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Power measurements with the PNA 8364B

Question asked by mjzee on Nov 15, 2011
Latest reply on Nov 16, 2011 by mjzee
I have an 8364B PNA (options 010, UNL, and 014)

I am wondering if there are any special considerations I need to account for when using the receivers to measure absolute power.

My specific application is I'm measuring a compressed T/R device with output power approaching 20dBm over an X-band range.  Typically we do this test with a power meter and it is very slow.  I wrote a script to measure power on "R1, 2" over frequency.  For what it's worth, I originally used receiver A, but the receiver seemed to be compressing.

The algorithm I use is as follows:

Note: I don't have a supported power meter to do the automated receiver cal unfortunately

1. Perform source calibration over fequency on port 2
2. Upload source calibration data
3. Measure R1, 2 measurement using thru, and record offset from source power.
4. Insert DUT and measure R1, 2 and apply loss values

This alogrithm is dead accurate when I use a thru as the DUT, but I notice a disagreement between my slow, direct power meter measurement and this one.  The PNA verison seems to be about a dB lower.

Now I'm thinking that the original measurement using the power sensor is summing the entire spectrum, and the PNA is discriminating over frequency, but when I look at the spectrum on the spectrum analyzer, I don't see enough harmonic/spurious content to account for a full dB extra of power as everything in the spectrum is at least 20dB down even in compression, so my question is if it is possible that the PNA receiver might have a soft compression curve?  Or is it very linear?  If it is not as linear as I assumed, is there a way to acount for any nonlinearity?  Where should I expect to start seeing compression on the A and B and R1 and R2 receivers?

Also, I should note that my original PNA power measurement algorithm took the measrement at 300Hz bandwidth.  When I widened that out a bit to 3kHz, I noticed the measurement difference shrunk.  In general, would a power measurement such as this be more accurate using the widest bandwidth possible and applying averaging? 

Basically any information that helps me take a more acurate measurement would be greatly appreciated.