Keysight Technologies CXA N9000B, Manual

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N9069C Noise Figure Measurement Guide 31
Making Noise Figure Measurements
Making Frequency Converter Measurements
Figure 2-7 DSB System Downconverter Measurements
— If the DUT bandwidth is greater than the LSB-USB separation, a system
downconverter can operate in USB, LSB, or DSB mode, and the same
circumstances occur in both the calibration and the measurements, hence
DSB sideband power addition corrections are not needed. Corrected
measurements cancel any sideband summation effects.
— If the DUT bandwidth is less than the LSB-USB separation and a DSB
system downconverter is still used, the calibration setup will operate in DSB
mode while the measurement setup will actually operate in SSB mode,
influenced by the DUT’s selectivity. For this case, a gain correction factor is
needed due to the DSB calibration and SSB measurement. Normally, a 3 dB
after DUT loss compensation should be set by pressing Meas Setup , Loss
Comp tab, Loss , After DUT , Loss Comp Mode , Fixed , Fixed Loss, 3 , dB .
For microwave measurements, above 3.6 GHz, the analyzer's input filter will
reject LO leakage from the downconverter, otherwise a filter is needed
between the system downconverter and the analyzer. Also, considerations
about mixer LO harmonic modes apply.
Measurement Modes with an SSB System Downconverter
The analyzer can perform frequency calculations for DSB, LSB, or USB system
downconverter conversions. The filtering requirements will be
measurement-specific.
Amplitude
Analyzer Input
Band
LSB Input
USB Input
F
LO
Frequency
Broad-band Noise
F
LO
-F
LSB
F
USB
-F
LO
For this case, the prerequisites of using the DSB system downconverter is that the
noise source ENR, gain, and DUT frequency response in the LSB-USB area are
constant. To verify this, you can experiment with different IFs to see if the
frequency variation errors are a problem. If the noise figure values change
significantly with the choice of IF, then the SSB system downconverter is
recommended.