5. TDR Measurement Theory
the difference between a 90 ps rise time TDR and an 800 ps rise time TDR (typical of low-cost
TDR instruments) for characterizing two 1 cm long, 75 ohm faults spaced 1 cm apart on a 50
ohm cable. The 800 ps rise time TDR detects only a single fault and grossly underestimates the
severity of the fault.
Figure 5.9.
Simulated comparison of a 90 ps rise time TDR (green) and an 800 ps TDR
(red) with respect to the ability to depict two 1 cm long, 75 ohm faults spaced 1 cm apart
on a 50 ohm cable. Notice how the faster TDR (90 ps) resolves each fault individually.
The blue lines represent a theoretically perfect response.
5.10. Timebase/Cursor/Horizontal Resolution
Timebase, cursor, or horizontal resolution refers to the horizontal spacing of samples along the
TDR trace. In order to faithfully depict the TDR profile of a connector or cable fault, the cursor
resolution should be at least 5 to 10 times higher than the spatial resolution. For the example
just shown in Figure 5.9, it is unlikely that the cursor resolution for the slower rise time
instrument would be adequate to reliably depict the fault in the first case.
Cursor resolution also comes into play when precise length or phase matching of cables is being
carried out, in which the cursor resolution must be sufficient to adequately depict the difference
in length between cables. Typically the cursor resolution should be 10 times higher than the
intended length accuracy.
With respect to frequency-domain measurements, very high cursor resolution ensures adequate
frequency bandwidth because the number of sampled data points in the time domain translates
into frequency resolution in the frequency domain.
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