Input Range; I-Q Measured Signal; I-Q Reference Signal - Agilent Technologies 89410A Operator's Manual

Video Demodulation Concepts (Opt. AYH)
The following may also cause loss of carrier lock:
l
A frequency span that is too narrow.
l
An incorrect range setting.
l
Using the wrong measured or reference filter.
l
Using [ freq spectrum normal ] when you should be using [ freq spectrum mirror ].
l
A pilot that is not in phase with the transmitted data. You may be able to
compensate for pilot-phase problems by using [ clock adjust ] to change when the
analyzer samples the I/Q trajectory (press [ Instrument Mode ], [ demodulation setup ],
[ more ], [ clock adjust ]).

Input Range

The input range must be set correctly to obtain accurate measurements. Input
ranges that are too low overload the analyzer's ADC. Input ranges that are too high
increase noise, which increases errors reported in error parameters, such as EVM.
To select the optimum input range when using the 89441A RF section (2-2650 MHz)
receiver, press [ Range ], [ ch1 range ] and decrease the range (using the down-arrow key)
until OV1 appears in the active trace. Then increase the range one step at a time
(by pressing the up-arrow key) until OV1 disappears.
To select the optimum input range when using the 89410A or the 89441A RF section
(0-10 MHz) or IF section (0-10 MHz) receiver, press [ Range ], [ ch1 range ] and decrease
the range (using the down-arrow key) until the Channel 1 Over and Half LEDs turn
on. Then increase the range one step at a time (by pressing the up-arrow key) until
the Over LED turns off.

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 system gain normalization, carrier locking, and
filtering to the incoming signal. The filtering is user-selectable. For QAM
measurements, the analyzer also applies I/Q origin offset compensation (similar to
pilot removal for 8 VSB and 16 VSB). I/Q origin offset compensation is not applied
to DVB QAM measurements.

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 is the same for both filters). Therefore, you can apply different
filters to the measured and reference signals to accommodate special test
requirements. For details, see the section on filtering later in this chapter.
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