VCO phase noise modeling using a transient noise simulation

This is with 3 tran-noise simulations and taking average 

Fnoise - max is 5GHz here

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