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Jun 14th, 2024, 6:05am
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 Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley Re: MATLAB DFT vs Cadence DFT Reply #1 - Oct 24th, 2012, 4:57pm   1 Vp corresponds to 30dBm? Is your reference impedance 500mΩ? Back to top IP Logged
 Sharath Raju Junior Member Offline Posts: 15 Re: MATLAB DFT vs Cadence DFT Reply #2 - Oct 24th, 2012, 6:26pm   Hi Ken,Thanks for the reply. The load impedance in my testbench is 1 kilo-ohm. I changed it to 50 ohm, bwut didn't see any difference in the power spectrum. (The peak tone continues to have a magnitude of 30 dBm and the noise floor is approximately -125 dBm).Snapshot of testbench: https://www.dropbox.com/s/lqkaieyf2kc152o/Power_Spectrum_Cadence_Testbench.pngThanks,Sharath Back to top IP Logged
 Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley Re: MATLAB DFT vs Cadence DFT Reply #3 - Oct 24th, 2012, 11:28pm   1 Vp with a 1kΩ reference resistance is -3 dBm. So your first step is to understand why both approaches are giving you the wrong power at the fundamental frequency.-Ken Back to top IP Logged
 rfidea Senior Member Offline Posts: 159 Europe Re: MATLAB DFT vs Cadence DFT Reply #4 - Oct 25th, 2012, 9:42am   What is causing your dft plots to be in dBm? You are talking about volts in the text. How is the conversion done? Back to top IP Logged
 Sharath Raju Junior Member Offline Posts: 15 Re: MATLAB DFT vs Cadence DFT Reply #5 - Oct 25th, 2012, 11:00am   The conversion to dBm is done as follows:Power in dBm = 20*log10(abs(Xk)) + 30where,Xk = FFT of xx = voltage input.The factor 30 causes the power to be in dBm. Otherwise, it would be in dB.Thanks Back to top IP Logged
 Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley Re: MATLAB DFT vs Cadence DFT Reply #6 - Oct 25th, 2012, 11:54am   dBm is decibels relative to 1mW. To convert voltage to dBm you need to know the load resistance, which is generally referred to as the reference resistance. The formula isdBm = 30 + 10 log(Vp2/(2R))where the voltage is assumed to be sinusoidal with a peak value of Vp. R is the reference resistance.I don't know how you are calculating dBm in Matlab (because even with your equation 1Vp does not correspond to 20 dBm), but I am pretty sure that ADE would be using the formula I gave above, but you would have to specify a value for the reference resistance.-Ken Back to top IP Logged
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