7. TDR Measurement Theory
where Z(t) is the impedance at time t, Z
S
is the source impedance, and Γ(t) is the reflection
coefficient at t. This is the basis for the CT100B ohms-at-cursor measurements. It is important to
note that impedance is nonlinear with respect to reflection coefficient, as shown in Figure 7.6.
Figure 7.6. Relationship of impedance (ohms) to reflection coefficient (rho) for 50 ohm
source impedance (marked by red circle).
Because TDRs measure reflection coefficient, noise and uncertainty on a TDR trace is in units of
millirho, not ohms. Given a fixed amount of uncertainty in reflection coefficient, the corresponding
impedance uncertainty will be low at impedances less than the TDR source impedance (typically
50 ohms) and increasingly larger and nonlinear for impedance values greater than the TDR source
impedance.
The CT100B factory calibration provides typical vertical accuracy of approximately 0.1 ohm near
50 ohms and approximately 10-20 ohms near 1000 ohms for most cable-testing applications. When
impedance measurements are very important, accuracy can be improved using Vert. Ref.
(Section 4.13.1) and normalized TDR traces (Section 5.7).
100 CT100B TDR Cable Analyzers Operator’s Manual
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