Intercarrier method: Difference between revisions

Revision as of 13:28, 9 November 2009

Revision as of 13:12, 13 November 2009

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{{main|Television set}}

In TV receivers, the received radio frequency signal is converted to IF in tuner and then demodulated. The output of the demodulator consists of a VF and an aural signal which is in fact an FM subcarrier modulated by AF. (The subcarrier is 5.5 MHz. in system B and 4.5 MHz. in system M ) The aural signal and the VF are separated by a simple filter. The only extra stage needed for AF (other than the [[loudspeaker]]) is an [[Demodulation|FM demodulator]].<ref> Bernard Grob,Charles E.Herndon:''Basic Television and Video Systems'',Glencoe,McGraw Hill,New York,6th ed,1999,pp 469 </ref> (It should be noted that although AF VF signals are combined in the IF stages of the transmitters, they are separated in baseband stages of the receivers.)

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{{main|Intermodulation}}

When VF and the aural signal modulate the same carrier the inevitable nonlinearity of the electronic circuits cause unwanted signals which are called intermodulation products. (An example of intermodulation products can be seen in the accompanying figure at right, taken from the article [[Intermodulation]].) The unwanted signals appear on [[RF spectrum]] at regular intervals, the interval being equal to the frequency difference of the two main signals.

[[Image:RF Intermodulation at 280 MHz.jpg|thumb|288px|Image:RF Intermodulation at 280 MHz.jpg|thumb|A [[frequency spectrum]] plot showing intermodulation between two injected signals at 270 and 275 MHz (the large spikes). Visible intermodulation products are seen as small spurs at 280 MHz and 265 MHz.]]

When VF and the aural signal modulate the same carrier the inevitable nonlinearity of the electronic circuits cause unwanted signals which are called intermodulation products. (An example of intermodulation products can be seen in the accompanying figure at right, taken from the article [[Intermodulation]].) The unwanted signals appear on [[RF spectrum]] at regular intervals, the interval being equal to the frequency difference of the two main signals.

In TV broadcasting the intermodulation products of the aural subcarrier and the main carrier appear out of the [[RF band]] of the [[TV channel]]. However, out of band product means an unwanted transmission in the neighbour TV channels. For example, intermodulation products of channel 7 appear in channel 5,6, 8 and 9. That is why notch filters are used in intercarrier system.

== References ==

<references/>

[[Category: television technology]]