Reflection Measurements In Time Domain Low Pass; Time Domain Low Pass Measurements Of An Unterminated Cable - Agilent Technologies 8719D User Manual

Reflection Measurements In Time Domain Low Pass

Figure 6-65 shows the time domain response of an unterminated cable in both the low-pass step
and low-pass impulse modes.

Time Domain Low Pass 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. To 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 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
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.
As described, the low pass mode can simulate the TDR response of the test device. This
response contains information useful in determinin g the type of discontinuity present.
Figure 6-66 illustrates the low pass responses of known d&continuities. Each circuit element
was simulated to show the corresponding low pass time domain Si1 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.
Figure 6-65.