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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

  • Dear Alex,

    I was unable to view your schematic. Perhaps you can upload it the Cadence site. It also looks as if you have posted the same question to that forum.

    In any case, it sounds as if your oscillation amplitude is not well controlled as it can vary from 3 to 550 mV. If your nominal supply voltage is 1.20 V, will not the 3.3V amplitude lead to a reliability issue? In order to use an MOS switch, you need to be able to quantify the maximum and minimum voltages applied to the side of the MOS switch exposed to the oscillator in order to verify the MOS switch stays in its desired region of operation for the specific gate voltage.

    Perhaps I need to see your circuit to provide a more complete response to your questions. Sorry!

    Shawn

Reply

  • Dear Alex,

    I was unable to view your schematic. Perhaps you can upload it the Cadence site. It also looks as if you have posted the same question to that forum.

    In any case, it sounds as if your oscillation amplitude is not well controlled as it can vary from 3 to 550 mV. If your nominal supply voltage is 1.20 V, will not the 3.3V amplitude lead to a reliability issue? In order to use an MOS switch, you need to be able to quantify the maximum and minimum voltages applied to the side of the MOS switch exposed to the oscillator in order to verify the MOS switch stays in its desired region of operation for the specific gate voltage.

    Perhaps I need to see your circuit to provide a more complete response to your questions. Sorry!

    Shawn

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