[SI-LIST] Re: Telephone Circuit EMC issues
- From: "John Matthews" <John.Matthews@xxxxxxxxxxxxxxxxx>
- To: "Bill Wurst" <billw@xxxxxxxxxxx>, "Saril" <saril_k@xxxxxxxxx>
- Date: Thu, 08 Jan 2009 18:12:14 +0000
We found that using capacitors between T/R and board grounds caused problems
in that noise from the board coupled onto the line. A common mode choke at
the output is better.
Most modems will have an isolation barrier and noise from the main circuit,
such as power supply harmonics, can couple across to the isolated part of
the circuit, from which it finds its way onto the phone line. A 100pF X
capacitor connected between the isolated section ground plane and the main
ground plane can help, as it recirculates the noise back into the main
ground. This has helped us on two occassions, once in our V.90 modem and
another time in an ISDN phantom power circuit.
rgds
John
-----Original Message-----
From: Bill Wurst <billw@xxxxxxxxxxx>
To: Saril <saril_k@xxxxxxxxx>
Cc: si-list@xxxxxxxxxxxxx
Date: Wed, 07 Jan 2009 16:01:01 -0500
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
>
>
> Download prohibited? No problem. CHAT from any browser, without
download. Go to http://in.webmessenger.yahoo.com/
> ------------------------------------------------------------------
> 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 technical documents are available at:
> http://www.si-list.net
>
> 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 technical documents are available at:
http://www.si-list.net
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 technical documents are available at:
http://www.si-list.net
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:
