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[boost]

Yep, I think it's been mentioned on here a few times

Problem is, I don't yet understand why it works so if I stick one on and it doesn't work, I will be furious with myself. Just heading myself off at the pass

[JGO]

OK Boost, you asked !

The primary use of a ferrite device is as a magnetic core with a known permeability and low loss, for making coils or transformers. There are design data sheet from the manufacturers, or for simple shapes it is a case of look  up the winding formula in a design handbook. The result is predictable within a few percent.
When leakage inductance also is important, as in RF3 etc it gets a bit more complicated as the data may not be available so a few experimental devices are needed.

The other use is to make a lossy coil. Ferrite loss depends on frequency so the loss will be low in the designed  range but  in the lossy frequency range you are on your own, since the ferrite will be designed to be low loss at least up to a specified frequency, but the onset of loss is an uncontrolled parameter. 

Such coils can reduce interference, by adding the equivalent of friction.
 
 One example was the FX1115, designed to thread on  transistor leads The aim was to have no effect on audio frequencies  up to say 20 kHz but to look lossy above say 100 MHz  where the transistor might do a parasitic oscillation.
Note the 5000:1 margin and that the device is about as long as an installed transistor'e leads so it can't move far.

In another case a ferrite ring was intended to attenuate RF on the outer sheathing of coax cable. In the lab it seemed OK but in the field it rapidly emerged that the loss was very dependent on exactly where it was on a long coax cable. Getting useful results was a laborious hit and miss process so the device was withdrawn.

These facts may explain why your contact won't reveal their design ?!

Unfortunately some traders advertise "a ferrite"  as a magic cure which suppresses interference at source.   It may reduce interference sufficiently for the particular case, but no way will it suppress the source;  shutting down Droitwich  would be frowned on or even next door's TV  !) 

   
 

[burakkucat]

[off topic]
Shutting down Droitwich or other UK based LW/MW broadcast transmitters would spoil my fun in attempting to deduce a fellow Kitizen's location from her/his QLN plot! 
[/off topic]

[boost]

OK Boost, you asked !

The primary use of a ferrite device is as a magnetic core with a known permeability and low loss, for making coils or transformers. There are design data sheet from the manufacturers, or for simple shapes it is a case of look  up the winding formula in a design handbook. The result is predictable within a few percent.
When leakage inductance also is important, as in RF3 etc it gets a bit more complicated as the data may not be available so a few experimental devices are needed.

The other use is to make a lossy coil. Ferrite loss depends on frequency so the loss will be low in the designed  range but  in the lossy frequency range you are on your own, since the ferrite will be designed to be low loss at least up to a specified frequency, but the onset of loss is an uncontrolled parameter. 

Such coils can reduce interference, by adding the equivalent of friction.
 
 One example was the FX1115, designed to thread on  transistor leads The aim was to have no effect on audio frequencies  up to say 20 kHz but to look lossy above say 100 MHz  where the transistor might do a parasitic oscillation.
Note the 5000:1 margin and that the device is about as long as an installed transistor'e leads so it can't move far.

In another case a ferrite ring was intended to attenuate RF on the outer sheathing of coax cable. In the lab it seemed OK but in the field it rapidly emerged that the loss was very dependent on exactly where it was on a long coax cable. Getting useful results was a laborious hit and miss process so the device was withdrawn.

These facts may explain why your contact won't reveal their design ?!

Unfortunately some traders advertise "a ferrite"  as a magic cure which suppresses interference at source.   It may reduce interference sufficiently for the particular case, but no way will it suppress the source;  shutting down Droitwich  would be frowned on or even next door's TV  !) 

 

Love your reply but you are still speaking at a technical level which is so far above my understanding.

Lossy?
The context of permeability?
Leakage inductance?
Parasitic oscillation??

I'm not suggesting you attempt to explain further or come down to my level (please do if you have the time/inclination) but these words are so completely out of context to what I deal with on a day to day, I can't even begin to 'fill in the gaps'

I think, what we need on here, are some stickies? You all probably have read this stuff so many times, it's second nature type thing. Us noobs, however, are reading this stuff with *wtf* style expressions

This stuff is absolutely key, IMHO, so I really would like to understand it better.

