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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 Shawn,

    I was intended to make my reply more sensible, but I couldn't find the 'edit my reply' function. So I am adding something what I wanted to say here in a more sensible way.

    1. For the (a) and (b) they are almost the same concern. For the (c) and (d) if my question on how to make MOSFET work stably in large signal case get solved, then they can be more reasonable to think of as they are mostly related to the accuracy problem if I am right on this.Now it seems the first two concern should be addressed. For (a) I am not sure if the negative resistance of the  sustaining amplifier refers to the M1 and M2 paris or other structures from other implementations. But is the 'negative resistance' refers to the gm or gds or might be gm || gds that controls the amplification of a signal? otherwise what does it mean? 

    2. Can the transient analysis operating point (TA-OP) help by running transient analysis? I always use it in diagnose problems in high frequency domain. It can print out a list of parameters. But I do not know how the Cadence get those information. Transient is run on a specific time range, the results must correspond to a sampling time. Are the printed TA-OP information revealing the simulation of the last sampling time or it always returns a average value?　Should not be the later case, as the TA-OP list includes the region of Mosfet, which is 0,1,2,3,4 for different meaning. This information cannot be averaged. Right?

    Sorry if I am asking irrelevant question here if it has nothing to do with diagnose problems.

    Thanks,

    Alex

Reply

  • Dear Shawn,

    I was intended to make my reply more sensible, but I couldn't find the 'edit my reply' function. So I am adding something what I wanted to say here in a more sensible way.

    1. For the (a) and (b) they are almost the same concern. For the (c) and (d) if my question on how to make MOSFET work stably in large signal case get solved, then they can be more reasonable to think of as they are mostly related to the accuracy problem if I am right on this.Now it seems the first two concern should be addressed. For (a) I am not sure if the negative resistance of the  sustaining amplifier refers to the M1 and M2 paris or other structures from other implementations. But is the 'negative resistance' refers to the gm or gds or might be gm || gds that controls the amplification of a signal? otherwise what does it mean? 

    2. Can the transient analysis operating point (TA-OP) help by running transient analysis? I always use it in diagnose problems in high frequency domain. It can print out a list of parameters. But I do not know how the Cadence get those information. Transient is run on a specific time range, the results must correspond to a sampling time. Are the printed TA-OP information revealing the simulation of the last sampling time or it always returns a average value?　Should not be the later case, as the TA-OP list includes the region of Mosfet, which is 0,1,2,3,4 for different meaning. This information cannot be averaged. Right?

    Sorry if I am asking irrelevant question here if it has nothing to do with diagnose problems.

    Thanks,

    Alex

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