Span considerations
When selecting a frequency span, select the narrowest span that includes all of your
signal components (select a span that is larger than the bandwidth of your signal).
If you select a span that is too narrow, your measurement may have excessive
errors or the analyzer may lose carrier lock. If you select a span that is too wide,
your measurement may be affected by excessive noise and slower speed. You may
want to select the Vector instrument mode and set the span while viewing the
spectrum of your signal.
The analyzer displays a warning message if the frequency span is less than the
symbol rate for QAM and DVB QAM formats, and less than half the symbol rate for
VSB formats. If you see this warning message, increase the frequency span until it
includes all components of your signal.
When using video demodulation, the symbol rate determines the maximum
frequency span that you can select, as follows:
Maximum Span (VSB)= 20
(SymbolRate)
2.56
Maximum Span (All Other Formats)= 20
(SymbolRate)
1.28
These formulas can be rewritten to obtain the maximum-span-to-symbol-rate ratio.
These ratios are fixed and cannot be changed.
VSB:
Maximum Span
Symbol Rate
=
20
2.56
OR
Maximum Span
Symbol Rate
= 7.8125
All Other Formats:
Maximum Span
Symbol Rate
=
20
1.28
OR
Maximum Span
Symbol Rate
= 15.625
For VSB measurements, you must use arbitrary spans; you cannot use cardinal
spans (cardinal spans are spans that are powers of two, such as 2.5 MHz, 5 MHz,
and 10 MHz). The analyzer displays an error message if you select a cardinal span
when using the VSB demodulation format.
You may notice that the frequency span for digitally demodulated spectrums
(the IQ measured spectrum, IQ reference spectrum, and error vector spectrum) is
different than that set with the [
span] softkey. The reason is that the digital
demodulation process derives its own frequency span and sample rate, as follows:
Demod Sample Rate =(symbol rate)×(points−per−symbol)
Demod Span =(Demod Sample Rate)⁄1.28
Video Demodulation Concepts (Opt. AYH)
23 - 12
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