AVNER,
        WHAT YOU MAY BE EXPERIENCING IS A LOT TO LOT DIFFERENCE WITHIN THE 
     TOLERANCE RANGE.  THE DIELECTRIC CONSTANT CAN AND OFTEN DOES VARY 
     ACROSS THE PWB ITSELF.  THIS IS A PROBLEM WE HAVE RUN INTO AT HIGHER 
     FREQs.  OUR ENGINEERS WOULD TUNE  A CIRCUIT (50 OR 75 Ohm >500Mhz). 
     AND FIND OUT THAT THE TWEAK WOULDN'T WORK ON AN INDENTICAL BOARD 
     BECAUSE OF THE DIELECTRIC CONSTANT TOLERANCE.  YOUR OPTIONS ARE TO USE 
     A BOARD MATERIAL WITH A TIGHTER TOLERANCE (MORE CONSTANT) OR USE 
     VARIABLE POTS, CAPS AND INDUCTORS EVERYWHERE.  NEITHER IS A HAPPY 
     SOLUTION.
     
     
     ROB BUTTERWORTH
     SR PWB DESIGNER
     ADC VIDEO SYSTEMS
     999 RESEARCH PARKWAY,
     MERIDEN, CT 06450
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: Controlled Impedance - Dielectric Constant
Author:  [log in to unmask] at internet-mail
Date:    5/21/96 8:15 AM


     
Subject: Controlled Impedance - Dielectric Constant
     
I like to know if anyone found that Dielectric Constant, for the same 
material (e.g. FR-4),
 might be different when used for calculation of Microstrip or for
calculation of Stripline.
     
Avner Drory