HP 8753E manual Reflection Measurements In Time Domain Low Pass

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. ‘Ihble 6-l 1 lists the minimm frequency range that can be used for each value of n for low pass time domain measurements

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.

Figure 6-65.

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. ‘Ib 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 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 smaIl 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 6-66 illustrates the low pass responses of known discontinuities Each circuit element was simulated to show the corresponding low pass time domain Sll 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.

AppiicationandOperationConoepts 6-131