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vp1953

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Posts: 172

How to plot instantaneous VCO frequency using Cadence Spectre
Jan 23^rd, 2013, 6:23pm

I am running a transient simulation (with noise) for a VCO. I want to see what the free running frequency drift of the VCO is. I could take the time interval between zero crossings (or a certain number of zero crossings) and estimate the instantaneous frequency - do this several times and then I can get an idea of the frequency drift of the VCO.

I am wondering if there is automated way to do this - say plot the inverse of the time interval between 10 zero crossings continuously along side the time domain VCO output?

Thank you.

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sheldon

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Posts: 751

Re: How to plot instantaneous VCO frequency using Cadence Spectre
Reply #1 - Jan 25^th, 2013, 4:24am

VP1953,

� I think what you want is the calculator function, freq, it converts
returns the frequency of the waveform. You can select the level to
use for the zero crossings or it will estimate the level for you.

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �Best Regards,

� � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � Sheldon

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vp1953 Senior Member Offline Posts: 172	Re: How to plot instantaneous VCO frequency using Cadence Spectre Reply #2 - Jan 28^th, 2013, 12:14pm Hi Sheldon, Thank you very much. This is exactly what i was looking for. Best regards..
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rf-design

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Reiner Franke

Posts: 165
Germany

Re: How to plot instantaneous VCO frequency using Cadence Spectre
Reply #3 - Feb 21^st, 2013, 2:40am

If the oscillator frequency is defined by oscillatory energy exchange between an inductor and a capacitor you can use a normalized state-space. The normalized state-space is defined by capacitor voltage and inductor current. You have to normalize so that the oscillatory behaviour gives a circle instead of an ellipse. Than you can extract the angular velocity and get the oscillation frequency in rad/s. Because oscillators also have some nonlinearities there is a ripple in the angular velocity. But you can extract the mean value. This measure is more accurate because finding the zero crossing point by interpolation from some time steps before and after gives additional errors which disturb detailed transient startup or supply impact analysis.

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