Agilent Technologies 8719D, User Manual

Page: 398 / 673

Time Domain Low Pass
This mode is used to simulate a traditional time domain reflectometry (TDR) measurement. It
provides information to determine the type of discontinuity (resistive, capacitive, or inductive)
that is present. Low pass provides the best resolution for a given bandwidth in the frequency
domain. It may be used to give either the step or impulse response of the test device.
The low pass mode is less general-purpose than the mode because it places strict
limitations on the measurement frequency range. The low pass mode requires that the
frequency domain data points are harmonically related from dc to the stop frequency. That is,
stop = n x start, where n = number of points For example, with a start frequency of 50 MHz
and 101 points, the stop frequency would be 5.05 Since the analyzer frequency range
starts at 50 MHz , the dc frequency response is extrapolated from the lower frequency data.
The requirement to pass dc is the same limitation that exists for traditional TDR.
Setting Frequency Range for Time Domain Low Pass
Before a low pass measurement is made, the measurement frequency range must meet the
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this function automatically: the stop frequency is set close to the entered stop frequency, and
the start frequency is set equal to stop/n.
If the low end of the measurement frequency range is critical, it is best to calculate
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and calibrating. This avoids distortion of the measurement results see an example, select
the preset values of 201 points and a 50 MHz to 13.5 frequency range. Now press
.. . .
to 13.499 and the start frequency changes to 67.164 MHz. This would cause a distortion
of measurement results for frequencies from 50 MHz to 67.164 MHz.
Note
If the start and stop frequencies do not conform to the low pass requirement
before a low pass mode (step or impulse) is selected and transform is turned
on, the analyzer resets the start and stop frequencies. If error-correction is on
when the frequency range is changed, this turns it off.
6-9. Minimum Frequency Ranges for Time Domain Low
of
3
11
21
61
101
201
401
801
Minimum Allowable Stop Frequencies.
The lowest analyzer measurement frequency is
50 MHz, therefore for each value of n there is a
allowable stop frequency that can
be used. That is, the minimum stop frequency x 50 MHz .
6-9 lists the
frequency range that can be used for each value of n for low pass time measurements.