The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Simulators >> System Simulators >> MATLAB DFT vs Cadence DFT https://designers-guide.org/forum/YaBB.pl?num=1351060146 Message started by Sharath Raju on Oct 23rd, 2012, 11:29pm

 Title: Re: MATLAB DFT vs Cadence DFT Post by Ken Kundert on Oct 24th, 2012, 4:57pm 1 Vp corresponds to 30dBm? Is your reference impedance 500mΩ?

 Title: Re: MATLAB DFT vs Cadence DFT Post by Sharath Raju on 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

 Title: Re: MATLAB DFT vs Cadence DFT Post by Ken Kundert on 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

 Title: Re: MATLAB DFT vs Cadence DFT Post by rfidea on 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?

 Title: Re: MATLAB DFT vs Cadence DFT Post by Sharath Raju on 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

 Title: Re: MATLAB DFT vs Cadence DFT Post by Ken Kundert on 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