how to display vrms and dBm values by using spectrum analyzer

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I am trying to generate random noise with 5uvrms and frequency 100khz after that conneccted to second order lowpass filter with cutoff 10khz .
so i selected random source block with these values ----->variance= 25e-12 sample time 1e-5 and connected to spectrum analyzer
rms spectrum analyzer parameter ------> sample rate= inherited , RBW =auto, span=full . results------>106dBm
power spectrum analyzer parameter ------> sample rate= inherited , RBW =auto, span=full .results------>16e-8 vrms
when i customize the spectrum parameter to:
rms spectrum analyzer parameter ------> sample rate= 4*100k , RBW =auto, span=0-20k . results------> -117dBm
power spectrum analyzer parameter ------> sample rate= 4*100k , RBW =auto, span=0-20k .results------> 5e-8 vrms
so i would like to know what is the right way to display the signal by using spectrum analyzer in dBm & Vrms and how to calculated.
the way to i used to calculate dBm mathematically
vrms=sqrt(kTRf)
find R=25e-12/(4*300*1.38e-23*100e3)
R=15096.6
find P (dBm)=10*log10(vrms^2/(R*0.001))= -117.8
i hope to help me how to use spectrum analyzer to display right result in dBm and vrms and how to calculated in dBm mathematically
and if its possible i wanna tested after lowpass filter with second order and cut off 10khz

Answers (1)

UDAYA PEDDIRAJU
UDAYA PEDDIRAJU on 31 Dec 2024
Hey ANAS,
Your calculation are theoritically correct. For estimating power and other parameters using spectrum analyser you should definitely consider referring to https://www.mathworks.com/help/dsp/ug/estimate-the-power-spectrum-in-matlab.html#:~:text=Spectrum%20Analyzer%20toolstrip.-,Convert%20the%20Power%20in%20dBm%20to%20RMS%20in%20Vrms,-Power%20in%20dBm and ofcourse you can test the input signal with low pass filter as per your model.
Let me know if you're able to understand.
Happy New Year!

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