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This table summarizes the difference in approach to alias protection between
the various sample modes:
Another concern in user rate mode is that you may see unwanted amplitude
variability in two channel measurements. This can occur if any part of the mea-
surement frequency span falls within the regions defi ned by:
(2*n+1) x F
S
/2 ± 0.1 x F
S
where n = 1, 2, 3, 4 …
where F
S
is the actual digitizer sample rate. Make sure you choose a sample rate
where such overlap does not occur.
4.3 Differences from 89600 VSA operation
• No Magnitude Trigger (however, Channel Trigger is available).
• Possible limitation of movement on the operating region curve to avoid alias
corruption of the measured signal. This depends on the settings of Center
Frequency, Span, Sample Mode, and Number of Frequency Points. There is
also potential interference due to the alias products of out-of-band signals.
(See Section 6.)
• No built-in wideband calibrator. The frequency response corrections are
determined from factory data.
Table 5. Varying approaches for alias protection
Full rate Both in-band and out-of-band aliasing are prevented through use
of the maximum sample rate and an appropriate lowpass fi lter.
User rate You are responsible to avoid in-band aliasing through appropriate
selection of sample rate based on center frequency and span. To
ensure no in-band aliasing is present the digitizer sample rate (FS)
must be selected to meet this criteria:
Baseband: F
STOP
≤ F
S
/2.56
Zoom: (n*F
S
)/2 ≤ F
CENTER
- (1.28 * F
SPAN
)/2
AND
((n+1)*F
S
)/2 ≥ F
CENTER
+ (1.28 * F
SPAN
)/2
where n = 1,2,3,4…
Out-of-band aliasing is minimized through use of appropriate
low-pass fi lter. Use of Alias Checker macro (Section 6) is required
to detect other unwanted out-of-band signal aliasing.
(Note: aliasing of unwanted signals can be eliminated through use
of appropriate bandpass filter.)
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