[SI-LIST] Re: Hanging Stub or via

Qazi, Neither option is very appealing.   I believe that you need to 
rethink your problem such that the end result is that any driven 
transmission line presents an essentially constant impedance appropriately 
matched to the termination scheme used.  As it stands, you describe:

Option 1:  Relay open- stub near the middle of a Tx line into the relay 
when the relay is open, and a "Y" branch so Z/2 more or less when the relay 
is closed, and the likely possibility that the relay is not a HF type.

Option 2. Same as one, but branch at one end of the line.

In either case, you have a stub when the relay is open, and Z versus Z/x 
where x >= 2 with the relay closed.   That's not good.

There are several alternatives depending on your specific needs / 
priorities.  The simplest that comes to mind is a low performance buffer 
and independent termination to the relay center contact since you report 
that timing in that path can be quite slow.

Steve
At 03:10 PM 4/27/2005 -0700, Qazi Arif Iqbal wrote:
>Hi Gurus,
>
>I need your opinion regarding which of the following would be better.
>
>1. From the DUT pin a 50 ohm Trace going to a pin of a relay from which it
>brances out to two different places. One is the high speed trace that goes
>straight to the tester pin and other going through the same relay (speed not
>a factor) when the relay is switched on. So if I keep the relay off, my high
>speed line would see a DUT pin to a relay pin (via) and then the tester pin.
>2. On the other possible option is to branch out the whole thing at the DUT
>pin. From DUT one branch would (my high speed line) go to the tester and the
>other branch would go to the relay ( a considerable trace length)through
>which it would go to the Tester. So if I keep the relay off, my high speed
>line would see a hanging stub of trace for the other branch.
>
>I was wondering which option would be better.
>
>Thanks in advance,
>Qazi
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
>Christopher.Jakubiec@xxxxxxxxxxxx
>Sent: Wednesday, April 27, 2005 8:17 AM
>To: CurtM@xxxxxxxxxxx; roymesi@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diode connected between source & drain - why?
>
>
>Vertical Power MOSFETs are designed specifically to handle a certain
>amount of energy through this diode per the data sheet.  Logic MOSFETs
>most likely do not care about this parameter.
>
>Chris
>
>-----Original Message-----
>From: Curt McNamara [mailto:CurtM@xxxxxxxxxxx]=20
>Sent: Wednesday, April 27, 2005 10:19 AM
>To: Jakubiec Christopher (IFNA AI QM CM); roymesi@xxxxxxxxx;
>si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: diode connected between source & drain - why?
>
>
>My experience with the body diode (as I understand it, a by-product of
>device construction) is that you *should not* depend on it to clamp an
>inductive kick-back. There is simply too much energy for it to=20
>dissipate, and you will destroy the FET. Use an external diode clamp
>which is rated for the energy -- these should only cost a few=20
>cents.
>
>                                       Curt
>
>Curt McNamara. P.E.=20
>Senior Electrical Engineer=20
>Logic Product Development=20
>411 Washington Ave. N Suite 101=20
>Minneapolis, MN 55401=20
>Tel 612-436-5178=20
>Fax 612-672-0443=20
>curtm@xxxxxxxxxxx=20
>www.logicpd.com=20
>This message (including any attachments) contains confidential
>information intended for a specific individual and purpose, and is
>protected by law. If you are not the intended recipient, you should
>delete this message and are hereby notified that any disclosure,
>copying, or distribution of this message, or the taking of any action
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>
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Christopher.Jakubiec@xxxxxxxxxxxx
>Sent: Wednesday, April 27, 2005 7:44 AM
>To: roymesi@xxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: diode connected between source & drain - why?
>
>
>Hi Roy,
>
>When current is switching through an inductive load, a reverse EMF is
>generated when the current is turned off.  This diode allows a path for
>this current in this situation.  This body diode comes built into the
>MOSFET for free, and is not an extra device.
>
>A diode connected MOSFET behaves like a diode, while a standard
>connected MOSFET behaves like a switch with a diode in the body.
>
>-Chris
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Roy M.
>Sent: Wednesday, April 27, 2005 9:21 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] diode connected between source & drain - why?
>
>
>Hi,
>Can someone explain to me why are they Transistors (mosfets), that has a
>diode connected between the source and the drain? I tried to find
>something on the internet, but nothing... i don't understand what's the
>difference between a regular mosfet and a
>diode-connected-source-drain-mosfet.
>Thanks!
>Roy
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