Span Considerations (Digital Demod)

When selecting a frequency span, select the narrowest span that includes all of the signal components (slightly larger than the bandwidth of the signal).

If a span that is too wide is selected, the measurement may be affected by excessive noise and slower speed. If a span that is too narrow is selected, the measurement may have excessive errors or the VSA may lose carrier lock. The span can be set while viewing the spectrum of the signal.

The VSA displays a warning message if the frequency span is:

 Frequency span < (1 + a) ´ (symbol rate)

where a is the filter alpha.

If this warning message appears, increase the frequency span until it includes all components of the signal.

For digital demodulation, the symbol rate determines the maximum frequency span that can be selected, as follows:

This formula can be rewritten to obtain the maximum span-to-symbol-rate ratio, as follows (for FSK Frequency Shift Keying: A form of modulation using multiple carrier frequencies to carry the digital information. The most common is the two frequency FSK system using the two frequencies to carry the binary ones and zeros. demodulation formats, the maximum span-to-symbol-rate ratio is 100):



 

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 for the VSA. The reason is that the digital demodulation process derives its own frequency span and sample rate, as follows:

 

 

When viewing digitally demodulated spectrums, aliasing may occur if the frequency span is too large. Aliasing only affects the validity of the spectral displays, not the accuracy of the demodulation. The VSA displays DATA? if it suspects aliasing in digitally demodulated spectrums. To prevent aliasing, be sure that: