enter image description here Only the left half is relevant.

Like mentioned before,I want to check out if reflection is one of the reason that deteriorates the signal integrity of high-speed signal.One way that has been come up is to reexamine the impedance of PCB traces(in this case,differential microstrips),connector and cable by TDR.

As can be seen from the pic,the impedance at the connector(around point1) bounces up and down.But then it turns back to normal(about 100ohm)when depicting pcb traces(between point1&2).

So my question is,now that I've been slightly aware of what the TDR wave means,how can I know if this path is eligible for the signal I've tried to transmit?

Are the bumps and dips all I can get from TDR,so that the only thing I can do is to root the problem and try to improve them(like adjust L&C,really roughly)?Or am I able to put the data into some kind of simulation tool and get something like a transfer function for the cable-connector-trace path?

  • \$\begingroup\$ And then the impedance jumps to 160 Ohms. Why it is so? Is your transceiver IC powered and properly terminated, or not? Unfortunately, we are not aware at what frequency your interface is expected to run, so it is not possible to answer your question, even roughly. \$\endgroup\$ – Ale..chenski Dec 13 '16 at 7:11
  • \$\begingroup\$ @Ali Well,this is actually a transceiver IC we designed last year,and it is supposed to work at 10GHz. Obviously we didn't do a great job in terms of termination(160 Ohms).And what I am gonna ask is,how can we use the data from TDR in general? \$\endgroup\$ – Xiao Xiang Dec 13 '16 at 8:58
  • \$\begingroup\$ You didn't answer the main question: is the trace done with the IC powered/configured/tuned, or it is in unpowered state? Modern interfaces must have impedance-controlled ports, and TDR tests are usually conducted in special test mode, with receiver-only, to test the actual termination value. \$\endgroup\$ – Ale..chenski Dec 13 '16 at 18:38
  • \$\begingroup\$ @Ali These results were produced in unpowered state because then we just wanted to check out if there was a impedance discontinuity problem with the traces. \$\endgroup\$ – Xiao Xiang Dec 14 '16 at 6:08
  • \$\begingroup\$ Ok then, see my assessment below. \$\endgroup\$ – Ale..chenski Dec 14 '16 at 8:00

There are several elements of the TDR technique that should be considered. First one is the selection of rise time of the TDR instrument. Theoretically the faster is better, but not always. Faster TDR step allows to resolve impedance discontinuities to finer length, better identify design problems.

But every communication signal tends to limit the edge rate, for EMI and other reasons. So too fine imperfections do not affect the actual signal propagation, and the rise time of step function in TDR instrument should be aligned with requirements of signal edges. Therefore for, say, the USB 2.0 TDR test specification dictates 400ps edge, which is achieved by imposing a sliding filter on data. For USB3.0 (5Gbps) the TDR edge is defined as 50ps.

Since your data rate is 10Gbps, I would say that the instrument needs the edge rate at 25ps. Your instrument shows 35ps edge, which is a bit below of what is needed to evaluate the trace quality/uniformity.

So, as you clarified, the 160-Ohm jump is due to some likely corrective passive resistor inside the IC, and the actual termination is unknown at this point. [usually an unpowered IC shows its impedance go to infinity, with some capacitance to ground]. So you can't blame the horrible 160-Ohm jump for the possible problems with your signal integrity (yet).

The discontinuity around the connector deserves some attention. First, as I explained above, 35ps is somewhat below the desired 25ps. So the trace would show larger excursions if faster edge will be applied. How much, I can't tell. The discontinuity is of L-C type, with L on outside part of the joint (bump), and some parasitic capacitor to ground (downward glitch) after that. For the 10Gbps rate the impedance inhomogeneity is not really horrible, but it is something that needs an attention and better modeling effort for the cable-connector-trace matching.

It also could be that this blip/dip is due to poor design of your test coupon.

The most important part of TDR evaluation would be when you power-on the IC, and make it to finish impedance self-adjustment, set the port in receive-only mode, and then to see what is really reflected back from the chip/package itself. This will be really important.

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    \$\begingroup\$ Regarding use of TDR for quality assessments, Tekronix has some tool and guidance how to do this. I can't remember the name of this tool. The following article can be of some help, literature.agilent.com/litweb/pdf/5988-6505EN.pdf \$\endgroup\$ – Ale..chenski Dec 14 '16 at 8:47
  • \$\begingroup\$ First to be clear,the clock works at a rate of 10GHz,so you can roughly regard this as 20Gbps data rate(but actually PAM4 signal is transmitted and the data rate could go as fast as 40Gbps).And could you give more details about the relation between the transfer data rate and edge rate of the instrument? \$\endgroup\$ – Xiao Xiang Dec 14 '16 at 10:47
  • \$\begingroup\$ @XiaoXiang, if you have your Unit Interval (UI), say, at 100ps, the eye can be reliably formed if edge transition is under 1/3 of that, or 30ps. There is no reason to make it much faster. To unfold transmission lines imperfections that can affect this time scales, the step edge of instrument should be faster than that. However, this applies to quick pass-fail test. To find a real problem at component/via level, you need faster instrument edge. \$\endgroup\$ – Ale..chenski Dec 14 '16 at 19:37
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    \$\begingroup\$ I can't edit comments, but the updated link to @Ale..chenski's article is literature.cdn.keysight.com/litweb/pdf/5988-6505EN.pdf (Measure Parasitic Capacitance and Inductance Using TDR) \$\endgroup\$ – Danilo Roascio Jan 1 at 18:09

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