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Jacki

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Posts: 237

How does Spectrum in Spectre calculate the SNR?
Oct 10^th, 2014, 5:54am

Hello,

� �I am comparing the results between measurement and simulation. In Spectre, it is possible to plot the SNR directly with the function "Spectrum", but I don't understand how the "Spectrum" calculates the noise if the noise floor/rtHz is -136dB. How much bandwidth should be considered in the noise integration? I get the measured signal power and distortion very closed to simulation, just have one more step left now.
� �By the way, the power spectral density in my measurement from spectrum analyzer is -128dBm/Hz (RBW=1kHz), and the power spectral density (noise floor) of spectrum analyzer is -144dBm/Hz, does it mean my actual noise floor is -158dBm/Hz since there is 30dB noise figure in the spectrum analyzer?
� �Thank you.
� �-Jacki

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Jacki Senior Member Offline Posts: 237	Re: How does Spectrum in Spectre calculate the SNR? Reply #1 - Oct 10^th, 2014, 7:10am By the way, I don't understand this noise floor/rtHz -136dB. Can anybody explain it? Normally we use nV/rtHz, how to convert it to dB (how much the resistor value should be referred? 50Ohm?)?
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Jacki Senior Member Offline Posts: 237	Re: How does Spectrum in Spectre calculate the SNR? Reply #2 - Oct 10^th, 2014, 8:13am Hi, By comparing the simulation and measurement, I find the noise figure in the spectrum analyzer is not important, I don't need to subtract it when I do the SNR calculation. -Jacki
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boe Community Fellow Offline Posts: 615	Re: How does Spectrum in Spectre calculate the SNR? Reply #3 - Oct 13^th, 2014, 3:53am Jacki, did you read up on SNR? - B O E
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Jacki Senior Member Offline Posts: 237	Re: How does Spectrum in Spectre calculate the SNR? Reply #4 - Oct 16^th, 2014, 2:03am Hello B O E, I try to find the user guide, but I am not able to find the new version which includes the description of this part. So I don't read any documents about the SNR calculation in Spectre, but I guess the integration window could be from 0Hz to fs/2. -Jacki
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sheldon

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Posts: 751

Re: How does Spectrum in Spectre calculate the SNR?
Reply #5 - Oct 17^th, 2014, 6:04am

Jacki,

You have not explained a lot about what you are doing or at least I
am missing the point. I think that the noise floor is indeed the total
noise divided by the Nyquist frequency, fsample/2, expressed in dB.
In the case of a data converter, the noise floor should be determined
by the quantization noise, unless you are doing a transient noise
analysis. So there is not much information that you can extract from
this results, that is, you can just directly calculate the noise floor
from the quantization noise and Nyquist frequency. No need to
simulate.

If you are really trying to simulating the noise floor, then you
probably should re-consider your approach:
1) You need to be using transient noise analysis
2) You should use the power spectral density function, psd
3) Convert from power to voltage
4) Use the IC616 spectrum function
In IC616, the SNR can be directly calculated from the frequency
spectrum. So in IC616, you can calculate the SINAD/SNR/...
from the Fourier Integral or psd results.

Also be aware that the transient simulation time needs to be
much longer for power spectral analysis.

Note:
Spectrum is a Calculator function, not a Spectre function, and it
has been upgraded in IC616.

Finally, you need to consider the transient simulation accuracy.
Numerical noise may limit the noise floor unless you set the
error preset appropriately.

Sheldon

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Jacki

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Posts: 237

Re: How does Spectrum in Spectre calculate the SNR?
Reply #6 - Oct 26^th, 2014, 8:39am

Hello Sheldon,

Thank you very much for your so detailed information. I didn't take care the noise for almost two years, and forget some basic knowledge like the relation between V/sqrt(Hz) and dBm, and now the knowledge is back again. Sorry to response you quite late since I am busy with measurement these day.
Jacki.

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weber8722

Community Member

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Posts: 95

Re: How does Spectrum in Spectre calculate the SNR?
Reply #7 - Feb 27^th, 2015, 11:10am

sheldon wrote on Oct 17^th, 2014, 6:04am:

Jacki,

� You have not explained a lot about what you are doing or at least I
am missing the point. I think that the noise floor is indeed the total
noise divided by the Nyquist frequency, fsample/2, expressed in dB.
In the case of a data converter, the noise floor should be determined
by the quantization noise, unless you are doing a transient noise
analysis. So there is not much information that you can extract from
this results, that is, you can just directly calculate the noise floor
from the quantization noise and Nyquist frequency. No need to
simulate.

� If you are really trying to simulating the noise floor, then you
probably should re-consider your approach:
1) You need to be using transient noise analysis
2) You should use the power spectral density function, psd
3) Convert from power to voltage
4) Use the IC616 spectrum function
� �In IC616, the SNR can be directly calculated from the frequency
� �spectrum. So in IC616, you can calculate the SINAD/SNR/...
� �from the Fourier Integral or psd results.

� �Also be aware that the transient simulation time needs to be
much longer for power spectral analysis.

Note:
Spectrum is a Calculator function, not a Spectre function, and it
has been upgraded in IC616.

� Finally, you need to consider the transient simulation accuracy.
Numerical noise may limit the noise floor unless you set the
error preset appropriately.

� �Sheldon

Hi, what I wonder a bit is how the Viva spectrum assistant is really working behind the scenery? E.g. is SNR only taking noise into account or also harmonic distortion, actually calculation SNDR? And e.g. in which freq ranges?
How is psd doing the averaging in detail? I played with some parameters like "smooth" but see no impact!

Bye Stephan

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sheldon Community Fellow Offline Posts: 751	Re: How does Spectrum in Spectre calculate the SNR? Reply #8 - Feb 28^th, 2015, 3:46am Stephan, If you don't define the number of harmonics to be considered as distortion, then the distortion is included with the noise. Sheldon
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