (frequencies cannot be used for DX):

• LW / MW / KW: mostly 455 kHz used
• FM: mostly 10.7 MHz used, also used by many LW / MW / KW double superhets
• TV: 33 to 40 MHz used
• SAT: first IF mostly 900 to 2150 MHz, second IF ~ 470 MHz

literature

• Gregor Häberle, Heinz Häberle, Thomas Kleiber: Expertise in radio, television and radio electronics . 3. Edition. Verlag Europa-Lehrmittel, Haan-Gruiten 1996, ISBN 3-8085-3263-7
• Helmuth Wilhelms, Dieter Blank, Hans Mohn: Electrical specialist 3 Telecommunications . 1st edition. BG Teubner Verlag, Stuttgart 1982, ISBN 3-519-06807-9