Atek HP 8753D, Руководство пользователя, Страница: 389 / 677

Minimum allowable stop frequencies . The lowest analyzer measurement frequency is 30
kHz, therefore for each value of n there is a minimum allowable stop frequency that can
be used. That is , the minimum stop frequency =n x 30 kHz. T able 6-10 lists the minimum
frequency range that can be used for each value of n for low pass time domain measurements .
Reection Measurements In Time Domain Low P ass
Figure 6-64 shows the time domain response of an unterminated cable in both the low-pass step
and low-pass impulse modes .
Figure 6-64.
Time Domain Low P ass Measurements of an Unterminated Cable
Interpreting the low pass response horizontal axis . The low pass measurement horizontal
axis is the two-way travel time to the discontinuity (as in the bandpass mode). The marker
displays both the two-way time and the electrical length along the trace . T o determine the
actual physical length, enter the appropriate velocity factor as described earlier in this section
under \Time domain bandpass ."
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 F ourier 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 reection coecient units .
This mode is similar to the traditional TDR response , which displays the reected 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.
F ault Location Measurements Using Low P ass
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 6-65 illustrates the low pass responses of known discontinuities . Each circuit element
was simulated to show the corresponding low pass time domain S
11
response 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 .
Application and Operation Concepts 6-123