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Simulators >> RF Simulators >> VCO pnoise problem
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Message started by chen on Aug 6^th, 2005, 6:03pm

Title: VCO pnoise problem
Post by chen on Aug 6^th, 2005, 6:03pm

Hi. I tried to simulate a differential LC VCO in spectreRF. However, the one with ideal L has the same (actually a bit higher!) phase noise than the one with real L (with finite Q) I use real transistors as varactor so the LC tank has finite Q in both cases. Here is my simulation setting:

pss:
# of harmonics=10
accuracy: conservative
tstab=50n
oscilator checked, (I have tried choosing nodes across the tank and 1 side of the tank with ref. to gnd. Both gives similar results)

pnoise:
relative harmonic=1
10k-10M
15 pts/decade
max sideband=10
voltage output, nodes same as pss
input:none
noise type: sources

Anyone has idea what dumb mistakes I have made?

Thx!

Title: Re: VCO pnoise problem
Post by Andrew Beckett on Aug 7^th, 2005, 1:21am

Well, your question is a bit light on details. Try (assuming you're using IC5141 or later) setting the highorder=yes option on the PSS options form to enable the Multi Interval Chebychev method - which should reduce the numerical noise if the oscillator is quite linear. See if that makes a difference (I'm not sure it will, but I wanted to ensure that what you're seeing is not related to numerical accuracy in the PSS).

Regards,

Andrew.

Title: Re: VCO pnoise problem
Post by chen on Aug 8^th, 2005, 10:30pm

Thx so much for your reply!

Unfortunately, I only have access to ic5033 right now and it seems that this version does not allow highorder for autonomous PSS.

I am not sure what kind of info will be useful to you. I am getting around 110dBc/Hz for both cases. When I looked at the spot noise summary, it does include the noise due to the inductor. (though it is not the dominant one) So I guess either it is because the MOS are too noisy or I am doing something wrong in my simulation.

Title: Re: VCO pnoise problem
Post by sheldon on Aug 15^th, 2005, 4:10am

Chen,

You can also try using relref=pointlocal to reduce the numerical noise
and that is available in IC5033. BTW, be careful pointlocal can slow
down the simulation time. Using the Noise Summary should tell
you where the noise is coming from. If you modify the circuit to reduce
the noise contribution from the noisy devices, what happens?

Sheldon

Title: Re: VCO pnoise problem
Post by Andrew Beckett on Aug 15^th, 2005, 12:37pm

The finitediff option can also help in reducing the numerical noise. relref=pointlocal can help, but usually I find it just slows the simulation down and doesn't improve the accuracy... but your mileage may vary.

Andrew.

Title: Re: VCO pnoise problem
Post by byang on Aug 15^th, 2005, 1:32pm

Another trick that may or may not help is to set maxstep. If you are lucky, maxstep can force uniform (euqally-spaced) timesteps and the numerical noise will be low.

Baolin

Title: Re: VCO pnoise problem
Post by chen on Aug 15^th, 2005, 10:07pm

Thx so much for the suggestions!

I will give them a try.