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Author Topic: Modems and mains cables - interference  (Read 1155 times)

Weaver

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Modems and mains cables - interference
« on: June 14, 2018, 08:48:11 PM »

After four weeks of begging, I managed to get Janet to volunteer to tidy up the layout of my three  modems, their dc cables and most importantly the cables from modems to wallsockets and get everything well away from any mains coming into the modems’ dc power bricks. (I didn’t want to stand upright for that long because it can make me feel dizzy and horrible.)

Four weeks ago two more ZyXEL VMG 1312-B10A modems were dropped in in place of the former DLink DSL-320B-Z1 units. One ZyXEL modem had been installed somewhat earlier still. But four weeks ago all three modems were just parked any old where in evidently a horrible physical arrangement because there was a huge performance drop evident on modem 1 for example the moment it was fired up after they were all moved.

Now Janet has moved mains away she tells me and downstream sync rates have improved this afternoon, just from this simple change, as follows
    modem cwcc@a.1 downstream sync 2861 -> 2975 kbps - +3.98%
    modem cwcc@a.3 downstream sync 2832 -> 2935 kbps - +3.64%
    modem cwcc@a.4 downstream sync 2821 -> 2894 kbps - +2.59%

However modem 1 is still not back to the sync rate it had 4 weeks ago, which was 3055 kbps, so still another 2.7% short. Back then I think Janet had the first modem well away from all other modems as well, and when she moved it, 6.35% of d/s sync rate was lost and we still have not got all of it back yet. (That last figure is a percentage of the higher, earlier number, in case you are noticing that the differences do not add up.)

So more needs to be done regarding placement, but I am not quite sure what. Could separate the modems physically, but I would not have thought they could be emitting any rf themselves. Won't they be shielded with some kind of conductive coating or some such inside the plastic casing? I could ask Janet to check where the dsl cables are right now just in case they are near to anything nasty.

The very first time ZyXEL #1 was plugged in, its downstream sync rate was a glorious 3139 kbps, quite a way above the 2848 kbps delivered by its predecessor a DLink DSL-320B-Z1, a 10.2% improvement in downstream sync rate. But 80k disappeared fairly soon, and then, over time, all the rest of that 10% eroded away too.

One thing that has not been done yet is putting ferrites on the ZyXELs’ DC cables. This was something that the earlier DLink modems had and so if ferrites give any benefit at all then there is another possibility for improvement. However, the first time one ZyXEL was plugged in it did not have the benefit (if any) of ferrites attached.

Another thing that I may have done in the past is folding up the DC cables to shorten them. The superstitious and optimistic idea here is to try and stop them picking up any interference, so what I have sometimes done in the past is to bend the cable into a u- or hairpin-shape, then I out a twist or plait into it and finally wrap the whole thing up into a compact volume. The wacky idea anyway is to prevent it from being a decent receiving antenna. Now I cannot even remember if that was done with the DLink modems. I suppose there might be a chance that the evidence is still there, depending on what Janet did with them when she put those modems back in their boxes in the spares dept.

Anyway, all a bit disappointing, frustrating and mysterious.

The ZyXELs are still a hit though, slightly more performance, no downside apart from the reduced upstream performance, which is really annoying and the much greater purchase cost, which is 4 or 5 times more. They are far more stable as their downstream SNR margins do not drop below the initial 3dB downstream target set and they never fail and resynch, they just carry on calmly forever. I don't know if this is to do with the ability to support monitored tones. It could be that the DLinks are set to be too aggressive and just actually cheat by going below the selected downstream target SNRM by design, because they show a lower SNRM than the target pretty much immediately and it makes you wonder if aggression and a bit of cheating is the secret of some of their speed. In that case, to compare like with like, I should level the real current downstream SNRM values on one or other or both modems by tweaking somehow, and the ZyXEL would then gain some extra speed relative to its rival as a result, because it currently always runs at or slightly above my chosen 3dB downstream target SNRM with no ‘cheating’.

I could do with some suggestions about physical shielding or location or anything like that that I can do.
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burakkucat

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Re: Modems and mains cables - interference
« Reply #1 on: June 14, 2018, 09:26:51 PM »

Two quick points --
  • There is no shielding inside the plastic case. (Remember, the VMG1312-B10A can be configured as a WAP.)
  • AC current carrying leads and xDSL signal carrying leads should cross at right angles. (If they must cross.)
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Weaver

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Re: Modems and mains cables - interference
« Reply #2 on: June 14, 2018, 09:30:38 PM »

Ah, forgot about that, there are no external antennae. What do they do about emc- is any if it in a tin can? That is a good point, they could be toxic to one another then. So perhaps I should put each in a small biscuit tin but then they would just overhead. Difficult to find the right container given that it needs apertures for cable egress too.
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spring

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Re: Modems and mains cables - interference
« Reply #3 on: June 15, 2018, 12:05:35 AM »

Do you use shielded twisted pair DSL cables with a length of 0.3 meters?

