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Plotting the frequency of a VCO versus time
Oct 09^th, 2006, 7:36pm

Hi Guys,

I want to examine the transient response of a quadrature oscillator as I force it to an initial unstable state. I run a transient analysis and I observe the oscillator behavior as it converges to its steady state. During this process, the oscillation frequency changes with time untill it stabilizes. I would like to get your opinion on how to plot the oscillation frequency versus time? Note when I use the calculator and choose the "Frequency" function I only get the steady state frequency. Thanks for your help in advance.

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ACWWong

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Re: Plotting the frequency of a VCO versus time
Reply #1 - Oct 10^th, 2006, 2:18am

I would use a verilogA block which counts crossings... if you use cadence there is a freq_meter in ahdlLib or try http://www.designers-guide.org/VerilogAMS/index.html

if using freq_meter there a couple of things you might want to bear in mind: the Nature Frequency blowup limit is default 1G. You might want to modify it to something bigger; and the accuracy will suffer depending on your timesteps... so using a maxstep might help.

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RFICInDaHouse

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Re: Plotting the frequency of a VCO versus time
Reply #2 - Oct 10^th, 2006, 11:14pm

Thanks, that worked just fine!. For you who are interested in this thread, you need to setup the "blowup" parameter of the "FF" above some critical value that depends on your circuit transient response. The syntax and the procedure for doing so in Spectre is as fllows:

1_ Create a blowFF.scs file in your favorite directory and add the following script:

temp quantity name="FF" units="Hz" blowup=15e+09

2_ Note that 15+e09 was the critical frequency for my circuit and you can adjust it as needed
3_ From the Analog Artist, go to Setup-->Model Libraries..., browse for the blowFF.scs file and Add it.
4_ you can adjust the abstol(FF) for better resolution by accessing the verilog.va file of the freq_meter component.

That worked for me.

Thanks again ACWWong

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Plotting the frequency of a VCO versus time Reply #3 - Oct 14^th, 2006, 11:27am A simple approach that may also suffice - Plot the VCO control voltage. That is going to be proportional to frequency, and show most of the performance issues associated with lock aquisition, overshoot, etc. Might suffice in most situations...
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