pulse average power measurement

Question asked by krebesz on Apr 27, 2009
Latest reply on Jun 7, 2010 by tabbott
Dear Agilent,

I read carefully your Application Note 150-2 and because for the first sight some parts of it was not clear I started to evaluate some equations to find the voltage law for desensitization factor and so on. Now I have some equations and everything seems to be good because they give me nearly the same results (difference is about 1dB or less) as the measurements (I use Agilent EXA N9010A) when I measure a pulse train. No ISI and the spectrum is also ok, so the RBW is set properly

I understand the pulse desensitization factor and its voltage law, the peak power measurements are also ok. But I had some problems with the average measurement and I would like to clear some things.

I found in the menu of the SA that I can set the averaging as voltage average or power average (RMS) or log power. I have the equations for the first and he second method but I would like to check if I am right with the following assumptions regarding to the operation of the SA.

I set the SA the lin scale instead of log because I used linear model for my equation. I checked the spectrum and after I set zero span to see the pulse train (actually the response of the IF filter, no video filter applied).  V_{peak}=27mV, f_{carrier}=10MHz T_{eff}=12.9us, T_{bin}=2.73ms, PRF=1/T_{bin}, RBW=3.3 kHz, VBW=1 MHz. I took into account the filter type of IF filter and its bandwidth definition (gaussian 3 dB bandwidth) in my equations. I measured -67,9 avg power and the calculation gives the same result. If I set half of the T_{eff} the power drops 6 dB. The situation is the same if I double the T_{bin}. So they follow the voltage rule and the RBW has no effect until I keep the rules (RBW<0.1/T_{eff}, etc.). It confirms my equation P_{voltage avg}=V^2_{peak}/2/Z_0*(T_{eff}/T_{bin})^2. I suppose that the SA first average the envelope of the voltage (peak detector) then square it and divide it by 2 (to get RMS) and by Z_0 to get power. When I measured the average I set video BW to 1 Hz.
Am I right?

In case of average power (RMS) mode I suppose the SA square the voltage and average it, divide it by 2 and by Z_0. Am I right? For this case I have P_{power ang}= V^2_{peak}/2/Z_0*(RBW)*T^2_{eff}/T_{bin}. Power law for RBW and 1/T_{bin} and voltage law for T_{eff}.  The measurement results and calculations are the same and confirm the equations. When I measured the average I set video BW to 1 Hz.

Your notes on page 8 says P_{avg}=P_{peak}*T_{eff}/T_{bin}. That is something else that I cannot understand.

Tom