Mitch;

What your describing is a choke to gnd.  The idea is to create inductance
on the thin trace before tying to gnd.  The length and distance to the gnd
plane
determines the inductance.  The problem is that it confuses the ratsnest
into thinking the whole net is gnd.  One way I have overcome this is to
create a two pad part with a pitch of maybe 50mil. The pads are the same
diameter as the trace. One pad goes to the RF path the other ties to gnd.
Then I place a non electrical item like a 2d line to short them out.  Then
I route  the gnd connection using two right angle bends like a digital
waveform.  This allows me to adjust the length by dragging the jog and
checking with the length checking feature of the program.  Remember to add
the length of the non electrical feature to the overall gnd path trace
length.  Most spacing checkers do not see non electrical items and
therefore will not identify a short.  Be careful not to inadvertently short
it by accident.

I design these types of high speed RF circuits all the time using PADS
PowerPCB.
GHZ RF and microwave is a very interesting specialty.

If you have any more questions please contact me.

Steve Ray
S.T.A.R. Design Service
(650) 369-6341

-----Original Message-----
From:   Mitch Morey [SMTP:[log in to unmask]]
Sent:   Friday, March 13, 1998 6:18 PM
To:     [log in to unmask]
Subject:        [DC] DES:"tuning" etch pattern

This question came up recently, and I'm unfamiliar with the practice.

A circuit was described to me that has a "few" traces going between
components, and then they go off the last (first?) component a "certain"
distance then tie to ground. Sound familiar? This "certain" distance is to
be determined during routing, and then the three or four (or more) traces
will be "adjusted" accordingly to satisfy the circuits function.

First off, this makes no sense to me.

Secondly, is there a preferred way to tie these traces to ground, and
what methods have been used to "trick" the software package you're
using into doing this (other than showing them as ground signals)?

Any help is appreciated.

Mitch Morey
Sr PCB Designer

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