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Author Topic: xDSL Filters  (Read 23567 times)

burakkucat

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xDSL Filters
« on: June 09, 2012, 03:02:23 AM »

Quite recently, Asbokid provided me with a link [1] to an interesting document. After reading it, I decided to examine the "FUN-JIN" branded plug-in microfilter that TalkTalk had given me, when I first subscribed to their service five years ago. That microfilter was only used for a short period of time, when it was replaced with a filtered faceplate fitted at the NTE5/A -- an NTE2005.

Below are four images that I managed to capture with my flat-bed scanner. Notice the severed green wire? That was exactly how it was received. It had clearly been done as an afterthought, in the factory, maybe?

So there are four wires, yellow, green, red & black, that originate from the plug. The green and red wires just connected the corresponding pins of the plug and the (telephone) socket together. In terms of IDC#s, of which we are all familiar, they are IDC3 and IDC4 -- the "ring" wire and the earth wire. The severed, green, is the "ring" wire and the red, the earth wire. The other two wires, yellow and black, are IDC2 and IDC5 (the B- & A-wires) respectively. The yellow and black wires are connected directly to the inner pair of the RJ11 socket. From there, they then continue to the PCB (the filter circuit) and then to corresponding pins at the telephone socket.

From the above, I deduce that there is just a low-pass filter (for frequencies from 0 up to about 3.5 kHz) between the line and the telephone. After a moments thought, I assume that every modem/router has a band-pass filter for ADSL/ADSL2/ADSL2+/VDSL2 frequencies, as appropriate, which block the low audio range and those radio frequencies above the top of the VDSL2 range.

The low-pass filter in the "FUN-JIN" device consists of three components. Two chokes (wound on a common core) and a capacitor. As of yet, I have not determined the exact circuit due to "gunk" on the PCB!

More to follow, once I've de-gunk'd the PCB.  ;)

[1] http://www.rsars.org.uk/ELIBRARY/BBCONNECT.pdf
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roseway

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Re: xDSL Filters
« Reply #1 on: June 09, 2012, 07:42:37 AM »

This is an interesting investigation. After reading the linked PDF I was reminded that ADSL Nation claim that their XF-1e filter and XTE-2005 faceplate have a sharper response because they are active filters, so it would also be interesting if someone with the appropriate gear could test their frequency response.
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  Eric

c6em

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Re: xDSL Filters
« Reply #2 on: June 09, 2012, 12:58:57 PM »


The current filters put the entire spectrum through to the router.
So the router's connection from the filter is direct connected to the incomming phone line and is totally unfiltered.

The document in the link details an experiment to pre-filter the incoming phone line.  So that what gets though to the normal internal faceplate filter (or the ADSLnation/BT type dangling ones) is cut off above the ADSL1/2/2+ upper frequency limits.
Isn't this what the RF3 pre-filter is intended to do? - where its fitted and where there is a problem with interference from frequencies higher than the upper limits of V/ADSL on to the ADSL line - such as RFI (though goodness knows where the RF3 starts cutting off)
If so this would suggest that the filtering inside the router for these higher frequencies is not so hot - I suppose its all down to cost in the manufacture/design process.
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burakkucat

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Re: xDSL Filters
« Reply #3 on: June 10, 2012, 02:34:23 AM »

I haven't had the time to "de-gunk" the PCB, so that little (in both senses of the word) mystery remains.

Having read both Eric's and c6em's posts, I guess we need to find someone who could provide a loan of both an ADSLNation XTE-2005 filtered faceplate and a BT80B-RF3. Somehow I think Walter will be that person.  ;D

Next, we need to consider who would have the technical expertise and ability to perform the analysis. We need an electronics wizard. Eric would be an ideal -- if he still had access to suitable test equipment. So my thoughts turn to Asbokid:-\

To be continued . . .
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asbokid

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Re: xDSL Filters
« Reply #4 on: June 10, 2012, 08:28:37 AM »

Hi, no I haven't got the kit, sorry.  I've got a DSO but to perform proper tests on the DSL filters, it would also require a signal generator to sweep up from voiceband frequencies right through to 30MHz, while measuring the response on the scope.

All the same, that's a very interesting article from Stephen Wilson, G3VMW, formerly "engineering manager in charge of telecommunications for Yorkshire Electricity". It was generous of him to put it into the public domain.   Has anyone encountered him on the internet?

Though it's not clear how a better filter would alleviate many of problems from amateur radio interference.  Wilson talks about 160m band, 80m band, right down to the 10-metre band.

160m is around 1.8MHz to 2.0MHz. So that's at the top end of the ADSL2+ bandplan.
80m is around 3.5MHz to 4.0 MHz.  That's going to interfere with VDSL2 signals in the DS1/US1 bands.
10m band goes right up to 30MHz. Again, VDSL2 (but only Profile 30) signals.

Ham radios have very narrow channel widths, so interference should only knock out a few DSL subcarriers.  Filtering out voiceband frequencies *and* frequencies above 2.2MHz (the top end of ADSL2+) is all fine and dandy (for ADSL2+).

But any ham radio transmissions on a frequency that falls within the DSL bandplan (e.g. a 1.8MHz signal from the 160-metre band), by definition, cannot be filtered out on the subscriber line.  To do so would filter out the affected subcarriers from the DSL signal itself.

As the rollout of VDSL2 slowly becomes more universal, the problem of RFI from radio ham kit can only become more evident.

