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N8975A input loss correction table not working as expected

Question asked by malibu on Apr 25, 2013
Latest reply on Jul 31, 2013 by malibu
The input loss correction functions, Fixed, Table, Temp are not giving me expected results.  I am testing a 16Ghz 16db gain amplifier with expected noise figure of 1.7db.  I am using an input loss table to correct for cable and probe losses.  The input loss at 16Ghz is 1.2db.  The output loss table entry is about 0.5db.  The cable connecting the output probe to the NFA was included in the user Calibration.  The meter measures gain of 16db which matched my s-parameter measurement.  The noise figure comes out at 2.3db. A figure that is 1/2 of the input loss above what is expected.  If I turn off the input loss correction the noise figure increases by 0.6db to 2.9db and the gain decreases by 1.2db.  The gain decrease is as expected but the noise should have gone up! by 1.2db NOT 0.6db!  I tried another self consistency check and put in a 3db pad between the noise source and my amp.  Leaving the input loss compensation table on.  I see the Noise Figure go up by 3db to 5.3db and the gain go down my 3db as expected.  Now I add 3db to the values in my input loss compensation table. 

The gain is correctly computed to be 16db but the noise is 3.8db... only 1.5db is removed from the computed noise figure!

Why is the noise figure only being corrected by 1/2 the input loss that I put in to the loss compensation system?

If I use the old school method of correcting for additional loss by reducing the ENR value by 1.2db I get both gain and noise figure numbers I expect!

Edited by: malibu on Apr 25, 2013 10:55 PM  

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