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Entry  Fri Apr 8 01:05:45 2011, Suresh, Update, RF System, RF Source Harmonics RF_Source_Harmonics_Sheet1.pdf
    Reply  Fri Apr 22 09:05:53 2011, Suresh, Update, RF System, RF Source Harmonics 
       Reply  Fri Apr 22 10:28:22 2011, rana, Update, RF System, RF Source Harmonics 
          Reply  Wed May 4 15:39:32 2011, Suresh, Update, RF System, RF Source Harmonics RF_Harmonics_Sheet1.pdf
Message ID: 4628     Entry time: Wed May 4 15:39:32 2011     In reply to: 4559
Author: Suresh 
Type: Update 
Category: RF System 
Subject: RF Source Harmonics 

I have measured the RF source harmonics in dBm using the HP 8591E spectrum analyser. There is a small discrepancy (< 1 dBm) in the value of RF power shown by the power meter and the HP8591E. This is probably due to the loss of calibration over time.

Initial problem I faced was that when we try to measure the weak harmonics, many below -50dBm we have to choose a small band as advised by Rana. However due to the large amplitide of the fundamental typically around 15dBm or so, the preamp on the spectrum analyser becomes saturated and nonlinear. This gives rise spurious harmonics not present in the source but are rather an aritifact of measurement. The power in harmonics to avoid this I used filters to selectively attenuate the fundamental component (11 or 55 MHz) and then measure the weak harmonics.

However the filters proved difficult to use, because over their stop-band they do not have an input impedance of 50 Ohm. As a result they produce unreliable power measurements for those frequency components which are within the stop band.

To get around this problem I used a suitable attenuator so that even when the internal attenuation is decreased the preamp does not saturate

All the measurements are recorded in the attached document. Pages 4 and 5 give the reliable measurements with the attenuator.

1) At times we can see the 29.5 MHz component reflected back from the triple resonant EOM driver.
2) In the 29.5 MHz source output there is a forest of peaks around 100 MHz, which disappear after passing through the AM stabiliser. This suggests that they are associated with AM modulation and have been removed by the stabilizer. But I did not check this further.

You should be able tosd resolve the other harmonics by decreasing the IF BW or RBW on the analyzer. Even though
they're OK, its useful to have the final measurement of all of them in some kinds of physical units (like dBm, but
not dBm/Hz or dB or dBcubits).
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