What is TDR?

TDR is the abbreviation of time domain reflectometry in English, which is called time domain reflectometer in Chinese. It is the main tool to measure the characteristic impedance of transmission line. TDR is mainly composed of three parts: fast edge signal generator, sampling oscilloscope and probe system.

TDR test principle

TDR sends a pulse or step signal to the transmission path. When the impedance changes in the transmission path, part of the energy will be reflected and the remaining energy will continue to transmit. As long as we know the amplitude of the transmitted wave and measure the amplitude of the reflected wave, we can calculate the change of impedance. At the same time, the phase of impedance change can be calculated by measuring the time difference from transmitting to reflected wave and then to transmitting point.

Figure (1) TDR diagram

According to the principle of reflection, the reflection coefficient

Formula (1)

In formula (1), zdut is the impedance of the device to be tested, Z0 is the output impedance of TDR, usually 50ohm standard resistance, vreflected and Vincent are the amplitude of reflected wave and incident wave respectively, which can be measured by oscilloscope to calculate the reflection coefficient ρ， The impedance zdut of the device to be tested is calculated.

Calculate the impedance of the device to be tested, and then look at how to calculate the electrical length of the device to be tested.

TDR generates a step signal to the device to be tested, which will produce an incident wave. After the incident wave passes through the time delay TD, when it meets the impedance discontinuity in the device to be tested, it will produce a transmitted wave. The reflected wave will be superimposed on the incident wave, and then reach the output end of TDR through the time delay TD.

TDR is simulated by simulation tools, as shown in figure (2)

Figure (2) simulated TDR

The voltage and impedance curves seen on the analog sampling oscilloscope are shown in Fig. (3) and Fig. (4)

Figure (3) voltage curve

Figure (4) impedance curve

As can be seen from figure (4), when the load is capacitive discontinuous, the impedance will be low; When the load is inductive discontinuous, the impedance will be high. The common impedance discontinuities in PCB, such as vias, pads and corners, are usually capacitive, while those across partitions and breakouts are usually inductive.

Fig. (5) inductive impedance discontinuity

Fig. (6) capacitive impedance discontinuity

So the question is, what is the resolution of TDR as a measuring instrument?

Editor in charge: GT