VCO phase noise modeling using a transient noise simulation

Dear asrf,

asrf said:

however when I run transient noise simulation and use PSD calculator I dont get a proper result

Achieving reasonable correlation in phase noise between a transient noise simulation and pss/pnoise simulation is absolutely possible. However, it requires a significant amount of care in your choice of transient noise simulation settings, PSD analysis, and pss/pnoise simulator settings. The amount of information your Forum post provides does not include enough for me to give you concrete suggestions as your PSD GUI settings are not relevant without knowing the details of your transient noise and pss/pnoise analysis.

However, I might suggest you study a similar question and the notes I assembled for each in the Forum posts at URL:

https://community.cadence.com/cadence_technology_forums/f/rf-design/51484/comparing-transient-noise-pnoise-and-pnoise-with-lorentian-approximation-of-a-ring-oscillator

There is a link to a set of notes that details considerations for comparing phase noise results for a transient noise simulation and pss/pnoise simulations.

https://www.dropbox.com/s/5zalilay6sqa06b/phase_noise_response_081422v1p0.pdf?dl=0 

https://community.cadence.com/cadence_technology_forums/f/custom-ic-design/51696/snr-calculation-wrong-by-3-db/1383427?focus=true

There is a link in this post to a set of notes describing some subtleties of using the Spectrum Assistant it produce PSD results from a transient noise simulation and a means to set your transient noise simulation parameters to produce valid Spectrum Assistant PSD results.

Shawn

Thank you for your time and effort.

These are my transient noise simulation setup

Thank you for the links you have provided. I will study them.

Some of these links are about phase noise. Here I am basically looking at voltage noise. (or integrated voltage noise)

I am not running pss/pnoise since my circuit has verilogA code and it does not work. 

I use noise simulation to check Vnoise of diode connected transistor

Dear asrf,

I think my response was not sufficiently clear and I apologize for not being more precise!

asrf said:

Its actual simulation data:

I defined a noise source and ran noise simulation:

My apologies! I thought you were simulating the small-signal noise of your diode connected transistor as you noted in your initial Forum post:

asrf said:

I started with transient noise simulation of diode connected transistor voltage (Vn or Vnoise)

I now understand  you are now simulating a component with a user supplied noise function.

asrf said:

My BWres is around 1k so at 100k I have 100 freq points isnt that enough ?

I should have been more clear. I was referring to the frequencies between 1 kHz and 10 kHz where there are only ten points.

asrf said:

Yes, I can run at least 3 simulation and take average PSD (My next plan)

Great! I would consider running more than 3 if that is possible.

asrf said:

If I dont take moving average how am I supposed to see the exact PSD number ?

As you noted above, you take the average of the PSD results (in linear power domain) at a given frequency. Actual spectrum and phase noise analyzers take multiple measurements and average the results to reduce the variance of the measurement (for example, the phase noise analyzer E5052A). 

Taking a moving average applies a first-order lowpass filter over a range of frequencies and really does not have a theoretical basis for its use in this application.

Finally, with respect to our statistical discussion and your final figure in this response, I noted you elected to choose two maximum values (shown as +8 dB and +10 dB) suggesting the PSD result is far in excess of your expected response. However, you could just as well choose the two minimum values suggesting the response was far below your expected response. This is why the use of multiple transient noise analyses is recommended to reduce the statistical noise to a level where the the average provides a reasonable estimate of performance.

There are other ways to attack this issue that avoid the need to perform a lot of transient noise analyses to estimate the average noise response, but I think their discussion is a bit too off topic.

Shawn

Dear asrf,

My response was just flagged as spam again...hopefully it will be allowed to post.

Shawn

Dear asrf.

asrf said:

This is with 3 tran-noise simulations and taking average 

Are you averaging the linear data and not the logarithmic data?

