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Simulators >> RF Simulators >> Ring-Osc based VCO pnoise varing with tstab.
https://designers-guide.org/forum/YaBB.pl?num=1422413567

Message started by Aman on Jan 27^th, 2015, 6:52pm

Title: Ring-Osc based VCO pnoise varing with tstab.
Post by Aman on Jan 27^th, 2015, 6:52pm

Hello,

I have a ring-osc based VCO which is biased with a heavily filtered current mirror(coarse control).
The issue is the modulated(PM) phase-noise varies a lot with tstab value chosen. I'm convinced this
is happening due to presence of low frequency poles in the circuit, which causes huge settling time
for various nodes. Is there a way around this problem, so that I can get reliable/repeatable results
without using a large tstab.

Thanks in advance.

-AS

Title: Re: Ring-Osc based VCO pnoise varing with tstab.
Post by sheldon on Jan 28^th, 2015, 5:35pm

AS,

1) Have you run a transient and plotted frequency versus time
to verify that there is as long settling time?
2) How are calculating the initial conditions?

Sheldon

Title: Re: Ring-Osc based VCO pnoise varing with tstab.
Post by AS on Jan 29^th, 2015, 12:05pm

Hi Sheldon,

Thanks for your reply.
Yes, there is a long settling time in the biasing circuit for various current and voltages to settle down
as I have a sub-KHz pole in the system to filter out a dominant noise source. Are you suggesting
taking .ic from a DC run for my PSS?

Best Regards,
AS

Title: Re: Ring-Osc based VCO pnoise varing with tstab.
Post by sheldon on Feb 8^th, 2015, 9:18pm

AS,

No, I was referring to the initial conditions option in the oscillator
analysis section. It was intended to speed up convergence for high
Q oscillators, for example, crystal oscillators. Have you considered
using harmonic balance analysis. Tested both analyses for a ring
oscillator and the results matched. Used a a lot of tones for HB
analysis, ~100 tones, that may be more than you will need.

Sheldon