文档介绍：Measuring Parasitic Capacitance and
Inductance Using TDR
Time-domain reflectometry (TDR) monly used as a
convenient method of determining the characteristic
impedance of a transmission line or quantifying reflections
caused by discontinuities along or at the termination of a
transmission line. TDR can also be used to measure quantities
such as the input capacitance of a voltage probe, the
inductance of a jumper wire, the end-to-end capacitance of a
resistor, or the effective loading of a PCI card. Element values
can be calculated directly from the integral of the reflected or
transmitted waveform.
by David J. Dascher
Why would anyone use TDR to measure an inductance or capacitance when there are many inductance-capacitance-
resistance (LCR) meters available that have excellent resolution and are easy to use? First of all, TDR allows
measurements to be made on devices or structures as they reside in the circuit. When measuring parasitic quantities,
the physical surroundings of a device may have a dominant effect on the quantity that is being measured. If the
measurement cannot be made on the device as it resides in the circuit, then the measurement may be invalid. Also,
when measuring the effects of devices or structures in systems containing transmission lines, TDR allows the user to
separate the characteristics of the transmission lines from the characteristics of the device or structure being
measured without physically separating anything in the circuit. To illustrate a case where TDR can directly measure a
quantity that is very difficult to measure with an LCR meter, consider the following example.
A printed circuit board has a long, narrow trace over a ground plane, which forms a microstrip transmission line. At
some point, the trace goes from the top of the printed circuit board, through a via, to the bottom of the printed circuit
board and continues on. The ground plane has a small opening where the via passes through it. Assuming that