Doug,

                The coupon outlined in the IPC spec is set up for a 
        "Nanohex" style probe. It works well for single line ("self coupled")
        transmission lines. The design has additionally considered ground
        vias around the perimeter of the TUT (trace under test). It ends up
        being rather large, however for high count multilayer PCBs. It also
        becomes very cumbersome if you try to incorporate differential lines.
        
        Rule of thumb: Make the trace length at least 4" in length. (This has
        to do with TDR pulse rates and allowance for waveform to settle and     
        provide a good "undisturbed interval". Place your ground connection
        (common to all reference planes) at a short distance away. Approx
        what your probe can easily reach and provide minimal ringing.           
        (Overshoot/Undershoot)  Terminate your trace with a similar ground
        via close by in case for some strange reason the other one gets         
         destroyed. 

        X   X-----------------------------X    X L1
        X   X-----------------------------X    X L3    Example
        X   X-----------------------------X    X L6
        X   X-----------------------------X    X L8


        Hope this helps you in what your looking for.

                Hadco Printed Circuits
                Tech Center Two / Watsonville                


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                Dave Hoover              (408) 728-6677
                Senior Process Engineer  (408) 728-1728 Fax
                [log in to unmask]

______________________________ Reply Separator _________________________________
Subject: TDR coupons
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    9/2/95 11:44 AM


-- [ From: Doug Jeffery * EMC.Ver #2.10P ] --


Hi,


I have been looking in IPC-D-275 and RB-276 for design criteria for a
TDR coupon.  I could only find a reference to IPC-D-317..Which, of
course, I do not have.(had to order)..

Does anyone know if this spec. has the coupon construction information
(number of holes, recommended sizes, which holes hook to
power/ground/signal for the varieties of Impedance styles? 

Is there a standard coupon, or does everyone make their own as they go?


Thanks,
Doug Jeffery
Electrotek, Inc.
[log in to unmask]
(414)762-1390