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How to use MOSFET as a switch to introduce a capacitor into a LC tank in VCO?

Alex Liao

Hi guys,

In my VCO design, if I introduce a fixed capacitance, Cap_fix into the C tank, it works fine and give me the target frequency I want. If I disconnect this path (in parallel with the total C) to disable the introduction of this Cap_fix, it gives me higher frequency and it is reasonable as it follows：
w = 1/sqrt(C*L).

But if I want to implement this on/off feature using a MOSFET it does not work.
It always generates strange frequency. I was observing the target frequency through Cadence DFT function of the output in the ADE panel.

Working as a switch, I treated the D and S ends as the switch's two ends. I biased the MOSEFT in triode (ohmic) region, which means,
give me a small Ron (1/gds) when it is on and a infinite large Ron when it is off. For MOSFET size, I tried several combinations, still not working. Either the harmonic signal's strength is high or sometimes output some unreasonable DFT waveform.

Is it such tricky on just using a triode region MOSFET as a simple on/off switch in RF circuit? Or was I implementing the switch using MOSFET in a wrong way? or any tips on bias or sizing this MOSFET? Shouldn't be the reason of my core design as it works fine by simply connecting/disconnect a regular capacitor into the LC tank.

Any reply is appreciated!
Thanks,
Alex

Parents
  • Andrew Beckett

    Alex,

    Transient can save the final operating point (at the end of the simulation) and that's what it may be displaying (it depends if you turn that on for the transient analysis). You can see in the results browser if there is a final transient operating point. It doesn't record the time for the operating point in that case - but normally it would just be at the very end of the transient (the name in the netlist is pretty clear too...). I thought the form for the annotate transient operating point said "final" in it somewhere but I'd have to check as my memory may be failing me at 9pm on a Friday night ;-)

    By the way, the extra images didn't bother me - it was just that posting the images as part of your post means that they show up inline which is a bit easier for users (I meant to include a smiley in my comment about extra pictures!)

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Alex,

    Transient can save the final operating point (at the end of the simulation) and that's what it may be displaying (it depends if you turn that on for the transient analysis). You can see in the results browser if there is a final transient operating point. It doesn't record the time for the operating point in that case - but normally it would just be at the very end of the transient (the name in the netlist is pretty clear too...). I thought the form for the annotate transient operating point said "final" in it somewhere but I'd have to check as my memory may be failing me at 9pm on a Friday night ;-)

    By the way, the extra images didn't bother me - it was just that posting the images as part of your post means that they show up inline which is a bit easier for users (I meant to include a smiley in my comment about extra pictures!)

    Regards,

    Andrew.

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