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  1. #1
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    PLL Charge Pumps and VCOs

    Could someone please explain how a PLL charge pump operates? And if I need to interface to a high voltage VCO with an opamp, can I simulate that?

    I get that it's a switching current source, biased by a voltage source at say 3.3V or 5V source and at it's simplest form it could be a BJT current mirror to provide the constant current, but if I have an active filter (w/ caps in the feedback) how can it source any current other than the tiny dV/dt across the caps?

    Also, I'm trying to interface to a 0:13V VCO, but I'd really like to verify the opamp in SPICE. I've got a reference design from Hittite (HMC704 PLL) and I checked the HMC-PLL Designer.exe using that reference design, and the eval schematic and PLLDesigner match (thanks HMC for keeping the two the same).

    If I build up that opamp in OrCad (using a pulsed current at the input, 0:3mA), it doesn't make much sense to me. If I use a pulsed voltage (0:4V) it at least kind of makes sense.

    Please, could someone try to learn me somethin good?

    •   Alt8th January 2016, 00:41

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  2. #2
    Advanced Member level 5
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    Re: PLL Charge Pumps and VCOs

    how? because even though there are capacitors in your active filter, there is a non-zero DC gain of the circuit. WHEN the loop is locked, that DC gain is used to tune the VCO to keep it locked. The capacitors, to some extent, just help the loop from oscillating/ringing if a perturbation happens.



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