Fault Location Measurements Using Low Pass - Agilent Technologies 8753ET User Manual

Making Time Domain Measurements
Time Domain Low Pass Mode
Interpreting the Low Pass Response Vertical Axis
The vertical axis depends on the chosen format. In the low pass mode, the frequency
domain data is taken at harmonically related frequencies and extrapolated to dc. Because
this results in the inverse Fourier transform having only a real part (the imaginary part is
zero), the most useful low pass step mode format in this application is the real format. It
displays the response in reflection coefficient units. This mode is similar to the traditional
TDR response, which displays the reflected signal in a real format (volts) versus time (or
distance) on the horizontal axis.
The real format can also be used in the low pass impulse mode, but for the best dynamic
range for simultaneously viewing large and small discontinuities, use the log magnitude
format.

Fault Location Measurements Using Low Pass

As described, the low pass mode can simulate the TDR response of the test device. This
response contains information useful in determining the type of discontinuity present.
Figure 3-13 illustrates the low pass responses of known discontinuities. Each circuit
element was simulated to show the corresponding low pass time domain S response
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waveform. The low pass mode gives the test device response either to a step or to an
impulse stimulus. Mathematically, the low pass impulse stimulus is the derivative of the
step stimulus.
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