The problem of an unsymmetrical noise distribution around the LO is more subtle. The noise figure of a mixer proceeded by broadband RF gain is degraded by converted image-frequency noise. Normally 3 dB is added to the measured noise figure of broadband mixers to account for image frequency noise; however, this may not be correct. If the image frequency gain of the RF amplifier is greater than the signal frequency gain (often the case when the signal frequency is above the image frequency), then the additional noise contribution due to image response may be higher than 3 dB — typically 4 dB or more.
Figure 6. The effect of sloping gain on noise
With no tuned circuits between the noise source used for noise testing and the device under test, there is no way to calibrate out this error. However, by measuring the gain at both the signal and image frequencies, the proper correction factor can be calculated and can be used to improve the accuracy of the measurment:
NFSSB(dB) = NFDSB(dB) + 10*log(1+GI/Gs)
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