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N9069C Noise Figure Measurement Guide 57
Measurement Related Tasks and Concepts
Setting the Measurement System
Effect of high level LO spurious signals and noise on mixer measurements
with low L-to-I rejection.
The spurious level of the LO also has to be low. At frequencies where there is a
high spurious signal, the noise figure measured will have a peak at that IF. For
example, ideally the LO’s noise, including spurious, needs to be below –90
dBm. If a mixer has higher isolation, then the noise of the LO can be higher
since the mixer will be better able to reject the LO’s noise.
This is especially necessary if the mixer has a poor balance, or LO to IF
isolation. With low isolation, the mixer is more likely to pass the LO noise
through and thus increase the measured noise figure.
Selecting a Local Oscillator
Here are several criteria that must be met when choosing the LO:
— It should have a frequency appropriate to the DUT’s frequency range, IF
range, and sideband chosen.
— It should have sufficient power to drive mixers (typically, +7 dBm).
— It should have excellent frequency accuracy and repeatability (typically, the
same as the analyzer you are using.)
The last point, frequency accuracy, deserves further comment. There are three
frequency-dependent components in a noise figure measurement that must all
be aligned to make an accurate measurement at the IF. The need for frequency
accuracy is the main reason for recommending a synthesized source for the
LO, such as a Keysight E8257D Analog Signal Generator with Option UNX.
Other LOs may be used, but should be tested to determine that their noise is
sufficiently low, as LO noise can cause an increase in noise figure for the
mixer/LO combination, and calibration of the system may not be possible. A
broad-band, high gain amplifier at the LO output usually generates
unacceptable noise. This is almost always the case when a heterodyne-type
sweep oscillator or signal generator is used.
L-to-I rejection is the mixer’s ability to reject the fundamental, harmonics
and spurious signals of the LO, and not allow them to pass through to the
IF output.
Measurement requirements will dictate what type of signal generator is
needed.