The right specific ferrite does give consistent gains. For example, banger or someone else can tell you which one he uses & his placement.
« Last Edit: June 15, 2018, 12:09:32 AM by spring »
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banger

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Re: Modems and mains cables - interference
« Reply #4 on: June 15, 2018, 01:25:03 AM »

I use 4mm TDK ferrite beads at the end of both dsl cable and power cables. You can place at both ends of the cable too. I found a Billion 7800 flat sliver cable the best to sync.
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Tim
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Weaver

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Re: Modems and mains cables - interference
« Reply #5 on: June 15, 2018, 04:28:20 AM »

I use 0.5m and 1m Tandy / ADSL nation RJ11-RJ11 DSL cables which are the best quality that I can find, from the evidence of comparison testing in the past. I have tried other cables too. An RJ11-RJ45 cable is a slightly better fit at the SSFP end, better ‘snap’ into place but RJ11 seems to work well enough.

I am always interested in cable recommendations. I just keep them as short as possible and always well away from any mains.

I would love to physically shield each entire modem and have been trying to find suitable metal enclosures with access holes and ventilation, plus they need grounding. But I really don't know where to start as googling for such things is a nightmare.
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burakkucat

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Re: Modems and mains cables - interference
« Reply #6 on: June 15, 2018, 03:08:34 PM »

I would love to physically shield each entire modem and have been trying to find suitable metal enclosures with access holes and ventilation, plus they need grounding. But I really don't know where to start as googling for such things is a nightmare.

Perhaps use the string "Faraday cage" in your searches?  :-\
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Weaver

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Re: Modems and mains cables - interference
« Reply #7 on: June 15, 2018, 03:47:11 PM »

@Burakkucat- Indeed. We had one at work, but it was really large.  I will try that again as I think I went there some years ago but I can’t remember what I found.
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Weaver

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Re: Modems and mains cables - interference
« Reply #8 on: June 17, 2018, 12:10:17 AM »

I found a shop on ebay - usa supplier unfortunately - that sells all kinds of Faraday cage type kit. They sell bags of various sizes made of plastic and double metal film. I do not know what I am supposed to do about getting cables in and out. I think these items are maybe for storage of equipment that is either turned off or is not unconnected to anything. Anyway I would like to try one of those, but leaving one end open for cables presumably invalidates the whole thing.

I think these bags have a fastener at one end so I could perhaps make small three cable holes in the one end, then put the cables through those holes into the bag and finally slide the modem in through the large open end and close it up. I think sticky copper tape is available so I could put that round the holes when all is done as a finishing touch.

First I will try some aluminium foil from the kitchen because if that does not work then I think is no point risking the money on one or more of these bags. Because that will tell me whether there is even any problem there to be fixed! (Assuming aluminium foil does work effectively.)

Because the frequencies we are dealing with are really low, so long wavelengths, eg 800 kHz is 375m, does that mean that I do not need to bother too much about small holes? Because the holes’ width is so very much smaller than one half-wavelength?

Any advice?
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banger

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Re: Modems and mains cables - interference
« Reply #9 on: June 17, 2018, 12:18:42 AM »

My advice would be ensure the modems have plenty ventilation or they might fry. :)
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Tim
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Weaver

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Re: Modems and mains cables - interference
« Reply #10 on: June 17, 2018, 02:48:08 AM »

That indeed is a big worry. The aluminium thing would have to be short-term. And then if it works and gives a good result, then the posh bags thing is out because of heat insulation anyway, so I think we have just killed this stupid idea.

A wire cage would be more like it. I would thing that ought to work because the wavelengths eg 375m+ for 800kHz are so enormous, no?
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banger

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Re: Modems and mains cables - interference
« Reply #11 on: June 17, 2018, 02:53:44 AM »

A business opportunity for Modem Faraday wire cages, perhaps?
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Tim
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Asus DSL-N55U and ZyXEL VMG1312-B10A Bridge on 80 Meg TTB Fibre

https://www.thinkbroadband.com/speedtest/1502566996147131655

Weaver

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Re: Modems and mains cables - interference
« Reply #12 on: June 17, 2018, 03:03:24 AM »

I have tried all over the place to find something suitable. Needs a grounding lead too.

Perhaps I could just make one out of wire fencing. Would need to spray paint it to make it a bit less ugly I think. I don't know how to attach the earth lead. I know someone who might make one for me.
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banger

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Re: Modems and mains cables - interference
« Reply #13 on: June 17, 2018, 03:07:38 AM »

When I used to mess about with radio I used to use a radiator pipe as a ground as it went to earth might be worth considering. I would try the foil first to test the theory as you may not gain much sync. I went to extreme lengths with modem cables on ebay spent about £30 on all sorts of special cables, ended up with a Billion 7800 flat cable that gave me the best sync.
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Tim
www.uno.net.uk & freenetname
Asus DSL-N55U and ZyXEL VMG1312-B10A Bridge on 80 Meg TTB Fibre

https://www.thinkbroadband.com/speedtest/1502566996147131655

Weaver

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Re: Modems and mains cables - interference
« Reply #14 on: June 17, 2018, 07:11:57 AM »

I am not sure where the ventilation holes are on the VMG 1312-B10A, I can see some holes on one short side. The entire top surface and the front long side appear to be free if holes.

So

I wonder if I could shield it very slightly by using copper tape on the top surface, front and right hand side. Again there is the problem of how to ground the whole lot. A nutty plan.
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