TDR is an acronym for time-domain Reflectometry. It is a remote measurement technology that analyzes reflected waves and learns the status of the measured object at the remote control position. In addition, there is time domain reflectometry; Time-delay relay; Transmit Data Register is mainly used in communication industry in the early stage to detect the breakpoint position of communication cable, so it is also called “cable detector”. A time domain reflectometer is an electronic instrument that uses a time domain reflectometer to characterize and locate faults in metal cables (for example, twisted pair or coaxial cables). It can also be used to locate discontinuities in connectors, printed circuit boards, or any other electrical path.
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Overview of signal transmission theory
In recent years, with the rapid improvement of the bit rate of digital communication standards, for example, the simplest consumer USB 3.1 bit rate even reached 10Gbps; USB4 gets 40Gbps; The improvement of bit rate makes the problems that have never been seen in traditional digital system begin to appear. Problems such as reflection and loss can cause digital signal distortion, resulting in bit errors; In addition, due to the decrease of the acceptable time margin to ensure the correct operation of the device, the timing deviation in the signal path becomes very important. The radiation electromagnetic wave and coupling produced by stray capacitance will lead to crosstalk and make the device work wrong. As circuits get smaller and tighter, this becomes more of a problem; To make matters worse, a reduction in the supply voltage will result in a lower signal-to-noise ratio, making the device more susceptible to noise;
The vertical coordinate of TDR is the impedance
TDR feeds a step wave from the port to the circuit, but why is the vertical unit of TDR not voltage but impedance? If it is impedance, why can you see the rising edge? What measurements are made by TDR based on Vector Network Analyzer (VNA)?
VNA is an instrument to measure the frequency response of the measured part (DUT). When measuring, a sinusoidal excitation signal is input to the measured device, and then the measurement results are obtained by calculating the vector amplitude ratio between the input signal and the transmission signal (S21) or the reflected signal (S11). The frequency response characteristics of the device can be obtained by scanning the input signal in the measured frequency range. Using band pass filter in measuring receiver can remove noise and unwanted signal from measuring result and improve measuring accuracy
Schematic diagram of input signal, reflected signal and transmission signal
After checking the data, IT was found that TDR’s instrument normalized the voltage amplitude of reflected wave, and then equivalent it to impedance. The reflection coefficient ρ is equal to the reflected voltage divided by the input voltage; Reflection occurs where the impedance is discontinuous, and the voltage reflected back is proportional to the difference between the impedances, and the input voltage is proportional to the sum of the impedances. So we have the following formula. Since the output port of TDR instrument is 50 ohms, Z0=50 ohms, so Z can be calculated, that is, the impedance curve of TDR obtained by plot.
Therefore, in the above figure, the impedance seen at the initial incident stage of the signal is much smaller than 50 ohms, and the slope is stable along the rising edge, indicating that the impedance seen is proportional to the distance traveled during the forward propagation of the signal. During this period, the impedance does not change. I think it is rather roundabout to say that it is regarded as though the rising edge was sucked up after the impedance reduction, and finally slowed down. In the subsequent path of low impedance, it began to show the characteristics of a rising edge and continued to rise. And then the impedance goes over 50 ohms, so the signal overshoots a little bit, then slowly comes back, and finally stabilizes at 50 ohms, and the signal has reached the opposite port. In general, the region where the impedance drops can be thought of as having a capacitive load on the ground. The region where the impedance suddenly increases can be thought of as having an inductor in series.
Post time: Aug-16-2022
