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[timdf911]

Bit of a long shot, but was wondering if anyone had done tests on the immunity of VDSL2 modems to HF transmissions ?

I'm in the UK and currently have a Huawei HG612 modem on BT Infinity 2 profile 17a and it provides 66M down and 12M up - but only when I'm not transmitting on the lower HF bands.

I've done all the usual stuff to protect it from RF - like torroids on all the cables going to it, twisted phone drop wiring etc.

I suspect it's just a case of the telephone line being too close to my antenna.

I'm thinking about buying a Draytek 2850n or a Fritzbox on the remote chance they have better immunity - but as they no doubt have similar modems I suspect I maybe out of luck !

Any ideas / suggestions ?

I've read quite a few threads on this topic here - but nothing about immunity between brands.

In theory if the telephone line was correctly balanced the modem would be fairly immune to common mode interference - however this is the real world and there's like some imbalance, despite my master socket being close the end of the drop line and my modem only inches from it.

I'm thinking I might have measure how much differential interference is leaking in and then try and balance it out !

Regards Tim

PS Great forum !

[Black Sheep]

Hi and welcome.

You could have the best balanced cable in the land, but if the 'noise' is higher than the received signal, you are going to be affected. I'm only talking from a phone technicians POV, and the standards we have to work to (ie:SIN349). There are a few resident experts on this kind of thing, and I'm sure they'll be along shortly to advise. 

[JGO]

Take it as read that ALL consumer equipment is designed on the assumprtion that NO other spectrum user exists !  So Best Modem will be Least Bad - look at the history of TVI.
This isn't to say something can't be done but don't rely on the manufacturers, most of them think the spectrum is empty except for their device.

Cancelling the interference as  you are suggesting is possible and indeed is routine for CWFM radars. It will need an automatic amplitude and phase balance to maintain an adequate null but that should be much easier at HF than say 3cm !  The ideal way would be to build a modem with this in mind but it may be simpler to make a "out phasing unit" to feed an out of phase sample into the VDSL and then feed the cleaned up VDSL to the existing modem. 100% screening would be needed and probably it would be useful to enclose the modem in it too, since the process assumes linearity i.e. if you are blasting the modem into cross modulation you can't cure that !

This sounds a useful but lengthy  project so I would be inclined to see what the RSGB and possibly the ARRL know about it first.

Alternativly, depending on your VDSL spectrum,  notch filters for the particular bands  may  notch the interference (and some VDSL) but leave sufficient VDSL bandwidth  (real bandwidth in Hz ! )  for the reduction in speed to be acceptable.

Roll on FFTP !