3-15
Making Time Domain Measurements
Time Domain Low Pass Mode
Time Domain Low Pass Mode
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 bandpass 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 × start, where n = number of points.
For example, with a start frequency of 50MHz and 101 points, the stop frequency would be
5.05 GHz. 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 the Frequency Range for Time Domain Low Pass

Before a low pass measurement is made, the measurement frequency range must meet the
(stop = n × start) requirement previously described. The softkey
performs 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
approximate values for the start and stop frequencies before pressing
and calibrating. This avoids distortion of the measurement
results. To see an example, select the preset values of 201 points and a 50 MHz to 13.5 GHz
frequency range. Now press and observe the change in frequency
values. The stop frequency changes to 13.499GHz, and the start frequency changes to
67.164 MHz. This would cause a distortion of measurement results for frequencies from
50 MHz to 67.164MHz.
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 correct ion i s on
when the frequency range is changed, this turns it off. Therefore, set the
frequency range for time domain low pass before performing a calibration.
Table3-2 M inimum Frequency Ranges for Time Domain Low Pass
Number of Points Minimum Frequency Range Number of Points Minimum Frequency Range
3 50 MHz to 150 MHz 101 50 MHz to 5.05 GHz
11 50 MHz to 550 MHz 201 50 MHz to 10.05 GHz
21 50 MHz to 1.05 GHz 401 50 MHz to 20.05 GHz
SET FREQ LOW PASS
SET FREQ LOW PASS
SET FREQ LOW PASS