Shawn

Thank you for your explanations:

yes in linear domain

10*log10((psd(leafValue( vtime('tran "/VNOISE") "FNOISE_MAX" 5e+09 "Iteration" 1 )  0.0 1.342m 67108864 ?windowName "Rectangular" ?windowSize 67108864)+psd(leafValue( vtime('tran "/VNOISE") "FNOISE_MAX" 5e+09 "Iteration" 2 )  0.0 1.342m 67108864 ?windowName "Rectangular" ?windowSize 67108864)+psd(leafValue( vtime('tran "/VNOISE") "FNOISE_MAX" 5e+09 "Iteration" 3 )  0.0 1.342m 67108864 ?windowName "Rectangular" ?windowSize 67108864))/3)

What if it is because my ResBw is a weird number ?  745.1 ?

This is out of control though we cannot make it a round number unless NFFT is not equal to 2^N

Dear asrf,

I studied your expressions and that is exactly the type of averaging I was suggesting - great!

asrf said:

What if it is because my ResBw is a weird number ?  745.1 ?

The only time the frequency bin width is considered important in a DFT/FFT/PSD analysis is if you are applying discrete tones at some frequency and are interesting in preventing leakage of the spectral components of the tone into adjacent frequency bins. In that case, the sampling rate relative to the frequency of the tone of interest and size of the DFT/FFT/PSD can be chosen to prevent spectral leakage using a conventional rectangular window. In your case, your noise is a wideband signal and one cannot choose a sample rate and DFT/FFT/PSD size to accommodate all the frequencies in your signal.

What I might suggest is you reduce your window size. At present, my understanding of your choice of window size is the length of your DFT (2^26). This will provide a maximum frequency resolution, but also maximum statistical error. If you reduce this value, multiple DFT are performed which will reduce the statistical noise.

Shawn

Dear ShawnLogan,

Thank you for your guidance.

I ran 5 simulations and took average. I reduced my window size this time.

It looks like I have less error this time. However, there is error below 1MHz. I think this is because we reduce resolution.

There is a trade off here unfortunately. 

Another strange problem is the integrator output noise is totally wrong !

Noise simulation results are precisely correct  but tran-noise is totally wrong. It does not even capture 1/f^3 region

Dear arf,

asrf said:

I ran 5 simulations and took average. I reduced my window size this time.

Very good and thank you for continuing your work and letting me know your results!

asrf said:

It looks like I have less error this time. However, there is error below 1MHz. I think this is because we reduce resolution.

It looks as if you made another change in that you are using a Blackman window. This impacts the gain of the PSD. The rectangular window you were using has unity gain and the Blackamn window has a gain of about 0.50. Hence, unless you accounted for the difference in gain, you need to scale your PSD results for direct comparison to small-signal noise results.

I don't think the difference in your noise below 1 MHz is due to the smaller frequency bin width. I think you need to re-examine the scaling factor.

Shawn

Dear arf,

asrf said:

I ran 5 simulations and took average. I reduced my window size this time.

Very good and thank you for continuing your work and letting me know your results!

asrf said:

It looks like I have less error this time. However, there is error below 1MHz. I think this is because we reduce resolution.

It looks as if you made another change in that you are using a Blackman window. This impacts the gain of the PSD. The rectangular window you were using has unity gain and the Blackamn window has a gain of about 0.50. Hence, unless you accounted for the difference in gain, you need to scale your PSD results for direct comparison to small-signal noise results.

I don't think the difference in your noise below 1 MHz is due to the smaller frequency bin width. I think you need to re-examine the scaling factor.

Shawn

ShawnLogan said:

It looks as if you made another change in that you are using a Blackman window. This impacts the gain of the PSD. The rectangular window you were using has unity gain and the Blackamn window has a gain of about 0.50. Hence, unless you accounted for the difference in gain, you need to scale your PSD results for direct comparison to small-signal noise results

Thank you for your valuable advice.

This is with rectangular window 2^23 window size 

It seems like its shifter by 3 dB. What if noise simulation shows a double side noise spectrum and transient returns single side ? (2*Power)