Discussing merits of Negative modulation for AGC circuit in TV transmission..!!

Discussion in 'General Electronics Chat' started by Himanshoo, Jun 23, 2016.

  1. Himanshoo

    Thread Starter Member

    Apr 3, 2015
    Hello friends..

    The work of an agc circuit is to measure the peak amplitude of the rf carrier and adjust the gain of the rest stage accordingly. To perform this operation the age needs a reference level (yardstick) by which it could measure the amplitude and decides the gain. The text says that in negative modulation this reference is provided by the peak of the sync pulse which remains at maximum voltage level of the signal whereas in positive modulation this constant ref level cease to exist or is at zero volts which probably is of no use.
    So my query is that…

    for an AGC circuit to work a a constant reference level is needed….but where is this “constant” level provided in negative modulated CVS, as sync pulse is of some duration lets say x sec..so just for the duration of x sec the reference is provided..but then what after x sec…??

    * What reference level will be the AGC circuit takes when each sync pulse is crossed..??

    * Why zero volts isn't considered a good reference level..??

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  2. crutschow


    Mar 14, 2008
    Analog video uses a negative sync pulse as shown here:


    The reference level is the tip of the sync pulse which is the negative-most voltage.
    Since the signal is AC coupled, zero volts varies, depending upon the signal content so can't be used as a reference level.
    To provide a DC reference level for the signal, the negative sync tip is clamped to ground (called a DC restore circuit), then the rest of the signal is positive with respect to ground, as discussed here.
    This provides a constant reference point, which allows accurate signal measurement for the AGC circuit.
    Himanshoo likes this.