how to display vrms and dBm values by using spectrum analyzer

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

Answer 1

[UDAYA PEDDIRAJU]

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!