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Modulated PXF results are confusing

shadiyoussef

I am trying to run modulated PXF to evaluate the supply sensitivity of a VCO. I went through the examples in the following documents:

- PSRR_Osc_AN

- Virtuoso Spectre Circuit Simulator and Accelerated Parallel Simulator Analysis User Guide (Version 15.1)

Below are the relevant snapshots of the simulation setup, direct plot form, and the AM to PM response from the tuning voltage source to the output.

The results are confusing:

[1] In direct plot form, the frequency ranges listed under USB do not make sense to me.

[2] The AM to PM plot plots both positive and negative frequencies, and that's partly why it looks weird. Is there a way to plot the response due to positive frequencies only? Modulated PXF options do not seem to have freqaxis setting.

I am running SPECTRE181

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