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frequency synthesizer basics

Discussion in 'Beginner Electronics' started by Lost'n Found, Aug 8, 2006.

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  1. Lost'n Found

    Lost'n Found Guest

    Hello,

    I was reading about frequency synthesizers and it seems I am missing some
    concept.

    These synthesizers can generate higher frequency. On the specification
    sheets, I read that we can divide the reference frequency by certain
    factors. My question is: If we are dividing the frequency, why are we
    getting much higher frequencies at the output?
     
  2. werty

    werty Guest

    Phase Locked Loops PLL

    VCO voltage controlled oscillator is controlled from a much lower freq
    ,

    This OSC is divided down so it can be compared to a Crystal CLOCK .


    OSC is 50 MegaHrtz , divided down 100 times to .5 Mhz .
    a Crystal OSC REF at same .5mhz is compared , and any phase
    difference
    produces a DC error voltage . This DC is sent up to the 50 mhz VCO .
    Say the error DC is an increase of .1 volts and VCO jumps 1 mhz ,

    They call it "gain" , which is 1mhz/.1 or 10mz per volt .
    But VCO has trouble stopping at any freq you like , it can only
    step by 1/2 megahertz .!
    So special chips do a complicated "divide down" and get more steps
    finer adjustment ..

    Dual modulus prescalers simply count down on one divider and when it
    figures it's at right place , starts couning down in the other divider
    ..

    Phase jitter can be a problem , so if you filter the DC more , you
    nix jitter
    but slow the responce , so now your Radio Shack scanner hops very slow
    !
    They have secrets to remove jitter .

    Freq Counters use PLL to improve the rading but also to put back in
    what the signal dropped . If signal is unreliable and it drops
    a bit , the PLL will simply add that missing bit to the freq counter
    and you'll get a good reading MPD Missing Pulse detectors...
     
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