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Author Topic: Modem power supplies and grounding  (Read 1009 times)

IanG

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Modem power supplies and grounding
« on: May 30, 2019, 08:53:41 PM »

Hello All. [Being new, please tell me if I am posting this in the wrong section.]

While trying to track down occasional interference that causes ADSL dropouts, I got interested in the ADSL modem grounding arrangements. As far as I can see, Ethernet and DSL inputs are normally transformer coupled, while the DC power supply is also floating, having no earth pin. So the internal chassis of the modem is likely to have RF signals capacitively coupled from all those inputs, which is not ideal. Or so I thought.

Changing the 12V power supply for another (different make from a different modem) had no effect, but replacing it with a linear lab power supply gave a notable improvment. Connection rate improved from 17.6 to 18.5 Mbps; at the same time FEC errors were much reduced, and the S/N ratio improved by a few dB over almost the whole spectrum.

Using the lab power supply is not ideal. For one thing, its internal fan makes a racket. Instead, I thought of buying a class 1 switcher (i.e. with earthed output) to see if it gives the same improvement.

So I have a few questions. Can someone confirm that it is safe to connect an earthed supply to the modem? And have others tried an earthed supply, and found it to be effective? The modem under test, by the way, was the Netgear DGND3700v2.
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burakkucat

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Re: Modem power supplies and grounding
« Reply #1 on: May 30, 2019, 10:43:46 PM »

Welcome to the Kitz forum.  :)

Thinking about the internals of the average CPE, there is no real, classical, ground-plane that could be earthed. Obviously there is a logical signal "chassis" . . . if there are header pins for a serial console, then one of them will be the signal ground.

The xDSL connection will be via capacitors, one in each leg of the pair. Essentially it is an RF feeder, capacitance coupled to the analogue front-end.

As for the LAN Ethernet connections, I am not to sure how the two (or four) pairs and the circuitry are coupled together.

Finally a switch-mode power supply will have no earthing and would be doubly insulated.

I am not surprised that a change of power supply shows a difference in the modem/router's performance. Most switch-mode PSUs are inherently noisy and some of the "budget class", Chinese, made devices are really poor quality. The best test would be to arrange a temporary DC supply . . . if the CPE requires 12V DC, then a car battery (or a power tool battery pack) would be ideal.
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sevenlayermuddle

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Re: Modem power supplies and grounding
« Reply #2 on: May 30, 2019, 10:48:38 PM »

My inclination would be to not interfere with the manufacturer’s earthing/grounding arrangements.

My reasoning is... The ‘earth’ conducter in home wiring is provided not as a “noise reduction wire” but as a safety feature to ensure that in the event of a fault, a fuse will do its job, melting and breaking the circuit, before the house catches fire.   Or (hopefully), breaking the circuit before an occupant touches a piece of exposed metalwork that has become accidentally connected to mains potential.

Worth mentioning too that for many domestic supplies, the earth conductor is directly connected (by the supplier) to the neutral conductor.   With that in mind, it maybe becomes more intuitive that (1) the earth conductor may actually be electrically quite noisy, and (2) messing around with earthing arrangements may have unintended and dangerous consequences.

Many modern home appliances simply do not need an earth connection, as they have other safety arrangements.   For example, my powerful and sensitive home AV amplifier has no earth, and works perfectly, with no susceptibility to interference.   In contrast, my old amplifier from 1970s does have its chassis connected to earth, which renders it susceptible to all sorts of “ground loop” interference problems when connected to other equipment, unless strict rules are followed.

...just my take.   But whilst I have a scroll of paper somewhere that proclaims I once knew a thing or two about electrical engineering, I am no expert these days, happy to be set right. ;)
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IanG

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Re: Modem power supplies and grounding
« Reply #3 on: June 02, 2019, 10:46:13 PM »

Many thanks for all for your responses. You are right to be cautious.

