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    LC VCO with Capacitor filter, what is the cap doing?

    Hi,
    Click image for larger version. 

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    The circuit in the picture is a complementary (NMOS & PMOS) Negative resistance core of an LC VCO. L, C and varactor are OFF-CHIP components connected across pins (IP,IP1B) and (IP1,IPB). The supply is a regulator output VREG and current in the oscillator is determined by resistors connected at top and bottom of the Negative resistance core. There is a capacitor (circled) connected across the common source point of PMOS and NMOS devices. This is what I am not sure of. I notice a significant improvement in phase noise for certain range of cap values. Below and above this range the phase noise degrades. My understanding is that this is some sort of a noise trap at twice the VCO oscialltion freq. Usually such a capacitor is accompanied by an inductor and the both resonate at twice the VCO freq and I don’t see any inductor here. May I know how this cap is helping improve phase noise and how to determine its value.

    Thank you,
    mvj

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    Re: LC VCO with Capacitor filter, what is the cap doing?

    Since you use an ideal voltage source, how this cap improves the PN ?? If you used a noisy-real voltage source, I could understand the filtering function of that cap but you didn't..
    I believe there is a loop/cap forms a loop with a -unknown/unseen- filter in VCO. It's just a guess but I'm not sure..



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    Re: LC VCO with Capacitor filter, what is the cap doing?

    Thank you for your inputs!! I have actually removed some of the circuitry in an attempt to keep it simple and not over crowd it. There is actually
    1. Bandgap voltage reference generator providing reference to a
    2. voltage regulator with about 20pF on chip output capacitor (no off chip capacitor present) and the output (VREG) of this regulator is applied to
    3. the oscillator.
    The capacitor in question seems to be filtering a lot of flicker noise from bandgap, regulator and VCO core transistors when i look at noise summary from phase noise sim. But, like i said, I am not sure how this happening that to for a certain range of cap values.

    Thank you!!



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