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Author Topic: what is an "internal noise filter", and how does it work ?  (Read 440 times)

jamesbob

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what is an "internal noise filter", and how does it work ?
« on: September 02, 2020, 11:41:27 PM »

Reading these forums it is clear that some modems have an internal (analogue?) noise filter, and that it can result in faster sync.

For example, it is reported that the Zyxel B10A have one but the cheaper B10D don't.

What is this noise filer, and how does it work?

How do I find out whether other modems have it? Does the TP-Link VR2800 have one?

Is it distinct from the filters we in the UK need to split the DSL signal from the voice signal coming over BT's copper wire?

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johnson

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Re: what is an "internal noise filter", and how does it work ?
« Reply #1 on: September 03, 2020, 02:01:51 AM »

All xDSL modems will have some kind of filter on their input from the phone line, some inductors and capacitors to attenuate signals not used for ADSL/VDSL.

The frequency response/properties of these filters is a bit of a dark art, even PCB trace layout will effect how they perform.

We have noticed that the possibly better designed (and certainly more visible) input filter on VMG1312/8x24 seems to work better with noisy/problematic lines.

There is an old thread which discussed this in more depth, but I'm struggling to find it.
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PhilipD

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Re: what is an "internal noise filter", and how does it work ?
« Reply #2 on: September 03, 2020, 09:28:49 AM »

Reading these forums it is clear that some modems have an internal (analogue?) noise filter, and that it can result in faster sync.

For example, it is reported that the Zyxel B10A have one but the cheaper B10D don't.

What is this noise filer, and how does it work?

How do I find out whether other modems have it? Does the TP-Link VR2800 have one?

Is it distinct from the filters we in the UK need to split the DSL signal from the voice signal coming over BT's copper wire?

The filter works as common mode filter, some excellent background to this is at Panasonic filters

This is also where twisted wires as a pair become helpful.  One thing twisted wires do is allow any noise picked up in the signal to affect both wires equally.  By using a simple filter as discussed here, it is possible to remove (well reduce) noise where that noise is exactly the same on both wires, leaving only differences between both wires to pass through, which in this context is the DSL signal.

What was seen is that some modems contained a common mode filter internally, this is very close to the input of the DSL signal into the chips, which is the best place to have one, as of course if it is in the telephone socket as a filter, you have a metre or so of wire that could pick up more noise before the signal gets to the modem.

Of course nothing gets given for free, the cost of passing the signal through the filter, which is nothing more than a coil of thin wire, is it attenuates the actual signal you want to keep, so if it benefits an application or not depends on the rest of the circuit design.

Regards

Phil

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