Intercarrier method: Difference between revisions

Revision as of 12:13, 28 October 2009

Revision as of 16:44, 28 October 2009

Line 20:

{{main|Television transmitter}}

In TV transmitters, Bothboth AF and VF [[modulate]] [[intermediate frequency]] (IF) carriers. Than the modulated IF signals are added either at the output of the [[Modulator|vision modulator]] or at the output of the vestigal sideband stage. In each case, the added signals are low level signals and no special combining circuitry is required.

[[Frequency conversion]] and [[amplification]] is common. So a [[Electronic mixer|frequency convertor]] (or a mixer) and a series of amplifiers for aural signal as well as an output [[combiner]] are spared, which reduce the cost of the transmitter and electricity consumption of the amplifiers considerably. (Although a [[Band-stop filter|notch filter]] to supress the intermodulation products is used at the output of the intercarrier transmitter, the cost of the notch filter is not comparable to the cost of extra amplifiers and the output combiner. (See the subsection '''Intermodulation products''' below)

== Intercarrier as used in receiver sets ==

Line 28:

{{main|Television set}}

In TV receivers, the received radio frequency signal is converted to IF in tuner and than 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]]. (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.)

== Intermodulation products and notch filters==

{{main|Intermodulation}}

[[Image:RF Intermodulation at 280 MHz.jpg|thumb|200px|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 accompaying illustration at right, taken from the article [[Intermodulation]].) The unwanted signals appear on [[RF spectrum]] at regular intervals where the frequency differance of two unwanted signals being equal to the frequency difference of the two main signals.

In TV broadcasting the intermodulation products appear out of the [[RF band]] of the [[TV channel]]. So this intermodulation doesn't cause much trouble to viewers . 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.

[[Category: television technology]]