[SI-LIST] Re: Remove Ground underneath Differential signal is deserved or not?
- From: pwelling@xxxxxxxxxxxxxx
- To: binchuan2000@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Sat, 27 Jul 2002 11:17:15 -0600
Hi,
One other thing to consider is that you can route against a plane or sets of
planes (Stripline structure) if the planes are quiet. For critical (both
functional and EMC requirements, spectral content considered), I will route
the pair against Chassis Ground Planes as opposed to Signal Ground Planes.
You need to remember that the reference plane is PART of the impedance of
the traces - even differential traces. If the plane is noisy, some of that
noise will be impressed onto the high impedance differential traces.
Often a product will fail Radiated Emissions when the pair is routed against
Signal Ground (Digital reference planes) they transfer the broadband noise
to the connector which exits the box on cables as common mode noise. The
broadband digital noise may be oscillator noise, bus transaction noise,
poorly decoupled power noise, etc. There are opportunities to reduce this
noise.
The common mode noise (unless it is dealt with at the connector egress and
returned back to Chassis Ground) will re-radiate off of the cable and may
fail emissions depending on what spec. you are testing to. If you do not
provide a "preferred path" for the common mode noise to go, it will find its
way to the lowest point of impedance to ground - usually by splitting and
going where you least want it to go.
By routing the traces against a quiet reference plane, the common mode
digital noise is usually much less, and it will be easier to meet your EMC
requirements. It will also help with Susceptibility and ESD.
Be sure to provide a method to connect Chassis Ground and Signal Ground
together on your board as well. But that's another topic.
Good luck,
Philip Ross Wellington
Mgr. Signal Integrity & EMI
L-3 Communications CSW
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of binchuan2000
> > Sent: Thursday, July 25, 2002 7:20 PM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Remove Ground underneath Differential signal is
> > deserved or not?
> >
> >
> >
> > I am confused that I read some book about remove Ground under
> > Differential could reduce the induced Current from
> Differential pair,
> > but the question is how we could control its impedance(Differential
> > impedance) although Differential doesn't reference to Ground. If I
> > follow this content to REMOVE Ground under Differential pair how
> > could I control its Zodd impedance and its Zdiff impedance. Does it
> > will generate any other problem and if it does, why some textbook
> > advise it?
> >
> > Remove Ground is worth or not for Differential signal ?
> >
> >
> >
> > Thanks,
> >
> >
> >
> > Binchuan.Wang
> >
> > ------------------------------------------------------------------
> > To unsubscribe from si-list:
> > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the
> Subject field
> >
> > or to administer your membership from a web page, go to:
> > //www.freelists.org/webpage/si-list
> >
> > For help:
> > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >
> > List archives are viewable at:
> > //www.freelists.org/archives/si-list
> > or at our remote archives:
> > http://groups.yahoo.com/group/si-list/messages
> > Old (prior to June 6, 2001) list archives are viewable at:
> > http://www.qsl.net/wb6tpu
> >
> >
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
