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User rate mode
The user rate setting allows you to directly control the digitizer’s sample rate. In
this mode you select the digitizer sample rate to be used. The actual sample rate
selected will be the highest rate that is less than or equal to the “user sample
rate” setting. By controlling the digitizer’s sample rate you can directly infl uence
the update rate of the VSA. This is because sample rate directly affects the num-
ber of points the digitizer must transfer to the PC. Certain measurements can be
made signifi cantly faster by limiting the digitizer sample rate due to reduced data
transfer requirements.
There are two types of signal aliasing to be concerned with when using the
89600 VSA application with an Agilent Acqiris digitizer. The fi rst is aliasing of
unwanted out-of-band signals into the measurement span. This will not be a
problem with the majority of clean, single-signal devices. However, devices with
multiple or spurious signals could have this type of aliasing. Section 6 describes
how to perform a check to determine if there are any unwanted signals present
that can alias down into the measurement span. A second type of aliasing hap-
pens when the sample rate causes the in-band signal to fold over on itself. This
in-band (or self-) aliasing is avoided in full rate mode by automatically selecting
the highest sample rate. However, in user rate mode in-band aliasing may be a
problem. To avoid in-band aliasing you must select a suitable sample rate based
on your measurement center frequency and span.
To avoid in-band aliasing the digitizer sample rate (F
S
) must be selected so that
the analysis span falls within confi ned regions. Staying within these regions
prevents the analysis span from being positioned too close to F
S
/2. When F
S
is selected so that the span is confi ned to be within a single gray box (see
diagrams below), in-band aliasing is avoided. See the following table for the
equations that describe these in-band alias-free regions.
The primary use of user rate mode is to increase maximum waveform length that
can be acquired or to increase the update rate. In all cases, user rate measure-
ments must be been carefully engineered to avoid unwanted alias signals.
NOTE: User Rate is the only available mode in HF operation. The VSA does not
attempt to avoid alias exposure regions when in User Rate mode.
Figure 2. Avoiding in-band aliasing by selecting FS within a confi ned span
Baseband:
Zoom:
DC
n * F
s
(n + 1/2) * F
s
(n + 1)F
s
F
s
/2.56
F
s
/2
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