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)
$ E Demod~Span~=~(Demod~Sample~Rate)~/~1.28
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 analyzer displays DATA? if it suspects aliasing
in digitally demodulated spectrums. To prevent aliasing, be sure that:
Frequency Span ≤
(Symbol Rate)×(points−per−symbol)
1.28
where: Frequency span is set by pressing [Frequency] [span].
symbol rate is set by pressing [
Instrument Mode], [demodulation setup], [symbol rate].
points-per-symbol is set by pressing [Time], [
points/symbol].
Data size considerations
Maximum data size for measured and reference IQ data is generally determined by
max time pts
++
which is user selectable, 64-2048 (up to 4096 samples with
extended RAM option UFG or UTH). A given measurement will generally use all
available memory as defined by max time pts:
max time pts≥ result length × points ⁄ symbol
Resolution bandwidth
Resolution bandwidth in digital demodulation is determined automatically by the
analyzer, and cannot be set independently. Resolution bandwidth is determined by
the time record length in the same manner as in Vector mode (see Fundamental
Measurement Interactions):
RBW = WBW ⁄ T
where RBW = resolution bandwidth
T = time record length
WBW = window bandwidth
However, in Digital Demodulation the time record length is determined as follows:
T ≈ Result Length (in symbols)⁄Symbol Rate
Display limitations
Digital Demodulation Concepts (Opt. AYA)
22 - 11
++ See [
max time points
] under [
System Utility
], [
memory usage
], [
configure meas memory
]
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