[SI-LIST] Re: Should the signals always return back through GND
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Bi Han <mike_bihan@xxxxxxxxxxxx>
- Date: Wed, 24 Aug 2005 08:47:35 -0700
Bi, yes I was looking at it as a PCB with the assumption that the aggressor
had some return path. In your example, it seems there was no defined
return path to start with. That is a bad place to be. You are right, I
would prefer to view the "shield" as the second half of a CPW.
Regards,
Steve.
At 08:41 AM 8/24/2005 -0700, Bi Han wrote:
>Steve:
>Thanks for pointing out the problem with so detailed illustration.
>There a key difference between our standing point. You assumed that the
>trace is on the PCB board, so there is GND plane below the signal trace.
>The GND plane will introduce a relative good return path for the signal
>trace, introducing another nearby return path will not help too much on EMI.
>I agree with your point on standing wave phenomenon in shielding trace,
>even both ends are grounded.
>I assumed that the signal trace is on chip, where there is no good GND
>plane on the chip. Introducing any nearby shielding trace will greatly
>help "collecting" return current. You could view shielding and sig trace
>as a pair of diff-trace, or a loop of current flow, differential trace
>will greatly reduce EMI, if the victim is not too close to the pair.
>thanks,
>Han
>
>steve weir <weirsi@xxxxxxxxxx> wrote:
>Bi, I see one of my sentences sounds circular: "That increases coupling
>because ... is itself a coupler to the victim." To make this very clear,
>when we insert that "guard" or any other trace, the field redistributes
>increasing the flux density in the region of the "guard", which is to say
>the region of the victim, and thus forming the coupling antenna.
>
>Steve.
>At 05:20 AM 8/24/2005 -0700, steve weir wrote:
> >Bi, no that is backwards. It is at high frequencies where we can't ignore
> >wave effects that parallel shield traces can offer little benefit or worse
> >create resonant couplers.
> >
> >There is an excellent treatment on this subject in Eric Bogatin's book,
> >"Signal Integrity Simplified"
> >
> >Let's start with a coaxial shield.all around the victim trace. As long as
> >the shield is thick enough skin effect in the shield prevents any
> >measurable aggressor energy from penetrating to the victim. The important
> >point here is that the aggressor fields have to penetrate the thickness of
> >the shield to reach the victim. An equivalent shield in a PCB would have
> >to be in the Z axis, such as a via fence.
> >
> >Now, look at what happens with a shield or guard trace on a PCB. It
> >doesn't surround the victim. What it does is to bring another chunk of
> >wave guide closer to the victim. That increases coupling from the
> >aggressor because unlike the coax shield where the aggressor fields have to
> >penetrate the shield to reach the victim, the parallel shield is itself a
> >coupler to the victim. Now if we don't attach the shield to anything, (
> >the evil of floating metal ), it becomes an increasingly efficient antenna
> >at rising frequencies until the point that it becomes a quarter wave
> >resonator. So, to make it an inefficient antenna and to prevent it from
> >resonating, we need to drive a bunch of vias through it spaced
> >substantially closer than one quarter wavelength of any strong harmonic in
> >the aggressor.
> >
> >Eric points-out, as has Dr. Johnson in his first book, that by the time you
> >allow enough space for the via clearances, and assuming you add enough
> >vias, the improvement in isolation is slight compared to what you get by
> >leaving the guard trace out. But if you are going to pay for those vias,
> >do yourself a favor and build a via fence inside of a cavity where both
> >planes are the return for the aggressor. This effectively builds a coax
> >shield around the aggressor. Leave the parallel trace out.
> >
> >Steve.
> >At 04:54 AM 8/24/2005 -0700, Bi Han wrote:
> > >Green:
> > >
> > >I agree that shielding could cause problem in "capacitive coupling case".
> > >However, if the inductive coupling dominate, shieling should help a lot.
> > >
> > >Since the current will mainly return in adjacent shieling trace
> instead of
> > >"relative" faraway victim trace, most of inductive coupling should be
> > >shielded. It will not be reflected in mutual inductance matrix directly,
> > >but will show its impact after matrix reduction.
> > >
> > >thanks,
> > >Han
> > >"Lynne D. Green" wrote:
> > >Hello, Hermann,
> > >
> > >Shielding can cause new problems. A shield trace can pick up
> crosstalk, and
> > >then couple that crosstalk onto a third trace. I know of at least one
> > >design that failed in this manner. The secondary coupling was in an
> > >unexpected area of the board, making it hard to debug.
> > >
> > >There are two common shielding approaches. First, one could remove the
> > >shield trace and leave the larger trace spacing in place (decreasing the
> > >capacitance between traces). Second, one could add enough vias (to the
> > >desired DC voltage) to make sure the shield does not conduct crosstalk
> noise
> > >very far.
> > >
> > >Good SI tools can be used to check the crosstalk conduction and secondary
> > >coupling.
