×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Filtering noise on tip ring line

Filtering noise on tip ring line

Filtering noise on tip ring line

(OP)
I have a floating tip ring line that is fed into a transformer on my telephone device.  The output of the transformer is referenced to ground.  I can filter the noise from the output side with an active bandpass filter.  However...

My question is:  How do I put the filter(s) on the floating side without messing up the line balance?

RE: Filtering noise on tip ring line

You could either use a passive filter or an active one with symmetric differential input and an insulated (low capacitance) PSU.

You need to careful about idle current and return loss as well. It is not an easy thing to do if you haven't done it before. The phone companies are kind of picky when it comes to adding non authorised equipment to the line.

Even if you put a filter on the transformer secondary, you need to make sure that return loss is OK. Primary impedance usually gets upset when you terminate the transformer other than it was designed for.

Gunnar Englund
www.gke.org

RE: Filtering noise on tip ring line

(OP)
Thanks.  I was a little weary of shunting the tip and ring lines to ground with caps, thinking about ground loops.

I wonder if just bridging from tip to ring using capacitors or inductors (for 60Hz noise) would work?  The signal is FSK.  I know the 60Hz is common mode, and should cancel, but it's there on the output.

RE: Filtering noise on tip ring line

Normal telephone band-width is somewhere between 300 Hz and 3400 Hz. So a capacitor that takes 60 Hz away will kill your FSK signal as well. An inductor may help, but it would represent a path for the DC current that probably upsets the station relay (assuming that we are talking about a PSTN aka POTS).

Why do you think that 60 Hz is a problem? Shouldn't be, considering the 300 - 3400 Hz bandwidth.

Gunnar Englund
www.gke.org

RE: Filtering noise on tip ring line

Nominal telephone VOICE bandwidth may be as stated, but don't forget about (for example) the AC ring voltage which can be 20 to 40 Hz and perhaps 90 VAC. In other words, think about all the other signals riding on the phone line besides just the voice before you start adding filters.


RE: Filtering noise on tip ring line

Yes, that's absolutely right. We have, for instance, a 25 Hz 110 V ring signal. It is not a nice experience to "hold the line" when the ring signal arrives.

Gunnar Englund
www.gke.org

RE: Filtering noise on tip ring line

I remember many moons ago as a kid hanging out at a friend's house on the back porch (screened in).  We were tasked with washing down the astroturf, so out came the garden hose and off came the shoes.  Now I was smart enough to stay away from electrical outlets, but what kid would think to stay away from a phone jack.  It was a mild tingle that we kept shooting the phone jack for, but it was years before I knew enough about POTS to understand where the tingling came from.

Sorry, just a side story this reminded me of... :)

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Filtering noise on tip ring line

(OP)
It's one modem to another using a dedicated twisted pair, and is on all the time.  I think the higher freq noise is riding in on 60Hz.  This is because you can attenuate it just enough with an additional transformer (with less bandwidth, no DC; i.e. cheaper), and it seems to help, or at least move the noise.  However, I don't think that's a consistent enough solution.

RE: Filtering noise on tip ring line

If the 60Hz is a differential signal on the balanced line before the transformer, then I don't think you can get rid of it very easily.  The transformer should get rid of common mode interference as you said.  If however it is induced through that ground connection on the secondary side then that's where you start looking.  A LPF is not an option so you need to look closely at balancing  techniques.

RE: Filtering noise on tip ring line

(OP)
Thanks.  I did try lifting the drain wire to the gnd, but no help.  In about a week, I'm going to try DC blocking caps like skogsgurra talked about on a different thread.  Although, I don't know how they will impact CMRR.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources