In the impedance graph, the impedance varies wildly because the signal reflected by the short termination adds or subtracts at different frequencies.The return loss is almost 0 dB because all energy is reflected by the short at the end.It’s an open impedance with increasing phase over frequency. In the Smith chart, the impedance is running circles around the perimeter.The graphs in Figure 2 are all different representations of the same S11 measurement on this network.Īt the top of Figure 2 are three graphs of the impedance over frequency: Figure 1 shows the setup of this measurement. The following graphs show a measurement of a piece of 50-Ω coax terminated with a short. The measurements were done with a MegiQ VNA-0460e 6-GHz vector network analyzer. This article describes frequency and TDR measurements of coax networks and the analysis of PCB trace impedances on a test PCB, which are some of the applications of TDR analysis. We can use a Fourier transform to convert this into a series of impedances over time, just as that produced by a TDR instrument. A VNA measures the (reflected) impedance over frequency. The same kind of measurement can be taken using a vector network analyzer (VNA), but in a roundabout way. And, because the signals travel with (nearly) the speed of light, the time can be translated to a distance through a cable or network. TDR yields information about the impedance as a function of the time it takes for a pulse to reflect or travel through the medium. Traditionally, this is done with a device that generates very fast pulses, injects them on the network, and measures the time and amplitude of reflected pulses.

Time-domain reflectometry (TDR) is a technique that measures and displays the impedance of a network (cable, filter, and so on) over time. Difference between frequency- and time-domain analysis.The basic concepts underpinning time-domain reflectometry.Members can download this article in PDF format. This article appeared in Microwaves & RF and has been published here with permission.

Vna smith chart