Differential Cable Run

[Chaos_Klaus]

Hey Folks,

I want to get a digital signal (5V, 12.188MHz) from one device to another via a twisted pair in a CAT5 Network-Cable.

The circuitry in both the transmitting and recieving device is single ended, so I want to generate a differential signal first before I send this through the cable. Then I obviously need to reconstruct a single ended signal at the recieving side.

So, I did some research on the net. The results were not very enlightening. Lots of interesting bits and pieces, but nothing that describes how to actually implement it.

I came to the conclusion that I can use a fully differential opamp to generate the differential pair, then send that through the twisted pair and use a comparator at the recieving end.

I have a few questions:

1) I have no idea how to judge which components suit my needs. There are hundrets of choices for diff amps and comparators. The components in the articles/videos where all very expensive. Are there "go-to" components?

2) What do I need to know about driving the cable. I read about matching impedances, but I don't really understand why and how to do it. Maybe someone has a good resource for that?

3) Given the fact that an opamp can work as a comparator, is it possible to generate a differential pair using general purpose opamps aswell? These would have to be quite fast with slewrates of 500V/µs to deal with the transients of the digital signal, I guess. One could probably generate a negative signal with a simple inverting amplifier. Would something like that work? Can someone recommend an inexpensive opamp that's fast enough so that I can start breadboarding some things?

Thanks for your time!

[Richard Crowley]

This is where I would start....

https://www.maximintegrated.com/en/products/interface/transceivers/rs-485-rs-422-transceivers.html

Consider also simply using the physical-layer of Ethernet, using the same connectors, pair identification and pin assignment, and pulse transformer isolation at the transmitting end and the receiving end.  That would make your system at least immune to accidental interconnection with an 802.3 Ethernet line.  It also makes transition to single-ended, and impedance matching, etc.  trivial as you can use off-the-shelf solutions which have been pre-engineered for much higher frequencies than you are talking about.

[T3sl4co1l]

1. What is the signal? Is it purely 12.188MHz, i.e. a sine wave with no modulation?  Or is it a square wave, with some required rise/fall time?  Duty cycle?  Does it carry data, or modulation (AM/FM/etc.), and the frequency is the bit rate or carrier frequency?  What's the total bandwidth (baud/2, or carrier +/- BW/2)?  Does it need to be DC-accurate, or can it be cap-coupled or transformer coupled?

2. What does the signal need to connect between, and for how long?  PCBs within a single enclosure?  Between enclosures in a lab?  Between enclosures in an industrial setting?  Between buildings or further?

3. What needs to be done about protection, filtering, error correcting, etc.?  This will depend on answers to #2 (a long cable run is likely to pick up more interference and surges than one within a lab or enclosure), but also depends on how reliable and robust you need it.  Likewise, how much noise the system is allowed to radiate is an important spec.

Offhand, it sounds like you want an RS-422 or LVDS transceiver pair, either with shielding or transformer isolation to provide immunity.  Analog hardware (op-amps and stuff) would be overkill for a mere digital signal.

But if you need to preserve analog properties of the signal, then an amplifier might be the way to go.

Tim

[Chaos_Klaus]

Consider also simply using the physical-layer of Ethernet, using the same connectors, pair identification and pin assignment, and pulse transformer isolation at the transmitting end and the receiving end.

Oh, wow. I didn't know there would actually be circuitry inside an Ethernet connector. Transformeres, LEDs and the impedance matching in one package. Unfortunately, I think I can't use this approach.

I actually posted something about this project some time ago but didn't have time to work on the project. The network calbe is supposed to carry both the digital signal and the power supply for the receiving device. That is one twisted pair for the differential signal, one for GND, one for +12VDC and one for -12VDC supply rail. I guess can't use Ethernet connectors with built in transformers to carry DC through the other twisted pairs.

But I do get the idea. If I were to use some transciever or transmitter/reciever pair and a 75Ohm resistor to match the cable resistance, I should be ok, right?

1. What is the signal?

The Signal is an ADAT digital audio signal. 12.188Mbit/s is the bitrate. I have an optical TOSLINK input in the transmitting device. It gives me an output signal between 0,5V (Low) and 4,5V (High). I want to take the output of this optical reciever, make it differential and send it to another device that has an ADAT decoder/receiver.

2. What does the signal need to connect between, and for how long?  PCBs within a single enclosure?  Between enclosures in a lab?  Between enclosures in an industrial setting?  Between buildings or further?

I want to connect two seperate devices. Seperate enclosures, 10m-20m maximum distance. In the same room or between two rooms. All of that happening in a music studio environment.

3. What needs to be done about protection, filtering, error correcting, etc.?  This will depend on answers to #2 (a long cable run is likely to pick up more interference and surges than one within a lab or enclosure), but also depends on how reliable and robust you need it.  Likewise, how much noise the system is allowed to radiate is an important spec.

