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Signal power calc. form spectrum vs time signal

Question asked by WSTR on Jan 16, 2005
Latest reply on Jan 20, 2005 by WSTR
I am interested in the exact signal power of a time signal. Hence I set up a QPSK source with a timed sink (50 Ohm) and a spectrum analyser in parallel at the output.
First I calculate the signal power in the time domain (which should lead to the exact value) from the rms value of the signal (T_V). ->
Ueff_V = sqrt(sum(mag(T_V)**2)/numberofsimpoints)
P_W = Ueff_V**2/50
P_dBm = 10*log10(P_V/1e-3)
This leads to a value which is 2.8 dB above the nominal power level of the QPSK modulator of 0 dBm. Using spec_power() on the output of the SpecAnalyser compontent leads as well to a value of 0 dBm.
What is wrong here? Any idea? Please let me know.

Thank you Wolfram???