To hopefully understand the hardware better, I made some measurements on two unpowered modems, one Netgear and one TP-Link. The attached diagram from an Analog Devices ADSL design guide is consistent with the measurements. On both modems, the input coupling capacitors (0.1 uF in the diagram) measured as 68 nF. I think they are present to stop the transformer shorting out the POTS signal at low frequencies. Also on both modems, the transformer inter-winding capacitance (measured from the DSL input to the modem 0V line)  was about 39 pF, which makes me happier about the modem surviving lightning surges. At a distance, of course.

Ethernet inputs similarly had no direct connection to 0V, but couple via a transformer. Input capacitance to 0V was 70 pF for Netgear, and 1 nF for TP-Link. With that measured, I felt happier that what I was proposing would not be unsafe, although it might well be pointless.

I completely take the point that mains earth is by no means an RF ground, and that all lines including the DSL common mode input are likely to have spurious RF on them. My concern is not with ripple voltage from the switching power supply, but with currents capacitively coupled from its mains voltage switching circuit to its output, which an earth return should loop back to the input, assuming the mains input has class Y capacitors to the earth line. Or so I hope. Anyway, I have ordered a class 1 switching supply, and will post the result here if it proves to be effective.
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burakkucat

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Re: Modem power supplies and grounding
« Reply #4 on: June 02, 2019, 11:23:52 PM »

On both modems, the input coupling capacitors (0.1 uF in the diagram) measured as 68 nF. I think they are present to stop the transformer shorting out the POTS signal at low frequencies.

The capacitors are the high-pass filters . . . passing the RF of the xDSL signal and blocking both the voice-band POTS at baseband frequency & the POTS DC component.

It will be interesting to see the results of your experiments. So please do post an update, in due course.
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PhilipD

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Re: Modem power supplies and grounding
« Reply #5 on: June 07, 2019, 02:18:08 PM »

Hi

Many thanks for all for your responses. You are right to be cautious.

To hopefully understand the hardware better, I made some measurements on two unpowered modems, one Netgear and one TP-Link. The attached diagram from an Analog Devices ADSL design guide is consistent with the measurements. On both modems, the input coupling capacitors (0.1 uF in the diagram) measured as 68 nF. I think they are present to stop the transformer shorting out the POTS signal at low frequencies. Also on both modems, the transformer inter-winding capacitance (measured from the DSL input to the modem 0V line)  was about 39 pF, which makes me happier about the modem surviving lightning surges. At a distance, of course.

Ethernet inputs similarly had no direct connection to 0V, but couple via a transformer. Input capacitance to 0V was 70 pF for Netgear, and 1 nF for TP-Link. With that measured, I felt happier that what I was proposing would not be unsafe, although it might well be pointless.

I completely take the point that mains earth is by no means an RF ground, and that all lines including the DSL common mode input are likely to have spurious RF on them. My concern is not with ripple voltage from the switching power supply, but with currents capacitively coupled from its mains voltage switching circuit to its output, which an earth return should loop back to the input, assuming the mains input has class Y capacitors to the earth line. Or so I hope. Anyway, I have ordered a class 1 switching supply, and will post the result here if it proves to be effective.

It's unlikely you are going to make any great improvements, you might have proved a power supply was not very good, but swapping that out doesn't improve the performance, it just puts it back to where it was when all was working well. 

Also the power supply could very well be okay, and working within specification, and what isn't working well or showing signs of age are the electrolytic capacitors in the modem itself.  With a switching power supply they need to be working well and have a low series equivalent resistance, if they aren't working 100% then a switching power supply will show that up more than a rectified 50Hz from a standard transformer/linear regulator.  Electrolytic capacitors can have working lifespans measured in just thousands of hours, and even in storage their performance drops.

A switch mode power supply also isn't 100% isolated from the mains at least where noise is concerned, most of them (certainly those supplied with the equipment itself as opposed to cheap rubbish from Ebay/China) will have a class Y capacitor across the primary and secondary side to allow noise a route to out to ground, therefore an earth/ground isn't required.

