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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
  • Alex Liao

    Shawn,

    smlogan said:
    Perhaps the expressions are not working if you have not saved the gate current and hence it returns "nil"?

    You are right, when I saved the node current it worked for me.

    But I am still confusing with several points that pointed by you. I will attach the testbench schematic and the resultant impedance along with the assignment of different fields of 'AC Isin' and R, L values. In such case you can easily see if I was doing right or wrong and can provide clear direction based on the schematic and settings.

    OK, in order to estimate of the small signal negative resistance characteristic, I may want to do the AC analysis and look at the impedance across the sustaining MOSFET.

    1. For the sustaining MOSFET, were you referring to the M5 & M6?

    2. In order to do the AC Analysis to get the impedance, I may need to add a AC current source between the nodes of Vsupply+ and Voutn (maybe or Voutp). Then set Ind. to be large enough like 1mH. Are the  testbench and settings correct?

    3. I tried to do as in 2. with the result shown as follows. I am wondering what is the negative resistance you were referring to?

    smlogan said:
    The magnitude of the negative resistance must exceed the magnitude of the real part of your inductance at the desired frequency of oscillation to both ensure oscillator start-up and steady-state oscillation. 

    As I have wiki-ed the definition of the negative resistance. It is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. But I am confused with the meaning in your context. Are you meaning (a) the Real part of the acquired Impedance, which is -6.408m as shown. Or the other case (b), the negative resistance, which I do not know what it is now, should be over than what I got from the real part (-6.408m) of the impedance (you said inductance as quoted, probably it is impedance)  from AC analysis.  Which case is the correct understanding?

    4. I believe what I did is a large signal AC analysis which is the only AC analysis from the 'Analyses' Menu of the ADE panel. What do you mean by small signal AC analysis? Is it a DC analysis? As far as I know, the DC analysis is performed based on a biased DC operating point with a small AC variation applied on. Or just if I apply a very small amplitude of current source, the AC signal is small enough to be called "small singal AC analysis" in your words?

    Thanks,

    Alex

Reply
  • Alex Liao

    Shawn,

    smlogan said:
    Perhaps the expressions are not working if you have not saved the gate current and hence it returns "nil"?

    You are right, when I saved the node current it worked for me.

    But I am still confusing with several points that pointed by you. I will attach the testbench schematic and the resultant impedance along with the assignment of different fields of 'AC Isin' and R, L values. In such case you can easily see if I was doing right or wrong and can provide clear direction based on the schematic and settings.

    OK, in order to estimate of the small signal negative resistance characteristic, I may want to do the AC analysis and look at the impedance across the sustaining MOSFET.

    1. For the sustaining MOSFET, were you referring to the M5 & M6?

    2. In order to do the AC Analysis to get the impedance, I may need to add a AC current source between the nodes of Vsupply+ and Voutn (maybe or Voutp). Then set Ind. to be large enough like 1mH. Are the  testbench and settings correct?

    3. I tried to do as in 2. with the result shown as follows. I am wondering what is the negative resistance you were referring to?

    smlogan said:
    The magnitude of the negative resistance must exceed the magnitude of the real part of your inductance at the desired frequency of oscillation to both ensure oscillator start-up and steady-state oscillation. 

    As I have wiki-ed the definition of the negative resistance. It is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. But I am confused with the meaning in your context. Are you meaning (a) the Real part of the acquired Impedance, which is -6.408m as shown. Or the other case (b), the negative resistance, which I do not know what it is now, should be over than what I got from the real part (-6.408m) of the impedance (you said inductance as quoted, probably it is impedance)  from AC analysis.  Which case is the correct understanding?

    4. I believe what I did is a large signal AC analysis which is the only AC analysis from the 'Analyses' Menu of the ADE panel. What do you mean by small signal AC analysis? Is it a DC analysis? As far as I know, the DC analysis is performed based on a biased DC operating point with a small AC variation applied on. Or just if I apply a very small amplitude of current source, the AC signal is small enough to be called "small singal AC analysis" in your words?

    Thanks,

    Alex

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