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SNR calculation wrong by 3 dB?

I did the following experiment to check the accuracy of the Spectrum assistant.

Simple VSIN with 1V peak amplitude series-connected to a 100kOhm resistor and done fft of the output after activating the transient noise up to 100kHz to avoid aliasing. Strobeperiod=1/24MHz was added. Sine frequency was chosen as 3*24e6/8192 to obtain a perfect fit of the signal on fft samples. Number of points is 8192. Hanging window selected. Number of signal samples = 1 as per manual.

My calculation of SNR over a 100kHz bandwidth gives: rms of signal = 0.707V, and rms of noise = sqrt(4*kT*R*100k) = 12.87uV. Hence, SNR = 20*log10(0.707/12.87uV) = 94.7 dB

However, the Spectrum Assistant gives around 3 dB better, i.e. 97.2 dB. It also reports signal power 1e-5 dB so 0 dB. We obtain close to this number if we do 1/12.87e-6.

So it appears to me that SNR function takes signal amplitude instead of signal rms. Where is the flaw in my reasoning if any?

Thanks, best regards.

ShawnLogan over 3 years ago in reply to Fede26

DearFede26,

Fede26 said:
In the end, the problem is related to the choice of fmax in Cadence transient noise analysis. Due to the noise generation algorithm, noise is attenuated by 3dB at fmax.

Yes -This is indeed the case. Thank you for letting me know of your findings! In fact, I just posted a note where this factor is quite obvious. The note is at URL:

https://www.dropbox.com/s/7vxzliml4uz6518/psd_study_sml_090622v1p0.pdf?dl=0

In case you are interested, and to save you some time, the PSD of a resistor is shown in Figure 1 computed ffom a transient noise simulation where the sample size is varied. For a large enough sample size (higher sample rate), the noisefmax chosen of 100 GHz results in the thermal noise being attenuated. The relevant figure from page 14 of the note is shown below.

Thank you again for your update Fede26!

Shawn

Figure 1

from page 14 of URL:https://www.dropbox.com/s/7vxzliml4uz6518/psd_study_sml_090622v1p0.pdf?dl=0
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