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Noise figure and gain calibration traces for a DownConverter

Question asked by sergbrus on Jun 17, 2014
Latest reply on Jun 23, 2014 by tabbott
Dear Colleagues,

Can anybody explain the following issue?

I am measuring an active downconverter on a PSA options 1DS, 219. 
The downconverter parameters are as follows:
LO fixed at 7028 MHz
RF is swept 6995MHz-7005MHz
IF is swept 33MHz-23MHz (centered around 28MHz)
RF image channel is supppressed by 40dB
LO leakage is suppressed by 50dB and is further attenuated by PSA low pass filter

Now the question:
Immeadiately after the calibration I get the noise figure trace offset by -0.78dB and the gain trace offset by +0.74dB. Is this a correct system behaviour? or will I get erroneous results during the measurement. What can cause this offset?

A qualified reply would be greatly appreciated!