I've lost track of the original question on this issue. If the concern is
just to make a circuit work at high frequencies, one of the customers we
have provided support to in the past uses lasers to automatically "trim"
chip caps while in circuit to attain the desired operating characteristics.
Each board was "tuned" prior to placement in the system. (MIL equipment,
high frequency monitoring stuff.)

>     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
>

                      Jack Crawford
                  HelpLine Manager
    Electronics Mfg. Productivity Facility
                     317.226.5616
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