Agilent Technologies E1437A, Руководство пользователя, Страница: 40 / 214

Frequency and Filtering
The E1437’s center frequency is normally set at zero (baseband measurement).
However, you may set the center frequency to a non-zero value in order to examine
a narrower span away from baseband (zoom measurement). The frequency band of
interest, represented by digitized time data samples from the ADC, is mixed with
the E1437 digital LO, a complex exponential, at the desired center frequency. As a
result the frequency band of interest in the input signal is shifted to a complex
signal centered around DC. See Synchronizing Changes in Multi-module Systems for
special considerations with respect to changing the center frequency in
multi-module systems.
The default filter for E1437 measurements is an analog anti-alias filter. However,
you may further isolate the frequency band of interest for more detailed analysis by
using digital filtering. A decimating digital filter simultaneously decreases the
bandwidth of the signal and decreases the sample rate. The built-in digital filters
conform to the Nyquist sampling theorem which guarantees that the output sample
rate may be reduced by th e same factor as th e signal bandwidth reduction wh ile still
maintaining a complete representation of the underlying bandlimited signal.
For each octave step in bandwidth reduction (except for the first octave) the E1437
digital filters automatically reduce the data rate by discarding alternate output
samples. This process, called decimation, results in an output sample rate which is
nominally four times the signal bandwidth whenever
sigBw >0. This is still double
the theoretical rate necessary to fully characterize the band limited signal. However,
because the digital filters do not have a perfectly abrupt cutoff, the sample rate
cannot be reduced to the theoretical limit without some aliasing of signals in the
transition frequency band of the filters. In many applications this limited aliasing
potential is not important. For this reason you may optionally choose to apply a final
factor-of-two decimation. See the Technical Specifications for detailed information
on the digital filter shapes.
The decimation process used to reduce the output sample rate is driven from a
“decimation counter” which keeps track of which samples to save and which ones to
discard for eachof the octave bandwidthreduction filter stages. In multi-module
systems where synchronous sampling is required, the decimation counters in all the
modules must be synchronous with each other. See Synchronizing Changes in
Multi-module Systems.
E1437A User's Guide
Using the E1437A
3-7
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