Agilent Spectrum Analysis Basics - Agilent Technologies

Amplitude calibration
So far, we have looked at how a harmonic mixing spectrum analyzer responds
to various input frequencies. What about amplitude?
The conversion loss of a mixer is a function of harmonic number, and the
loss goes up as the harmonic number goes up. This means that signals of
equal amplitude would appear at different levels on the display if they
involved different mixing modes. To preserve amplitude calibration, then,
something must be done. In Agilent spectrum analyzers, the IF gain is
changed. The increased conversion loss at higher LO harmonics causes a
loss of sensitivity just as if we had increased the input attenuator. And since
the IF gain change occurs after the conversion loss, the gain change is
reflected by a corresponding change in the displayed noise level. So we
can determine analyzer sensitivity on the harmonic-mixing ranges by noting
the average displayed noise level just as we did on fundamental mixing.
In older spectrum analyzers, the increase in displayed average noise level
with each harmonic band was very noticeable. More recent models of Agilent
spectrum analyzers use a double-balanced, image-enhanced harmonic mixer
to minimize the increased conversion loss when using higher harmonics.
Thus, the “stair step” effect on DANL has been replaced by a gentle sloping
increase with higher frequencies. This can be seen in Figure 7-9.
Figure 7-9. Rise in noise floor indicates changes in sensitivity with
changes in LO harmonic used
Phase noise
In Chapter 2, we noted that instability of an analyzer LO appears as phase
noise around signals that are displayed far enough above the noise floor.
We also noted that this phase noise can impose a limit on our ability to
measure closely spaced signals that differ in amplitude. The level of the phase
noise indicates the angular, or frequency, deviation of the LO. What happens
to phase noise when a harmonic of the LO is used in the mixing process?
Relative to fundamental mixing, phase noise (in decibels) increases by:
where
20 log(N),
N = harmonic of the LO
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