> > >
> > >Best regards,
> > >Lynne
> > >
> > >PS: more history on GND: Breadboard circuits had much higher
> impedances in
> > >the circuit that in the connecting wires, so students (and junior
> engineers)
> > >treated all points along the wire as GND. Given this earlier training,
> > >learning distributed transmission line theory was very difficult for
> most of
> > >them.
> > >
> > >
> > >
> > >"IBIS training when you need it, where you need it."
> > >
> > >Dr. Lynne Green
> > >Green Streak Programs
> > >http://www.greenstreakprograms.com
> > >425-788-0412
> > >lgreen22@xxxxxxxxxxxxxx
> > >
> > >
> > >-----Original Message-----
> > >From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> > >Behalf Of hermann.ruckerbauer@xxxxxxxxxxxx
> > >Sent: Tuesday, August 23, 2005 2:14 AM
> > >To: a.ingraham@xxxxxxxx; nikitanivan@xxxxxxxxxxx
> > >Cc: si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Should the signals always return back through GND
> > >
> > >=20
> > >
> > >
> > >Hi Andy,
> > >
> > >Good answer!
> > >
> > >That points me to one question that I have had quite a while ago ...
> > >Just from a high level point of view I would try to get the same current
> > >returen for DC and AC. So If I do have a high level terminated signal
> (e. g.
> > >to VDD) I would try to do the referencing/shielding also with VDD to
> avoid
> > >any referencing/return crossing even if it is just between DC
> > >and AC (btw. What is DC what is AC in this discussion ;-) )!
> > >
> > >Of course I might get some disadvantages by doing so. Usually the most
> > >stable signal is called GND (whatever it really is ...). So any
> referencing
> > >to VDD might disturb my signal by talking to it due to any noise on the
> > >reference!
> > >
> > >What is your/the groups opinion on this ?
> > >
> > >Thanks
> > >
> > >Hermann
> > >
> > >=20
> > >
> > >
> > > >
> > >
> > >-----Original Message-----
> > >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > >On Behalf Of Andrew Ingraham
> > >Sent: Monday, August 22, 2005 6:24 PM
> > >To: nikitanivan@xxxxxxxxxxx
> > >Cc: si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Should the signals always return back through GND
> > >
> > > > Its always said that GND acts as a return path for a signal.
> > >
> > >Really? I wasn't aware of this. If everyone says that where you come
> from,
> > >then maybe you should educate your teachers and/or co-workers that
> they are
> > >wrong.
> > >
> > >What we call GND is often used as a reference point for VOLTAGE, but this
> > >doesn't mean it is where CURRENT flows.
> > >
> > > > Do all the signals know that they
> > > > have to return back through GND and not through anyother track in
> > > > the=20 signal layer having lower impedance? What are the factors taken
> > > > into=20 consideration to make sure that signals return back through
> GND.
> > >
> > >At low frequencies and DC, signal currents return to wherever they came
> > >from, to complete the loop.
> > >
> > >If you take a signal source and connect one end of the source to GND
> and the
> > >other end of it to a wire that goes off somewhere to some load, then
> the DC
> > >return path will have to get back to GND to complete that loop.
> > >
> > >If you take that same source and connect the first end of it to VDD
> rather
> > >than to GND, then the DC return path will have to end up at VDD and
> not GND.
> > >It might go by way of GND in order to get back to VDD, but that
> depends on
> > >the topology, the load, etc. The exact path (GND plane vs. VDD plane,
> etc.)
> > >that it takes, depends on the relative DC resistances and low frequency
> > >impedances of those paths.
> > >
> > >Thus, when the pull-up transistor in a pull-up/pull-down (or
> > >"totem-pole") pair is on, the return path (at DC and low frequencies)
> will
> > >have to get back to the VDD supply net that connects to that output
> driver.
> > >GND might not be involved at all.
> > >
> > >At high frequencies, signal switching current return paths for a wire
> or a
> > >trace, are by way of any and every conductor that is nearby, to which
> field
> > >lines can be drawn. Most of the high frequency current chooses the
> path(s)
> > >with the lowest impedance. The signal propagates via an electro-magnetic
> > >field in the dielectric, which just happens to cause currents to flow
> in all
> > >conductors that touch the E-M field.
> > >
> > >If a trace runs over a solid plane that connects to, say, some VTT
> voltage,
> > >then return current will be in that VTT plane. If a trace runs between
> two
> > >planes, the high frequency return current will be shared between both of
> > >them, regardless of what voltage each one is connected to. If a trace
> runs
> > >over one plane for several inches, then a different plane for several
> > >inches, and so on, then the return current will have to (or try to)
> find its
> > >way on each of these planes, in series.
> > >
> > >The DC path may take a totally different route. That is, the initial
> > >switching wavefront has a return path associated with it, determined
> by the
> > >E-M field around and between the conductors; but after a few
> nanoseconds the
> > >return current may take a different route, eventually determined by DC
> > >resistances and not the E-M field anymore.
> > >
> > >Regards,
> > >Andy
> > >
> > >
> > >
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