Specifications

Pluses and minuses of preselection

We have seen the pluses of preselection: simpler analyzer operation,

uncluttered displays, improved dynamic range, and wide spans. But there

are some minuses, relative to an unpreselected analyzer, as well.

First of all, the preselector has insertion loss, typically 6 to 8 dB. This loss

comes prior to the first stage of gain, so system sensitivity is degraded by the

full loss. In addition, when a preselector is connected directly to a mixer, the

interaction of the mismatch of the preselector with that of the input mixer

can cause a degradation of frequency response. Proper calibration techniques

must be used to compensate for this ripple. Another approach to minimize

this interaction would be to insert a matching pad (fixed attenuator) or

isolator between the preselector and mixer. In this case, sensitivity would

be degraded by the full value of the pad or isolator.

Some spectrum analyzer architectures eliminate the need for the matching

pad or isolator. As the electrical length between the preselector and mixer

increases, the rate of change of phase of the reflected and re-reflected signals

becomes more rapid for a given change in input frequency. The result is a

more exaggerated ripple effect on flatness. Architectures such as those used

in the ESA Series and PSA Series include the mixer diodes as an integral

part of the preselector/mixer assembly. In such an assembly, there is minimal

electrical length between the preselector and mixer. This architecture thus

removes the ripple effect on frequency response and improves sensitivity by

eliminating the matching pad or isolator.

Even aside from its interaction with the mixer, a preselector causes some

degradation of frequency response. The preselector filter pass band is

never perfectly flat, but rather exhibits a certain amount of ripple. In most

configurations, the tuning ramp for the preselector and local oscillator come

from the same source, but there is no feedback mechanism to ensure that

the preselector exactly tracks the tuning of the analyzer. Another source

of post-tuning drift is the self-heating caused by current flowing in the

preselector circuitry. The center of the preselector passband will depend

on its temperature and temperature gradients. These will depend on the

history of the preselector tuning. As a result, the best flatness is obtained

by centering the preselector at each signal. The centering function is typically

built into the spectrum analyzer firmware and selected either by a front panel

key in manual measurement applications, or programmatically in automated

test systems. When activated, the centering function adjusts the preselector

tuning DAC to center the preselector pass band on the signal. The frequency

response specification for most microwave analyzers only applies after

centering the preselector, and it is generally a best practice to perform this

function (to mitigate the effects of post-tuning drift) before making amplitude

measurements of microwave signals.