Re: [SI-LIST] : TDR Measurements

Mark McKee (
Fri, 12 Dec 1997 09:35:44 -0800 (PST)

Thanks for all your recommendations and comments.

Lessa write:

>A quick way to check this is to decrease the TDR pulse repetition rate
>and see if the "impedance" changes.

What would this indicate? Is it the repetition rate I want to decrease or
the rise time? Am I interested in the impedance as a function of frequency
using this approach. See my question regarding TDR's at the end of this email.

I've redone my measurements and found the following:

My board that did not appear to have loss actually does have a few ohms
of loss (roughly 4 ohms over a 20 inch trace). This is still roughly 1/2
the loss that I'm seeing on the other board.

So it seems there are 2 possible reasons based on all comments I've received

1) The roughening process for the board with more loss is much rougher.
2) The board material has a much higher loss tangent.

(or some combination of these) going to try a couple of different
board venders before trying to unravel this more.

Another brief question regarding TDR measurements.

It occured to me that I'm using a 38 psec rise time TDR to check thhe impedance
of boards where the fundamental frequency is less than a GHz (so harmonics of
interest can extend upward towards 10 GHz where I know FR-4 is extrememly
lossy). It seems that I should be checking the board impedance at the
fundamental, and a few harmonics to really understand the performance of the
board. Not knowing very much about how a TDR works, or what the bandwidth of
the sampling circuit is, I'm realizing that I'm no longer sure what it is I'm
looking at.

Can someone give me a brief overview of how a TDR works, and why using a 38
psec sampling head is OK (or better) when fundamental frequencies are well
below 1 GHz? A network analyzer tells all, but what exactly can I determine
about the performance of my boards at various frequencies when using a TDR?