[SI-LIST] : Re: delay lines with PCB traces

Paul Galloway (pgjr@Cadence.COM)
Wed, 24 Sep 1997 22:48:48 -0400

Aren't there any old microwave circuit designers here? You may want
to look for some of the old design guides for Schiffman phase shifters.
This tight self coupling is a well known circuit design principle.


>Errors-To: si-list-approval@silab.Eng.Sun.COM
>Date: Wed, 24 Sep 1997 14:59:53 -0700
>From: Michael Chin <mchin@cisco.com>
>To: si-list@silab.Eng.Sun.COM, smithh@VNET.IBM.COM
>Subject: [SI-LIST] : Re: delay lines with PCB traces
>Cc: mchin@cisco.com
>Sender: owner-si-list@silab.Eng.Sun.COM
>I would like to echo that this zip-zag trace pattern has been known
>and oberserved in the lab to reduce the total expected delay. I
>came across a situation when I was using PCB trace to add some "skew"
>into the clock nets. The actual clock skew that was induced thru this
>kind of topology was measured to be less than the min. expected delay
>by 5% to 10%.
>I ended up running the serpentine in a wider gap (24 mil to 30 mil)
>to allow an accurate clock skew distribution. But, this costed us
>more space on the PCB to delay the clocks.
>Michael Chin
>Cisco Systems, Inc
>> From owner-si-list@silab.Eng.Sun.COM Wed Sep 24 14:35:44 1997
>> The 2nd order effect that Andy alludes to is what I call current jumping
>> in a zig-zag pattern. Primarily due to inductive coupling, the active
>> signal induces a return current on itself which due to the zig-zag pattern
>> propagates in the same direction as the active. The outcome of this effect
>> is a net delay decrease.
>> So when you force wire in a zig-zag pattern to obtain a certain net delay
>> (i.e. for clock balancing, etc.) and expect a To*L result, be careful.
>> You may have just added an unwanted skew in your clock distribution or
>> pop up early mode surprise in your design.
>> > ...
>> > What about second-order effects; for example, does the little zig-zag
>> > approach allow a weaker wave mode to zip right along as if the
>> > zig-zags weren't there?