Wanting to use ethernet cables for low frequency, down to DC - Page 1
Forum section: electronics / projects

author: haxby
author_rank: Contributor
timestamp: May 04, 2022, 12:39:08 am
content: Hi all,I want to send low frequency signals, around 4khz down to DC, from one place to another, around 3m max, in a very electrically noisy environment.I thought about RS485, but while those are differential signals, the chips still are not completely isolated per-se.I thought about optical fibres such as Avago HFBR-1414TZ but for close to $100 for a tx-rx pair, plus the fibre itself, it gets a bit pricey for the 20 odd pairs that I need.So I wonder if using ethernet cables and the associated magnetics built in to the sockets will work. They are after all, truly magnetically isolated at both ends.What I'm looking for is a purpose built IC transmitter that could modulate the 4khz frequency to whatever higher frequency the ethernet system uses, then a receiver IC could demodulate back to the original signal. Is there such a chip in existence? The signals are real-time critical, so I can't implement real ethernet TCP/IP connection. I need something much lower level than that. Has anyone done this before?

author: haxby
author_rank: Contributor
timestamp: May 04, 2022, 12:41:21 am
content: I should say, I've also tried toslink (SPDIF) tx and RX pairs, but found that in practice, they have a minimum frequency of around 5Khz.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 04, 2022, 12:56:25 am
content: Take a look at MIDI current loops to get an idea of how to do it with optoisolators. Note that MIDI is UART and the idea can be applied to other protocols over UART.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 04, 2022, 01:05:35 am
content: I doubt you can get any satisfactory signal below somewhere between 100kHz and 1MHz using a typical 10/100 Ethernet MagJack.And DC is certainly out of the question!You have a couple options. One is that you could modulate some carrier with your input signal, transmit that, and demodulate at the other end.Another is to use something like the ADUM6201:https://www.analog.com/en/products/adum6201.htmlIt works down to DC and generates its own isolated power, so could directly be used with a cable. You get 5kV isolation, pretty good. You can use the two channels to drive a differential pair if needed.Now I'm assuming you want to transmit a digital signal. If it's analog signal, the above will not work. (Or you'd have to convert it to a digital signal first, which could be done with some modulator.)

author: pigrew
author_rank: Frequent Contributor
timestamp: May 04, 2022, 01:07:45 am
content: So, how much latency can you afford?I've never done it, but can think of a few different options:Use an audio ADC with S/PDIF output, your TOSLINK cable, and an audio ADC on the other side.Use an isolation amplifier IC (with your ethernet cables), such as the ADUM3190Use a RF modulator IC (FM?), and modulate to be in the passband of your magnetic transformerI like digitization the best, since it will be the most immune to noise. When using Ethernet cable, you'd want to use differential signaling.

author: BrianHG
author_rank: Super Contributor
timestamp: May 04, 2022, 01:16:13 am
content: Why not just FSK encode your serial data into a 10MHz carrier using a RS485 transmitter/receiver?The 10MHz carrier should be compatible with normal Ethernet 10/100 magnetics.There are even enough gates in a single GAL20V8 or 2-3 74HCxx logic ICs to do the FSK encode/decode. Medium sized PLDs with 64 macrocells would be overkill, but you can add multiple layered redundancy running the FSK at 50 MHz.Though, this would be unidirectional unless you use more than 1 pair on the Ethernet cable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2022, 01:20:48 am
content: How noisy are we talking? At what frequencies? If you don't know, then what are the expected sources? How many, what kind, how distant, how are they connected, etc.?How are the end points connected? Grounded? Isolated? Connected to other equipment in turn?What kind of signals are these? Large voltages? Tiny? Current? What noise floor / dynamic range is required? Is it even analog (as I would guess from "signals"), or is this digital, and if so what protocol (e.g. async serial)?How much propagation delay is actually tolerable ("real-time critical")? A bandwidth of 4kHz implies quite a lot would be needed to become a problem (i.e. on par with 1/(4kHz) ~ 250µs, a ridiculous amount for most any cabling solution), so I'm a bit dubious of this requirement -- is the bandwidth in fact 4kHz, or is it actually much higher?And finally, what quantity and cost are we talking? If this is one-offs, it would be fairly trivial to solve with some relatively pricey op-amps and a bench supply (i.e. to give enough common mode range to receive the signals through the noise). Or an isolated ADC-DAC pair (digitize the signal, isolate that, and convert it back). Or if it's digital to begin with, any old isolator will do for very little cost and effort, even 4N35 is fast enough here (<100µs propagation delay).Tim

author: Haenk
author_rank: Super Contributor
timestamp: May 04, 2022, 07:28:23 am
content: ADC-DAC via SPDIF sounds like a good idea.The parts should be rather cheap and widely available, including the cables.Or something to try out first:https://www.ebay.de/itm/362972780479combined withhttps://www.ebay.de/itm/403287396837That's already 2 channels. Maybe just crack them open to check for the ICs and build a couple of them on your own PCB.(the ebay items are just picked as a sample, there are plenty more sellers of similar items...)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 04, 2022, 02:10:35 pm
content: You can use RS485 on one twisted pair and DC power on another pair.Then everything on the cable works with the same GND reference.If you need galvanic isolation you can add opto couplers or use galvanically isolated RS485 drivers.another possibility is to use audio transformers.but what is "real time" in your timeframe?If required bandwidth is only 4kHz.RS485 can easily go upto several Mbit/s and if you can still actually buy a microcontroller somewhere it can receive and process quite some bytes in those 250us.You can also have a look at MIDI.MIDI uses a current loop with an opto coupler on the receiving end.

