[SI-LIST] Re: Attaching coax cable to board
- From: "Michael Khusid" <mkhusid@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 24 May 2002 10:10:38 -0500
Ideally, you would have only odd mode on your differential pair. That
is, in the ideal case there will be no total current in the microstrip
ground. The return currents that flow in coax grounds will cancel each
other as long as they reach the PCB.
So, in the ideal case, it does not matter whether you connect SMA ground
to IC ground, keep them isolated or AC decouple them.
In the real world case, you will likely have some even mode on the
microstrip differential pair (there was a lengthy detailed discussion on
the odd/even modes so I'll avoid repeating it). I would recommend to
evaluate your board and check how much even mode you are seeing.
If you do keep SMA and IC grounds isolated, the even mode will be
completely reflected back into microstrip. I do not know whether that is
a desired outcome in your application, but I assume it is not.
If you have weak even mode, then the simplest solution is probably just
to let it dissipate. In that case, I would connect the SMA and IC
grounds. The resistance of cables will decrease the reflection by
dissipating some of the even mode energy.
It is an engineering judgment for you -- whether complete suppression of
the even mode is worth your time or not, and whether there are other
noise sources to consider.
Lastly, it is possible to analyze the even mode reflection at the SMA
ground/IC ground junction. I would recommend considering not only
capacitor(s) between SMA and IC grounds, but also resistor(s). I would
suggest to run a few simulations assuming only even mode excitation of a
coupled 3-conductor model for microstrip connected to ideal 50 Ohm
transmission lines respesenting cables.
-----------------------------------------------------------------
Michael Khusid
Ansoft Corporation
HF/SI Application Engineer
25 Burlington Mall Road, 6th floor
Burlington, MA 01803-4100
Tel 781-229-8900 Ext. 34
Fax 781-229-8624
--------------------- http://www.ansoft.com ---------------------
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of mittalr@xxxxxxxxxx
Sent: Thursday, May 23, 2002 4:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Attaching coax cable to board
Suppose you attach 50ohm cables from a synthesiser to you board through
SMA . Its 10G differential setup. The board contains a
microstrip/coplanar waveguide connecting the SMA to your IC which has
50ohm termination connected to Vcc
It seems to me you have field lines in the coax between the center
conductor and the sheathing. This then somehow has to transfer to fields
between the differential microstrip/coplanar wg on your board. The field
lines between the surface line and the ground provides the common mode
impedence whereas the field lines between the conductors provides the
differential mode Z.
Now my question is: should you connect your SMA ground to the IC ground
or should you keep it isolated. If you keep it isolated, should you
connect decoupling caps between your IC ground (ie the board ground) and
the SMA ground.
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