Tektronix TDR
Tektronix TDR
Tektronix simple toolset combines the ultra fast acquisition of the Tektronix DSA8200 Sampling Oscilloscope to capture critical TDR edges with automated setup & calibration routines allowing you to efficiently perform measurements for cables, connectors, and backplanes.
A Time Domain Reflectometer (TDR) is used to measure the impedance and path loss of a lumped element or a transmission line. The TDR emits a short pulse, typically 25 ps. Let us assume that emitted pulse flows down a transmission line of infinite length or a transmission line terminated in its characteristics impedance. If there are no discontinuity along the path of the transmission line, there will be no reflection. The TDR screen will just show an incident signal. There will no signal from reflection.
Now let us assume that a finite transmission line of say 10 inches in length is left open on the far end. If a short pulse is sent out to this transmission line, it will reach to the end of the transmission line and get reflected. The polarity of this reflected signal will be same as the incident signal. The two signals will add and travel back towards the TDR. The reflected signal can be observed in on the TDR screen.
TDR has been used in telephony to find the break positions of a cable. It has been used to detect and estimate the distance where the cable has possibly broken. This is done by calculating the distance using the formula c x t, where c is speed of the wave propagation in the medium and t is the time it takes to traverse the medium. A marker is placed at the launch time and another at the time when the reflected signal comes back from broken cable. The TDR screen shows waves as it traverses the cable and come back. By dividing the time between the launched signal and the reflected signal by 2 we get the time it took for the signal to reach from launches end to the broken end. Multiplying this time by the signal propagation velocity gives the distance.
In high speed digital designs, TDR is used to measure the characteristic impedance of traces. It can also be used to measure the values of lumped circuit element, for example, load capacitance of an IC.
Let us assume that a transmission line is terminated in a lumped element of impedance Zt.. As a result there is reflection from the end of the transmission line. The magnitude of the reflection is referred to as the reflection coefficient or ρ. The reflection coefficient is calculated as follows:
ρ = (Zt-Zo)/(Zt+Zo)
Where Zo is defined as the characteristic impedance of the transmission medium and Zt is the impedance of the termination at the far end of the transmission line.
ρ can be computed by taking the ratio of the incoming reflected signal with respect to the incident signal. We can then find the value of Zt.
TDR can also be used to find the discontinuities along the transmission line as the signal propagates from one end of the transmission line to the other. At each discontinuity there will be reflection. These reflections can be observed on the TDR.
A part of this blog is taken from the book “Signal Integrity for PCB Designers”, by Vikas Shukla