Stats

  • Locked Locked
  • Replies 18
  • Subscribers 63
  • Views 26583
  • Members are here 0
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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

  • Hi Andrew,

    Andrew Beckett said:
    If you use the "infotimes" parameter of the tran analysis to save the operating point at a list of times (what you call "TA-OP", which is a term I've never heard anyone use before!) then you get the operating point at those times - it's not averaged. I'm not sure why you would think it was averaged - you get an output at each and every time you ask to save at.

    I have practiced the infotimes option and did get different OPs.  Thanks for your reply. The reason why I think it is averaged is because even though I did not set this 'infotimes', I can still print the transient operating point and no time information is shown. This must corresponds to a time, most likely the end time of the tran analysis. But it did not say this is for the end time Op info. or give a warning saying something like 'Since you did not specify the time, the system will return you the information of the last sampling time which is the end time by default". That's why I think this OP info. is for a specific case (or incorrectly, average case) for tran analysis. Thus I strangely call it "TA-OP". Maybe the spectre or Cadence manual has everything but at the first glance nobody gets it. Most people trying new things and only gets stuck and then resort to the Manual. It is suggested to prompt some information or warning on improper operations for people who are not the experts of Cadence tools.

    Thanks,

    Alex

     

Reply

  • Hi Andrew,

    Andrew Beckett said:
    If you use the "infotimes" parameter of the tran analysis to save the operating point at a list of times (what you call "TA-OP", which is a term I've never heard anyone use before!) then you get the operating point at those times - it's not averaged. I'm not sure why you would think it was averaged - you get an output at each and every time you ask to save at.

    I have practiced the infotimes option and did get different OPs.  Thanks for your reply. The reason why I think it is averaged is because even though I did not set this 'infotimes', I can still print the transient operating point and no time information is shown. This must corresponds to a time, most likely the end time of the tran analysis. But it did not say this is for the end time Op info. or give a warning saying something like 'Since you did not specify the time, the system will return you the information of the last sampling time which is the end time by default". That's why I think this OP info. is for a specific case (or incorrectly, average case) for tran analysis. Thus I strangely call it "TA-OP". Maybe the spectre or Cadence manual has everything but at the first glance nobody gets it. Most people trying new things and only gets stuck and then resort to the Manual. It is suggested to prompt some information or warning on improper operations for people who are not the experts of Cadence tools.

    Thanks,

    Alex

     

Children
No Data

Community Guidelines

The Cadence Design Communities support Cadence users and technologists interacting to exchange ideas, news, technical information, and best practices to solve problems and get the most from Cadence technology. The community is open to everyone, and to provide the most value, we require participants to follow our Community Guidelines that facilitate a quality exchange of ideas and information. By accessing, contributing, using or downloading any materials from the site, you agree to be bound by the full Community Guidelines.