Problem with ADSL filter

I'm having a problem with what I thought would be a simple minor project: A noise filter for a phone-ADSL combo line.

For some 15 years, I've been using separate landlines for voice calls and for ADSL+ internet. This is because I use a landline phone more than I do mobiles and I don't like the hiss on combo lines. It belatedly dawned on me that the noise must be caused by inadequate filtering at the phone/modem splitter. Sure enough, the one I have has just a single 1mH inductor and one 39nF capacitor, and those that I can find in the local market are no better.

So I thought I'd make a more effective micro-filter and, considering the wide separation between the <4kHz bandwidth for voice and >25kHz for ADSL signals, I expected it to be a breeze. I was wrong.

I thought I'd start with a simple one-stage LC filter to see what effect I get. As I don't stock inductors in the range I wanted, I wound one on a small ferrite toroid salvaged from a broken device. I made two 6mH windings, bifilar-wound on the same core, connected them anti-phase, one on each side of the phone line with a capacitor at the output side. Like this -



Well, the result is that the modem gets disconnected whenever I take the phone handset off-hook. I changed the phase of one inductor, tried various capacitor values, placed series resistors after the capacitor, removed one inductor to make an unbalanced LC filter. Nothing works.

I know there are more complex filters but I've also seen schematics of cheap commercial filters with configurations similar to mine. What am I doing wrong?

here I can give you the schematic of a french commercial adsl filter to compare to your.
good luck.

Core saturation due to line current could be a possible problem in off-hook state.

Kripton2035 said:

here I can give you the schematic of a french commercial adsl filter to compare to your.
good luck.

Click to expand...

Thanks for your interest. The basic idea behind that circuit is the same as mine. The smaller inductors are intended to spread out the effects of self resonance, but some some filters work without them. That is, as far as stability is concerned. Whether it filters out noise adequately is another matter.

FvM said:

Core saturation due to line current could be a possible problem in off-hook state.

Click to expand...

That could be it. I've already ordered some drum type inductors that wouldn't saturate easily. In the meantime, I'll see if I can scrounge up some old inductors with drum cores of suitable size.