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Simulators >> RF Simulators >> Facing an issue with the noise summary tool of cadence
https://designers-guide.org/forum/YaBB.pl?num=1460179891

Message started by iVenky on Apr 8^th, 2016, 10:31pm

Title: Facing an issue with the noise summary tool of cadence
Post by iVenky on Apr 8^th, 2016, 10:31pm

Hi,

I am designing a VCO. While debugging the phase noise issues, I am using the noise summary tool of virtuoso to find the contribution of individual transistors. There is one thing that is really confusing and I am not able to understand. My VCO has a capacitor bank, varactor, tail current source, cross coupled transistors (for negative resistance) etc. So I was initially placing everything as ideal elements and then replaced them one by one by the real elements to find out the addition of phase noise due to these elements along with their individual contributions on the noise summary window.
Case 1) with tail current source only
In this case, I had the tail current source and replaced the varactor with an analog lib cap (just for checking at a particular frequency). The cap bank was also present. I found on the noise summary window that the tail current source was contributing most of the percentage while cross-coupled pair was contributing less noise. Cap bank was also contributing some noise.
Case 2) with ideal cap bank
I used an ideal cap bank and everything else was same as before. Now when I looked up the noise summary, the cross coupled transistor contribution got increased and the tail current source contribution got reduced! I couldn't understand how this could happen.

Is "noise summary" tool reliable? I have attached schematic of the VCO (in fact it's a QVCO) that i used

Thanks! Please let me know if you have any questions.