Specifications

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Thus, the “stair step” effect on DANL has been replaced by a gentle sloping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.

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:

20 log(N),

where N = harmonic of the LO

Figure 7-9. Rise in noise floor indicates changes in sensitivity with

changes in LO harmonic used