Mr Wilson is very fortunate.. To put up with internet disconnections every time he keys up, he must be blessed with very tolerant neighbours!

It's surprising that no independent tests have been published on DSL filters and faceplates.  It's surely not a difficult set of tests to perform.  And as a framework for the tests, there is already a BT 'SIN' (346) specifying how DSL filters should perform.

cheers, a
« Last Edit: June 10, 2012, 11:28:42 AM by asbokid »
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c6em

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Re: xDSL Filters
« Reply #5 on: June 10, 2012, 06:53:40 PM »


You could always try asking ADSLnation for the data on their own filters
I'm sure there must have been some certification done on them to comply with a BT SINE standard before being approved for connection into the UK telephone system.  It seems likely, given their function, that a full spectrum analysis would have been done.

They may claim its proprietary information - but it's not like you are asking for part numbers of the components and the data could be obtained with the right equipment anyway.
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Blackeagle

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Re: xDSL Filters
« Reply #6 on: June 10, 2012, 09:53:13 PM »

Front of the ADSLNation PCB



And the rear

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burakkucat

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Re: xDSL Filters
« Reply #7 on: June 10, 2012, 10:32:16 PM »

 :hmm:  Hmm . . . I assume you are now busy drawing out the schematic diagram for us?  :P
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roseway

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Re: xDSL Filters
« Reply #8 on: June 10, 2012, 10:54:46 PM »

According to the ADSLNation site, it contains:

1 Spark Gap
4 Ferrite cores
5 Diodes
5 Capacitors
7 Resistors
2 Transistors

(But that's probably obvious :) )
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  Eric

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Re: xDSL Filters
« Reply #9 on: June 11, 2012, 08:43:30 AM »

Interesting stuff.  I've always been a smidgen sceptical of 'wonder-filters', but I'm open-minded too.

I could've sworn there used to be a web page somewhere (maybe adslnation's own site) that published schematics for various filters, including the adslnation 'active' filters.  But I'm darned if I can find it.

Anybody else remember seeing such a thing, or shall we put it down to a 7LM hallucination?
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les-70

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Re: xDSL Filters
« Reply #10 on: June 11, 2012, 09:16:15 AM »

  I have always been under the impression that a face plate filter, or another filter if connected effectively as the face plate filter (with all other telephone wires on the phone side of the filter) is easy to test. 

Basically disconnect all of the telephone side so you just have the adsl/vdsl socket straight to the router (i.e. nothing to filter) and note the statistics (they should be the same as on a direct master test socket connection) or better monitor continuously with routerstats or dmt.  Then add back the telephone side to the filter while keeping connected.  If nothing changes then the filter is good enough.  With the sync fixed, if the SNR drops a better filter may help.  Infrequent noise sources are as always harder to test. A Pressac face plate passed this test for me with no discernible drop even with a phone call. An Adslnation filter was nearly as good but totally failed after a while! Other filters I had gave a drops of between 0.2 and 0.6 db in SNR on my wiring.

As an easier test which I have not tried---Possibly not messing with the basic wiring but monitoring the SNR and connecting to the telephone side with a long length of un-twisted or separated pair would simulate adding a bad home wiring situation. Equally some phones are good test noise sources and when a call is made or answered will impact the SNR with a poor filter.  A phone test is good confirmation that filter is good but if the phone test fails the line may be faulty.
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roseway

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Re: xDSL Filters
« Reply #11 on: June 11, 2012, 09:33:07 AM »

I could've sworn there used to be a web page somewhere (maybe adslnation's own site) that published schematics for various filters, including the adslnation 'active' filters.  But I'm darned if I can find it.

There are a couple of schematics at the bottom of this ADSLNation page but that's all I've seen.
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  Eric

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Re: xDSL Filters
« Reply #12 on: June 12, 2012, 02:51:19 PM »

So its basically a tuned circuit amplifier (really a switch in this instance) with some negative feedback then. ie one of the first transistor circuits every electronics engineer learns.

I can't see that doing much in terms of sharpening the response curve - it will simply remove some of the rolloff compared to a passive filter. Its not going to improve anything other than a faulty dect baseunit/early sky box and even then you'd probably do much better putting two passive filters in-line.

I can see an additional and quite unnecessary extra failure mode though - ie the transistors. The soldering on those pictures is appalling as well :)
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sevenlayermuddle

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Re: xDSL Filters
« Reply #13 on: June 12, 2012, 04:02:36 PM »

One other point about the transistorised circuits interests me, and that is where do they draw power from?

I was under the vague impression that it would not be legal to deliberately draw current from the line's DC voltage when the phones were quiescent, or for any other purpose than powering the phones when a handset is taken off-hook? 

Otherwise, we'd have 'LED night lights' and all manner of naughties plugged into phone sockets.   If that would be legal, then perhaps 'Dragons Den summons'   :D
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sheddyian

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Re: xDSL Filters
« Reply #14 on: June 12, 2012, 06:49:22 PM »

My experience of using ADSL Nation faceplate :

                                 Attenuation
                                    Up      Down

D-Link filter [1]               11.5     22
ADSL Nation faceplate      11.4     22

Though it also tidied up my bodged wiring and resolved the DSL noise on voice phone problem I've discussed in another thread.

So it did make a slight difference, but not much.

[1] D-Link filter as supplied with D-Link DSL-2780 modem from Talk-Talk. Exterior build quality looks good, I've never opened it up though.

Ian
« Last Edit: June 12, 2012, 06:53:29 PM by sheddyian »
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