The Mixed-Measurement Model - HP 3563A Operating Manual

The Mixed-Measurement Model

For mixed measurements, it's usually necessary to model the analyzer as a sampled system in order to
design a measurement and interpret the results. In an all-analog measurement, this "modelling" was
unnecessary because the internal input anti-alias and source reconstruction filters were completely
effective at the 256 kHz internal sample clock rate. Although simple models described here are not
intended to accurately represent actual signal paths and data operations, they do provide some
insight into the effects and interactions of the various clocks and filters.
The Analog Input Model
The analog input is modeled a sampler preceded by an analog anti-alias filter as shown in figure
The sampler operates at the sample clock rate, which is the digital rate times the value of the mixed
ratio. The low pass anti-alias filter begins to roll-off at
kHz.
156 The model shows that corrections, which compensate for the effects of the anti-alias filter,
are applied after the sampling process. The corrections are applied with the assumption that the
input signal is bandlimited to one-half the sample clock rate (not necessarily 256kHz). If this
assumption is violated, then the corrections will be in error and the filter will affect the measured
results over the portion of the spectrum that is aliased.
The effects of the input anti-alias filter should be considered whenever the input signal is not
bandlimited to one-half the sample clock rate. For example, a rectangular pulse (which is not band
limited) is delayed as it passes through the filter. This changes the timing relationship between the
pulse and the sample clock. If the pulse width is less than one sample period, then the pulse may or
may not be sampled.
+
(ADC
A nt i- al ia s
filter
Sample clock
rate =
Figure 14-7. Model of a n Analog Input Channel
Digttal and Mixed Measurements
kHz and is down at least
100
StH)
C or re ct io ns
F sa
Mixed Measurements
14-7.
dB by
80
1 4-1 1