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Simulating an autonomous circuit in addition to a voltage source

Hello to the community,

I have designed a 14 GHz VCO on schematic view that I usually simulate using harmonic balance (HB) analysis and everything works fine for the extraction of its typical figures of merit.

My goal is to study the effect of various small perturbations, with different frequencies, on different nodes of the circuit. Say the perturbations I want to inject are in the MHz range, my hope is that I can see spurs generated in the output spectrum of the VCO that would be there either because of FM or AM.

I use a sinusoidal voltage source to generate these perturbations but unfortunately, HB analysis doesn't work on an autonomous circuit when a voltage source is present on the design so I can't simulate my circuit. I also tried using ' tran ' analysis but the VCO won't start up on its own when I'm not using HB. Other analysis types for periodic circuits require an input fundamental frequency but I effectively have two in my circuit : the 14 GHz signal generated by the VCO itself, and the MHz signal I inject on different nodes.

Has anyone been in this situation before ? If anyone could give me advice on how to simulate such an effect I would be grateful.

Thank you.

Youssef

Andrew Beckett over 2 years ago

Youssef,

You absolutely can do this with hb analysis (unless you're using an extremely old version - semi-autonomous harmonic balance has been supported for many, many years). in ADE you need to do this via the hb analysis (not pss in harmonic balance mode) and specify that you have two tones. Check the oscillator checkbox on the form, and the first tone is marked as "osc!" and then you specify the estimated oscillation frequency for that. For the second tone you specify the frequency of your perturbation source and the number of harmonics, etc.

Andrew
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Youssef Bendou over 2 years ago in reply to Andrew Beckett

Hello Andrew,

Thank you for your answer. You are right, I have activated the two tones mode and it seems to be working.

Thank you again and have a nice day.

Youssef
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ShawnLogan over 2 years ago in reply to Youssef Bendou

Dear Youssef Bendou,

I am not sure of your final goal, but as I read your post, it sounded as if you were trying to perform an ISF analysis for your VCO. If so, this feature is included in a PSS analysis and the function can be viewed in ViVA from a PSS analysis. This is clearly a LOT faster than injecting many phases of a tone into the VCO and measuring its resultant phase perturbation at each applied phase.

If that is your interest, you might take a look at my comments in the Forum posts at URL:

https://community.cadence.com/cadence_technology_forums/f/rf-design/47454/isf-simulation-using-transient-analysis-by-injecting-impulse

https://community.cadence.com/cadence_technology_forums/f/custom-ic-design/43969/isf-function-extraction-in-cadence-virtuoso

Shawn
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illaoi over 2 years ago

"I also tried using ' tran ' analysis but the VCO won't start up on its own when I'm not using HB" this issue can be resolved if you use initial condition for a node (to create asymmetry) or use a pulse that starts from 0 and stays at its final value for power supply.
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