author: haxby
author_rank: Contributor
timestamp: May 04, 2022, 10:57:33 pm
content: Thanks all for the input. Very much appreciated. I'll have a good look at MIDI. That might be the simplest answer.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 05, 2022, 01:56:31 pm
content: you need simplex or full duplex ?many talkers ? many listners ?what is the voltage supply ?power requirement ?is each unit also supposed to run from the power in the ETH cable ?you say 4kHz is this your bit rate ?constantly ?or small bytes, send a few times pr minute or pr hr ?The more you can tell, the better guess for a solution you get..also : if you need slow speed, higt imunity , why not use RS232 and the old drivers +12 and -12Vone tx and one rx each end for full dublex,use the 4 pairs in the cable correctly, so each signal have a propper return line, and not uses another data / signal line as its return way

author: coppice
author_rank: Super Contributor
timestamp: May 05, 2022, 02:31:25 pm
content: Quote from: haxby on May 04, 2022, 12:41:21 amI assume we are talking about a low frequency digital signal, if you looked at RS485. TOSlink is reasonably high bandwidth. You could use various modulation schemes to push your DC to 4kHz signal to above 5kHz, but you could make it as easy as ANDing with a fairly fast square wave, and suppressing the high frequency content at the receiver. A simple single pole RC filter should be enough, if you square wave is reasonably fast.

author: Marco
author_rank: Super Contributor
timestamp: May 05, 2022, 02:52:32 pm
content: Ignoring for a moment how to transmit it, ye olde way of turning a DC signal into an AC signal for transmission was V-F + F-V.Another option with digital fiber connections is PWM :https://www.farnell.com/datasheets/1884376.pdfhttps://docs.rs-online.com/06c2/0900766b8129b22e.pdf(Skyworks doesn't have the ISOLinear app notes on their website any more, so these come from different places.)At high enough frequency PWM apparently couples across isolation transformers, and if you're made of money you can use a bunch of Analog ICs to do it :https://www.edn.com/isolated-pwm-suits-low-frequencies/

author: antenna
author_rank: Frequent Contributor
timestamp: May 05, 2022, 04:02:02 pm
content: Gonna have to disagree with the common assumptions again, I actually tested it for you. Here I ran a sine wave from 30mHz (almost DC) to 4kHz. The blue trace is through the ethernet cable (and connectors) and the red trace is probing the generator output directly. The part people seemed to have missed is the 3m of cable part, and my test used way more than thatGood luck!Forgive the messy desk, I was in the middle of cleaning out desk drawers...

author: coppice
author_rank: Super Contributor
timestamp: May 05, 2022, 04:09:24 pm
content: Quote from: antenna on May 05, 2022, 04:02:02 pmYou used a plain connector, right? The OP wants isolation. He was talklnig about the connectors with integrated transformers.

author: antenna
author_rank: Frequent Contributor
timestamp: May 05, 2022, 04:15:39 pm
content: Yea, my bad. Plain connectors.But ethernet cable can be used with optoisolators....Here is a crosstalk test. This time, the blue trace is probing an unterminated pair with a 10x probe. There is still enough isolation there to work with... Is it worth the time to test the crosstalk with a small load attached? Let me know.

author: antenna
author_rank: Frequent Contributor
timestamp: May 05, 2022, 04:21:29 pm
content: I assume the magnetic isolation is just like a balun, removing the common mode current. Is physical isolation even necessary? Why not use a differential amplifier? Then you get analog capability too (and that crosstalk and common-mode between wires goes to nothing too).My point was that ethernet cablecanhandle it, not what happens once you get the signal there. There are plenty ways to isolate if needed once you get the data from point A to point B...

author: CaptDon
author_rank: Super Contributor
timestamp: May 05, 2022, 11:02:55 pm
content: O.P., your statement of purpose contradicts itself in so many ways!!! Can you explain your system a bit better? "I want to pass a signal from 4KHZ down to D.C." well, that rules out magnetics which don't work at D.C. Then "I want to modulate the 4KHZ signal up to the ethernet speed"?? What does that mean??? Any wire is suitable for D.C. to 4KHZ generally speaking. Do you maybe want to sample a signal at perhaps 100K samples per second, convert with an ADC and ship it down your twisted pair wires to a DAC at the other end to re-create the signal. We can really solve your problem without a bit more explanation of what you are trying to do and how much conversion delay is tolerable.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 05, 2022, 11:38:08 pm
content: If it's unidirectional, a passive-input isolator like the NVE IL610 might be worth a look

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 06, 2022, 01:12:56 am
content: For FSK modulation using simple "jellybean" parts, you can get inspiration from this app note:https://www.ti.com/lit/pdf/slaa618They use a 74HC4046A both for modulating and demodulating.Additional benefit, this would be a fun project.