[SI-LIST] Re: Telephone Circuit EMC issues
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: <saril_k@xxxxxxxxx>, si list freelist <si-list@xxxxxxxxxxxxx>
- Date: Wed, 7 Jan 2009 13:43:45 -0800
Saril.
It seems to me that its not a good idea to filter the phone lines or connect
them via capacitors to the digital ground plane. I would try taking them out
and running the EMC test again to see if common mode noise has gone down. It
may take a few itterations like this to eventually get to the source of the
problem.
I think that where the 2 caps come together and get connected to Digital ground
needs to be disconnected and re-routed to an area near the ground where the
power enters the board. What is the ground signal for power that enters the
board? This is area where I would recommend connecting the decoupling caps
that also touch the phone lines.
One other thing to consider is putting new decoupling caps between the Digital
ground and the system chassis ground. This can also be done for the analog
ground plane to supress EMC noise.
Richard Jungert
> Date: Wed, 7 Jan 2009 16:01:01 -0500
> From: billw@xxxxxxxxxxx
> To: saril_k@xxxxxxxxx
> CC: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Telephone Circuit EMC issues
>
> Saril,
>
> You haven't provided much information to go on. If I understand your
> problem correctly, you have a conducted emissions problem which I'm
> assuming is measured at the power Line Impedance Stabilization Network
> (LISN). Then, when you connect the telephone line the conducted power
> emissions increase.
>
> I can only guess, but my hunch is that conducted emissions on the
> telephone line are radiating and being picked up by the power cord. You
> might try re-arranging the telephone cord to see if that has an effect.
> I doubt that the filter capacitors on the Tip & Ring could increase
> the common-mode noise on the digital side, but if not implemented
> properly (noisy ground connection and so forth) could certainly increase
> conducted emissions on the telephone line.
>
> Hope this helps.
>
> Regards,
> -Bill
>
> /************************************
> / William C. Wurst, PE /
> / billw@xxxxxxxxxxx /
> / Advanced Electronic Concepts, LLC /
> / www.aec-lab.com /
> ************************************
> ===============================================================> Saril wrote:
> > Hi All,
> > I'm working on a dial up modem board. There are some conducted emissions. I
> > have to do some wrok around to get it suppressed.
> > we have two capacitors (0.1uF)Â as shown below, connected from the TIP and
> > RING to the Digital ground.
> > Â
> > TIPÂ Â ------------------------
> > Â Â Â Â Â Â Â |
> > Â Â Â Â Â Â |
> > Â Â Â Â Â Â Â _ C1
> > Â Â Â Â Â Â Â |Â Â Â Â Â Â 0 Ohm Res
> > Â Â Â Â Â Â Â Â --------^^^^--------- DIG_GROUND
> > Â Â Â Â Â Â |
> > Â Â Â Â Â Â Â _ C2
> > Â Â Â Â Â Â Â |
> > Â Â Â Â Â Â |
> > RING ------------------------
> >
> >
> > These capactiors are provided as per the modem chip vendors advice. when
> > the telephone line is connected to the modem the emissions seems to be
> > increasing.
> > Â
> > Does this capactors have any role on increasing the common mode noise on
> > the digital side and causing the emmission to aggravate.
> > Â
> > Thanks in advance.
> > Â
> > Regards,
> > Saril Kaiprath
> > Hardware Design Engineer
> >
> >
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