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Simulators >> Circuit Simulators >> Cadence-Calculator-psdbb
https://designers-guide.org/forum/YaBB.pl?num=1113645562

Message started by svensl on Apr 16^th, 2005, 2:59am

Title: Cadence-Calculator-psdbb
Post by svensl on Apr 16^th, 2005, 2:59am

When using the power spectral density base band function, there always seems to be an offset in magnitude. For example, while my input signal is 200mV, the psdbb function returns -38dB after taking dB10. The spectrum itself is correct, its is just that there is an offset I cannot explain. Even when I play around with the scaling factor, I cannot get the right dB value. Any thoughts or comments would be appreciated.

Thanks.

Title: Re: Cadence-Calculator-psdbb
Post by pauloau on Jun 25^th, 2009, 11:04am

I think that this has to do with the frequency resolution of the PSD and with the fact that the orignal PSD is displayed in Volts²/Hz.
I identified this by making a two-tone simulation (using envelope following analysis) and using the PSDBB function. I was expecting two tones of 5dBm (or 0.316V²) each at the output, but I got two tones of 24.7e-6V² each instead. However, when I zoomed in near the frequency of interest, I noticed that the peak was 24.7e-6V², but the frequencies nearby had also nonzero amplitudes. I decided then to use the integ function to add up all amplitudes within a certain bandwidth. First, I divided the orignal value by (2*RL), where RL is the load resistance, to get the correct power at each frequency. Afterwards, I integrated the resulting power over the desired bandwidth. By doing that, I got the expected value of 3.1mW, which is equivalent to 5dBm in a 50ohm load.

Hence, before you apply db10 or dbm, you must be sure that your system impedance is correctly taken into account and that your PSD frequency resolution is OK. The later has to do with the envelope following analysis setup (stop time, etc).