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Author Topic: Old DSL phone filters good for phone and answering machine RFI?  (Read 6825 times)
WA2ISE
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« on: March 23, 2014, 03:16:26 PM »

It looks like those DSL filters meant for use on old style telephones (to keep the DSL signals out of the phone) may also keep our RF out of those phones.  These are low pass filters where the stopband is around 100KNz and above, around 60dB down.  They look like this:

DSL signals ranged from about 100KHz to around 1.5MHz, and if I understand this below diagram right, the filter also attenuates above that. 

from http://www.emfields.org/filters/adsl.asp though you should be able to find these things at garage sales and hamfests for almost nothing. 

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WX7G
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« Reply #1 on: March 25, 2014, 05:18:28 AM »

That is a differential-mode filter and might not be the most effective for RFI.

RFI is primarily common-mode. The filter will exhibit a degree of common-mode attenuation by it's CM inductance of 8.4 mH working against the phone TIP-to-GND and RING-to-GND capacitance. However, without a common-mode inductor there will be common-mode to differential-mode conversion and this could make things worse than no filter at all.
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WA2ISE
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« Reply #2 on: April 03, 2014, 01:50:11 PM »

Well, http://support.2wire.com/index.php?page=view&article=135 says it might help RFI from AM radio stations (and maybe ham radio stations).

"Reasons for Using DSL Phoneline Filters

If you have a home phoneline network adapter (HomePNA) connected to your standard telephone wiring, 2Wire DSL phoneline filters preserve the quality and clarity of telephone conversations. The DSL filters help whether you have DSL, HomePNA, or both. There are also instances where AM radio stations interfere with telephone conversations; the 2Wire DSL filter also clears up this disturbance as well."

Looking at the guts of one of these, the coils L1 and L2 are on the same ferrite core, and L3 and L4 are on another ferrite core.  Of course it matters which way the coils are phased, but if in the right way, they should do common mode rejection.  Anyway, it would be quick to see if these filters would work or not. 
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WX7G
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« Reply #3 on: April 03, 2014, 02:12:47 PM »

To filter DSL out of the phone yet present a high impedance to DSL signals L1-L2 is phased as a differential-mode inductor. Being wound on one core it has essentially zero common-mode inductance. With the two caps placed as they are, and with the steep roll-off, L3-L4 is also a differential-mode inductor. This filter should be quite ineffective against RFI.

The FCC (part 68) requires telephones to be immune to 3 V/m common-mode signals from 150 kHz to 50 MHz. And they encourage, but don't require, extra immunity in the AM broadcast band.
« Last Edit: April 03, 2014, 02:28:35 PM by WX7G » Logged
WA2ISE
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« Reply #4 on: April 04, 2014, 01:15:03 PM »

To filter DSL out of the phone yet present a high impedance to DSL signals L1-L2 is phased as a differential-mode inductor. Being wound on one core it has essentially zero common-mode inductance. With the two caps placed as they are, and with the steep roll-off, L3-L4 is also a differential-mode inductor. This filter should be quite ineffective against RFI.


Duh, of course.  The DSL signal is a differential signal on the phone line twisted pair, and RFI would mostly not be.  Well, so much for that.   Embarrassed   
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WX7G
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« Reply #5 on: April 04, 2014, 01:32:34 PM »

Just a couple of weeks ago I did some consulting on a new telephone product that was failing FCC part 68 immunity. From that job I learned a bit about the various regulations and how these problems have been overcome. Apparently AM BC interference to telephones has been a problem for years and that's why the FCC wants good immunity in that band.
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WA2ISE
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« Reply #6 on: April 05, 2014, 01:09:45 PM »


With the two caps placed as they are, and with the steep roll-off, L3-L4 is also a differential-mode inductor. This filter should be quite ineffective against RFI.


I pried apart the filter, and I reversed one of the two coil connections on each coil form, that should convert it from differential mode to common mode. And maybe do something with possible RFI.  One coil is a pair of 1.5mH coils on the same ferrite core (with 0.015uf caps), the 2nd one is a pair of 2.4mH coils on its own ferrite core (with 0.012uf caps). But at this point, it's likely more trouble than it's worth...
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