At 03:06 PM 26/02/99 +0000, you wrote:
>Hi ryu,
>
>I hope this will be of some guidence for you.
>For microstrip tracks (single transmission line on the outer
>surface of the substrate with a ground plane on the reverse side)
>at microwave frequencies I have always used the following rule of
>thumb;
>                NOTHING CLOSER THAN 3 x TRACK WIDTH
>Regards,
>                Barry Bates
>
>> -----Original Message-----
>> From: ryu [SMTP:[log in to unmask]]
>> Sent: 24 February 1999 16:19
>> To:   [log in to unmask]
>> Subject:      [TN] electrical clearance
>>
>> I saw the term in the IPC-610.  I think it has something to do
>> with the design of a product for cross talk elimination.  But can
>> someone tell me if there is a minimal or maximum requirement
>> for the distance, is there any number a designer needs to follow?
>> if the answer is no, then how the designer comes up with the
>> number ?  Thanks.
>>
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Dear Ryu & Barry,

If you take a microstrip trace and gradually reduce the width of the
"infinite" ground plane underneath the trace by removing the earth
plane equally (symmetrically) from either side of the centre line of
the trace and make TDR impedance measurements, you will find
that the impedance stays almost constant until the width of the
remaining grond plane is less than three times trace width.
This is backed up in the Motorola MECL System Design Handbook
ref: HB205, rev 1, page 137 to 138

This indicates the extent to which the electromagnetic field spreads
out from the microstrip trace to its associated ground plane.  So in
theory there would be only small coupling between microstrip traces
of width W, with a gap of 2 x W, as the field patterns would not
significantly overlap (interact) with each other.  However, as stated
above, 3 x W gives room for error in this simple analogy.

Hope this helps to simplify things.


Best regards

Andy Burkhardt
Product Manager
Email: [log in to unmask]
Tel: + 44 1481 253081
Fax: + 44 1481 252476
http://www.polar.co.uk
=====================================================
World leaders in PCB faultfinding and controlled impedance measurement
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