Hopefully this doesn't come across as ungrateful. The alternative is the forum equivalent of 'nod and smile' which I absolutely abhor because no one wins.

Thank you

[JGO]

OK Boost,

Thanks for making your position clear, it is always a problem at what level to pitch things with a complete stranger.

Firstly suggest you have a look in your local library for an Amateur Radio handbook, preferably a RSGB one but the ARRL versions are OK. These explain the theory but always with half an eye on practical applications, including getting rid of interference. "O" level Physics covers much the same ground, but it isn't always practically based or, sometimes,  accurate.

Now about what ferrites are good for in Broadband.

1) Making coils of a specific inductance for a filter. (I've only heard of one instance of this and maybe a RF3 would have done as well !).  This is to reject interference picked up on the 'phone wire, which implies higher frequencies say 1 MHz plus, so smaller coils. This is fairly straight forward - follow the recipe.

2) Making coils for a power line filter. These tends to apply to lower frequencies including electric i.e. not electronic sources. As it has to pass 50Hz but clobber anything above about 10 kHz the coils are larger so ferrites are essential to make the number of turns manageable.  Even so you may be in for say 100 turns. For this reason and electrical safety, my preference would be to buy.

3) Clip on ferrites seem to be aimed at the second example in my last post, where it is a case of minimum modification to an existing setup. This worked to a degree at say 20 MHz, but at say 10 kHz for a power line filter, it would need to be scaled up by 2000. As it is only a one turn "coil" the ferrite will have to do a miraculous job !
 

 

[guest]

I'm fairly certain we had this discussion a couple of years back (treadmill splattering noise around) & I was under the distinct impression that people realised that designing a functional RF filter (ie one which filters the desired noise but doesn't attenuate the ADSL signal) is not simply a case of slapping a ferrite on the outside of a cable?

The noise will usually be induced via the mains rather than via the phoneline (ADSL front-end is differential so a ferrite will do bugger all IMHO unless your home wiring is garbage).

As such you'd be looking at an in-line mains filter like the one we identifed for the guy with the noisy treadmill. In fact I actually had to buy another of these recently as a second-hand PS3 caused enough mains noise to drop my VDSL sync by ~10%. Two weeks later the DLM reinstated fastpath and 79987kbps sync.

Unless you actually have filter design experience any bodge you try (& a single ferrite IS a bodge in this instance) is unlikely to work, especially when you have no test equipment to verify the response curve. I DO actually have filter design experience from LF to 80GHz+ & there's no way I'd be buggering around with a ferrite

tl;dr buy something properly designed for the job.

[funkydan]

Like boost I am not that technically minded in these matters.
This is a link to an home-made REIN filter. The writer claims it helps but does not provide any proof that it does work.

http://www.wirelesswaffle.com/index.php?m=02&y=12&entry=entry120211-055210

 I have found the ferrite ring on ebay. Before I buy it would some more technical members give their views on whether it is a viable solution or just mumbo jumbo?

 

[guest]

What you linked to there is a double-pole inline filter & the guy has obviously checked the response curve.

A clip-on ferrite (assuming that's what we're talking about) is IMHO going to do absolutely bugger all given the differential nature of ADSL/VDSL.

Worth noting that any of the filters shown in that link will have an extremely adverse effect on VDSL (FTTC) services.

[funkydan]

What you linked to there is a double-pole inline filter & the guy has obviously checked the response curve.

A clip-on ferrite (assuming that's what we're talking about) is IMHO going to do absolutely bugger all given the differential nature of ADSL/VDSL.

Worth noting that any of the filters shown in that link will have an extremely adverse effect on VDSL (FTTC) services.

Thanks for the reply rizla.
Sorry I should of said I have an ADSL2+ service.
I have tried clip on ferrite beads on my internal telephone line and RJ11 cable between my telephone socket and router and you are correct that it made no difference at all.

 

[boost]

Thanks for the posts chaps. Some epic info that has already helped my understanding, I think!

