Agilent Spectrum Analysis Basics - Agilent Technologies

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.
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