I-Q measured signal
The IQ measured signal is the result of resampling the data to an integer number of
points per symbol and applying IQ origin offset compensation (FSK and MSK type 1
do not compensate for IQ origin offset or droop), system gain normalization, carrier
locking, and filtering to the incoming signal. The filtering is user-selectable.
I-Q reference signal
A powerful analysis technique involves comparing a demodulated signal with an
ideal signal generated from detected bits. The analyzer detects bits from the
measured IQ signal and reconstructs a sequence of ideal I and Q states. These are
then treated as ideal impulses and are baseband filtered according to a reference
filter selected by the user. The resultant IQ reference can be overlaid or compared
side-by-side with the IQ measured signal.
The reference filter can be selected independently from the measured signal
(although the alpha/BT is the same for both filters). Therefore, you can apply
different filters to the measured and reference signals to accommodate special test
requirements. See the section on filtering later in this chapter.
Special considerations for FSK demodulation
Separate I and Q signals do not exist for FSK demodulated signals. There is no
phase information (the imaginary data is zero) because FSK demodulation
performs FM demodulation on the input signal. Consequently, the FSK
demodulator produces real rather than complex data. The display result is a
baseband, single-sided spectrum and a time display representing frequency versus
time.
Digital Demodulation Concepts (Opt. AYA)
22 - 9
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