Something that has become abundantly clear, however:

[JGO]

Like the picture !

What you linked to there is a double-pole inline filter & the guy has obviously checked the response curve.

Worth noting that any of the filters shown in that link will have an extremely adverse effect on VDSL (FTTC) services.

I agree with your last statement 99% of the time, but at times one is forced into sacrificing some signal along with interference to improve the S/(I+N) ratio - like jettisoning cargo to stay afloat.
Filters are a bit like medicine; what cured Joe's loose bowels will do no good to Fred's constipation !!   

[roseway]

I think I'll add that last line to my collection of quotable quotes.

[guest]

Aye but look at the rolloff of the filters - it'll massacre a 17a profile

Frankly an active filter (simple op-amp circuit) would work better for specific interference spikes on xDSL. That way you could tune the notch filter in nice & tight with a pot. Could probably run it off the line voltage too. Just have to watch the gain so you don't start distorting the xDSL signal or increase insertion loss.

Probably not for the average user though

[JGO]

Aye but look at the rolloff of the filters - it'll massacre a 17a profile

Frankly an active filter (simple op-amp circuit) would work better for specific interference spikes on xDSL. That way you could tune the notch filter in nice & tight with a pot. Could probably run it off the line voltage too. Just have to watch the gain so you don't start distorting the xDSL signal or increase insertion loss.

Probably not for the average user though

I didn't say it was good filter !   is it good enough ? 

I have heard of something like it  and apparently people were happy with the result, i.e. their ADSL1 was no longer wiped out completely after it was installed !

Hadn't thought of active filter; only time I used them was at sub-audio frequencies to avoid making enormous coils, thanks for the idea.

[guest]

Not teaching anyone to suck eggs & I'm sure I've explained this better on previous posts in this locale* but it may be helpful to readers.

Re passive filters :

They all depend on the rolloff/response curve. The ones linked above will probably wipe 50% off a VDSL2 connection minimum but provided you aim for a rolloff (-3dB) at about 3MHz then it will have no adverse effect on ADSL of any type. You could aim a bit tighter if required, a rolloff @ 2.5MHz is unlikely to trouble any ADSL2+ connection other than the highest syncing ones.

The other problem is the type of passive filter - in general you get a fast rolloff (slope on graph would be a cliff) with a lot of ripple (not good) or a slow rolloff with a nice flat response curve. You can ameliorate this by increasing the number of poles in the filter which increases cost/size & probably has very limited effect in most instances. Its all a trade-off though.

Re active filter :

First time I came across this was working on Rapier B1MLI upgrades where basically the target radar returns were sorted for doppler shift through a load of notch filters (10kHz IIRC) implemented with op-amps.

Sounds totally insane these days but back then you couldn't do the DSP stuff now possible so it was just better to do this stuff in (analogue) hardware, then digitise it & put it into a 68020 cpu running at 25MHz to do the logic. As an OT point Rapier B2 (would slew vertically onto target automatically) had 32-bit FFT butterfly filters implemented in discrete logic - you've never seen so many 16-pin ICs in your life & the heat generated was incredible (was all TTL, some ECL ). I have huge respect for the man who designed those PECs (panel electronic circuit in milspeak).

Its perfectly possible to build a wee opamp (I loved ua741s ) circuit to cover the ADSL2+ frequencies and have it capable of only knocking out one or two bins if required. Loads of resources online. You'd probably need to have a look how to power it though. Line voltage might do, it isn't going to draw much power but the variations in voltage on POTS are unlikely to be helpful.

Real problem would be multiple noise spikes where I wouldn't be happy cascading the signal though multiple notch filters (another few opamps basically) because of gain & heat issues possibly distorting the ADSL2+ carrier. There's probably a way of sorting that but its getting towards the stage of reinventing the wheel because the road has potholes I'd be inclined just to tune it as tight as possible either side of the spikes & accept the losses.

*something with asbokid maybe? There was a fairly extensive & technical discussion which will be in my posting history.

Edit - the post above refers to filtering the phoneline, not the mains power supply just in case there's any (probably short-lived) confusion