Specifications

Chapter 15 159
Concepts
Stimulus Response Measurement Concepts
Concepts
• Source attenuation may be set incorrectly (select Attenuation (Auto)
for optimum setting).
• The source power may be set too high or too low, use
Amplitude (Off)
then
Amplitude (On) to reset it.
• The source power sweep may be set too high, resulting in an
unleveled condition at the end of the sweep. Use
Power Sweep (Off)
then
Power Sweep (On) to decrease the amplitude.
• Reverse RF power from the device under test detected by the
tracking generator ALC (automatic level control) system.
Sweeping in Stimulus Response Auto Coupled Mode
Auto coupled sweep times are usually much faster for stimulus
response measurements than they are for spectrum analyzer (SA)
measurements.
In the stimulus response mode, the Q of the DUT can determine the
fastest rate at which the analyzer can be swept. (Q is the quality factor,
which is the center frequency of the DUT divided by the bandwidth of
the DUT.) To determine whether the analyzer is sweeping too fast, slow
the sweep and note whether there is a frequency or amplitude shift of
the trace. Continue to slow the sweep until there is no longer a
frequency or amplitude shift.
Normalization Concepts
To make a transmission measurement accurately, the frequency
response of the test system must be known. Normalization is used to
eliminate this error from the measurement. To measure the frequency
response of the test system, connect the cable (but not the DUT) from
the tracking generator output to the analyzer input.
Press
View/Trace, More, Normalize, Store Ref (1→3), Normalize (On).
The frequency response of the test system is automatically stored in
trace 3 and a normalization is performed. This means that the active
displayed trace is now the ratio of the input data to the data stored in
trace 3. (The reference trace is Trace 3 with firmware revision A.04.00
and later)
When normalization is on, trace math is being performed on the active
trace. The trace math performed is (trace 1
− trace 3 + the normalized
reference position), with the result placed into trace 1. Remember that
trace 1 contains the measurement trace, trace 3 contains the stored
reference trace of the system frequency response, and normalized
reference position is indicated by arrowheads at the edges of the
graticule.
NOTE Since the reference trace is stored in trace 3, changing trace 3 to
Clear Write will invalidate the normalization.