TDR response tells a story

-December 12, 2014

Some of the most versatile and intuitive tools in a signal integrity engineer's arsenal are TDR (time domain reflectometry) and TDT (time domain transmission). Time domain analysis gives an insight into how electronic signals propagate through different dielectric media through a conductor whose geometric dimensions also influence these responses.

As a signal propagates through PCB vias, connectors, interfaces, ground-plane interruptions, etc., TDR and TDT are commonly used to identify and quantify these environments. Let's look at how one can associate different environments to TDR responses.

It must be noted that, when it comes to interpreting complex TDR responses, there is no substitute for experience. With that said, fundamental TDR responses to simple environments provide the building blocks for interpreting complex TDR responses. Let's start with the simplest of TDR responses: the open, the short, and impedance greater than and less than the TDR's system impedance. Figure 1 depicts these fundamental TDR responses for both a step response and an impulse response.

Figure 1. Responses from an open, short, and impedances > Z0 and < Z0.

For an impedance equal to TDR system impedance (normally 50 Ω for single-ended measurements), the TDR response is a flat line.

We will now progress to the next level of simple environments -- inductive and capacitive -- and their corresponding TDR responses. For this, I will defer to the following application note diagram originally published by TDA Systems, TDR Interconnect Modeling Quick Guide. (Tektronix later acquired TDA Systems.)

Figure 2. Inductance and capacitance affect TDR step responses.

From these classic TDR responses, you can begin to decipher a complex TDR response, a section at a time. You should know the general layout of the structure being measured to identify landmarks such as connector interfaces. The figure below shows a complex single-end TDR response that incorporates several of the aforementioned TDR responses. This response is of an interface between two PCBs connected through a high-density, shielded connector that exhibited a relatively high serial inductance -- after a relatively small capacitive via transition -- with the first board exhibiting a favorable controlled impedance of 50Ω.

Figure 3. A connector interface between two PCBs produces the spike in impedance above the nominal 50 Ω.

From the above TDR response, you can see that there was excessive ground-loop inductance through the connector, because the connector interfaces were not mechanically flush with each other. Understanding TDR responses for relatively simple environments is key to deciphering more complex responses and the environments they represent.

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