Regards

Phil




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Alex Atkin UK

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Re: Modem power supplies and grounding
« Reply #6 on: June 24, 2019, 11:48:54 PM »

Its certainly curious though that two different PSUs give the same sub-optimal behaviour.
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IanG

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Re: Modem power supplies and grounding
« Reply #7 on: June 29, 2019, 06:26:57 PM »

I have done some more tests, now that the line has been stable for a few days. Previous erratic results might have had something to do with Openreach vans being around this area like wasps at a picnic.

If one unplugs one power supply and plugs in another, the modem retrains, normally to a different data rate, making it difficult to compare SNR values. Each retrain gives a new QLN spectrum, but as these are single measurements they may not be repeatable. To get around this, I wired up a DPDT switch with make-before-break action, which allowed me to switch 12V and 0V supply lines without interrupting the operation of the modem.

The results in this post come from a single Netgear modem, with 5 different supply configurations. The supplies were two +12V switchers that came with Netgear and TP-Link modems, a class I supply that I purchased separately, and the same Netgear & TP-Link supplies with an added strap between 0V and mains earth. NOTE: I do NOT recommend the strapping procedure to anyone unfamiliar with the hazards of domestic wiring, and probably not to anyone else either.

@PhilipD: All three supplies were made in China. Not that I have any fondness for such. I do have a pukka British power supply, but it was excused on grounds of age and infirmity. See the attached inspection label.

For the test, I switched supplies every 5 minutes, as plotted in the attached time chart with X axis in minutes. Since a measurement was taken every 2 seconds, each 5 minute section has 150 fairly consistent readings. For both Netgear and TP-Link switching supplies, adding the earth strap improved upstream SNR (labelled "^ SNR") by 1.0 dB, and downstream SNR by 2.0 dB. [Y axis labelling is a division count rather than a dB count]

In the upper graph of the separately attached SNR spectrum, the red and dark green spectra are from the Netgear power supply with and without an earth strap. The lower graph shows the improvement from adding the earth strap, i.e. the difference between the two upper graphs. The spectrum is ADSL2+, with 512 samples.

As far as possible I avoided moving cables during the test, and as cable layout may be a significant factor I do not claim that anyone else will see the same result.

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jack21

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Re: Modem power supplies and grounding
« Reply #8 on: July 01, 2019, 02:36:01 PM »

@IanG
What an excellent well-performed test, with illuminating results. Would it be possible for you to add a further source to the mix - a stand-alone 12v battery (at somewhere between 12 and 13v DC).....I'd love to know if that was beneficial compared to the others.
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IanG

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Re: Modem power supplies and grounding
« Reply #9 on: July 01, 2019, 08:33:58 PM »

I'd love to know too, but I don't have a suitable battery, and am unwilling to disconnect and drag in the battery from the over-electronic car. :(
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jack21

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Re: Modem power supplies and grounding
« Reply #10 on: July 02, 2019, 06:36:12 AM »

I completely understand; I wouldn't want to disturb my car's electronic equilibrium either.

I did some experimenting with several (switching/linear) wall-warts, class 1 psu, laptop chargers, 12v battery etc some years ago, but my observations were always affected by the modem-resync issue you described. And I had no idea of the possibility re the DPDT make-before-break arrangement you've used in your work, so all my observations had that variability about them. I didn't do any grounding work.

I did subjectively conclude - for my line/modem - that battery gave the best results, followed by non-switching laptop charger (stepped down to 12V). My current long-term arrangement of APC line-interactive 230>52 UPS stepped down to 12v is just as good as the first 2, and when it is forced onto battery I don't see a noticeable difference.

For my line, cross-talk is the biggest issue; I was the third person connected via a new/nearer (second-in-village) AIO and initially had a max-obtainable of 95M.....over the 2 years since, that has dropped to 59M (I subscribe to a 55M service). However, when there is a village-wide power cut, the max-obtainable leaps to close to 115M.
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