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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 30 kHz and 101 points, the stop frequency would be 3.03 MHz. Since
the analyzer frequency range starts at 30 kHz, 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 300
kHz to 3 GHz frequency range. Now press
and observe the change in frequency
values. The stop frequency changes to 2.999 GHz, and the start frequency changes to 14.925 MHz. This
would cause a distortion of measurement results for frequencies from 300 kHz to 14.925
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. Therefore, set the frequency range for time domain low pass before
performing a calibration.
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