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Log averaging and RMS averaging

Question asked by Leyo on Feb 24, 2013
Latest reply on Feb 27, 2013 by tabbott
This is regarding IM  measurements  in dBC using PXA spectrum analyzer mode for 6 carriers GMSK modulation. The detector selected is "Average". I am comparing the IM (in dBc) between the Average types of "Log_Power"  and "Power". Trace averaging is selected with count of 100. 
When the average type is "Power",  RMS value of all the samples in a bucket is  calculated and displayed in dBm ( My  dsiplay scale is dBm). Since trace averaging with 100 is activated, the bucket values in the sweeps are averaged in  W , converted to dBm and displayed as final trace (I am assuming single sweep). 

When the average type is "Log_Power",  average of  log of all samples in the bucket is calculated  and displayed in dBm.Since trace averaging with 100 is activated, the bucket values in the sweeps are averaged in  log mode  , converted to dBm and displayed as final trace (I am assuming single sweep).  Here the numbers displayed will be around 2.5 dB less than the real numbers (The numbers displayed with "Power" mode will be the real numbers). 

So, whenever I change sthe  average type for "Log_Power" to "Power_RMS", both carriers and IM products should chnage by 2.5dB and the IM in dBc should not change.  But in my case, the  carrier amplitudes are changing by 2dB and the IM products by 2.9 dB  and the IM dBc is degrading by .9 dB. 
Following are the numbers I am getting==> 

                              Carrier                  IM                      IM (DBC) 

Log_Power               -6.33 dBm           -77.91 dBm            -71.58 

Power_RMS              -4.32 dBm         -75.04 dBm              -70.72 

Why the carrier and IM products are behaving differently with different average type?. The IM products are close to PXA Noise Floor. Is this contributing?  

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