Puh. Hm. I have no idea. The reason that got me thinking about a differential/balanced connection was to not flood my studio with electromagnetic fields from the digital signal cable. There is lots of analog circuitry running through cables in the room. most of it is balanced/differential though.


Thanks for the answers so far. Looking through those transceiver ICs now.

[Richard Crowley]

Oh, wow. I didn't know there would actually be circuitry inside an Ethernet connector. Transformeres, LEDs and the impedance matching in one package.

There are many levels of integration. If I were doing that project, I would strongly consider using an RJ45 jack which includes the data transformers. There are several models from HanRun and similar vendors. Many of the popular models are even available on Amazon for less than the cost of postage.

I guess can't use Ethernet connectors with built in transformers to carry DC through the other twisted pairs.

Quite to the contrary. There are standards for "POE" (Power over Ethernet) which more than meet your requirements. No reason to re-invent the wheel.  Furthermore, since this is a custom interface, you can simply use the third and fourth pair as the power and ground conductors out to the remote end.  And use local voltage regulator(s) as appropriate.  You don't even need the POE protocol for something so simple.

The reason that got me thinking about a differential connection was to not flood my studio with electromagnetic fields from the digital signal cable. There is lots of analog circuitry running through cables in the room. most of it is balanced/differential though.

And the 802.3 Ethernet physical layer is fully balanced, also and works quite nicely in close proximity to balanced mic lines, etc.

[Chaos_Klaus]

Haha. Damn. PoE ... so I'm not the first one trying to do this. 

So if I were to use this kind of jack (which is still quite expensive):

http://belfuse.com/pdfs/0813-1X1T-57-F.pdf

then I could transmit my signal on the first twisted pair. The negative power rail could be applied to the center tab of the transformer for the second twisted pair. The other twiste pairs could be GND and positive rail. Is that correct?

The impedance is matched in the jack itself.

And the transformers already make a differential signal, right? So do I need to take special care to drive that line?

Honestly ... this all sounds way too easy now.

[Richard Crowley]

Well, $5 doesn't seem "quite expensive" for an integrated connector/transformer solution.  And of course the price goes down in quantity.

I'm not sure I would even fool around with the signal center taps (VC12 and VC36).
I would just use the VC45 and VC78 for power and ground.
And you could use VC12 and VC36 as additional grounds for good measure.
Do you need the second signal pair (3/6) for clock or anything?  Or is your data-stream self-clocking?

Yes the built-in transformers provide both impedance matching and differential transmission.  Quite convenient, IMHO.

[Chaos_Klaus]

Well. The receiving side has analog audio circuitry with a few opamps, so I want to have a negative power rail aswell. That makes three power rails.

The Receiver/Decoder generates the clock directly from the ADAT-Stream, so no additional clock is required.

This is a great and easy to use solution. Thanks!


EDIT:

This one should work aswell, right?

http://www.mouser.com/ds/2/336/-610985.pdf

It's actually cheaper with shipping and VAT. It has these extra transformers on the power lines. What do they do?

[Richard Crowley]

P7/P8/P9/P10 are routed through a common-mode choke which is part of the filter circuit (along with the 22nF capacitors and the 75 ohm resistors).
I suspect that is why the specs say that the current is limited to 350mA. You will have to figure out your power budget to see whether that would work for you.
I would use P9/P10 for ground, and P7 for one voltage rail and P8 for the other.
If that 350mA limits you, there are probably other similar products where you have direct access to J4/J5 and J7/J8

[Chaos_Klaus]

Well, if that is 350mA per pin, then that is fine.

I think this choke might even be useful to me. Actually, I plan to have one primary device that has the transformer and the optical ADAT input. This device distributes both power and signal to up to 8 secondary devices that are connected to the primary device via network cable.

Since there is digital circuitry in the secondary devices, it probably nice to have these chokes filter the supply net so that the secondary devices don't pollute eachothers supplys. I guess I just have to have the voltage regulators in the secondary devices and be generous with filter caps.

Thanks again. You've really helped.   

One last question: Just checking the options here. If I were to use a bare bone RJ45 jack, then I'd only really lack the one signal transformer and the impedance matching per jack. What specs would I have to look for when choosing this transformer?

[Richard Crowley]

This is the transformer I would use.  It is only $1
http://www.goldmine-elec-products.com/prodinfo.asp?number=G17078
http://datasheet.octopart.com/PE-68025-Pulse-datasheet-8399033.pdf

I would probably use a bare jack and the transformer. Just to have direct access to the other 6 pins.

[Chaos_Klaus]

I would probably use a bare jack and the transformer. Just to have direct access to the other 6 pins.

That was my thought aswell.

I have some space constraints though. The one you linked is 18mm x 10mm x 6mm and has both transmit and receive circuitry. Would it be sufficient to just use a bare signal transformer with a center tab on one side to add that 75Ohm resistor?

Like that: (see attachment)

The recieving side could be the same, just "backwards".

What general properties would this transformer need? 1:1 windings? High impedance, low impedance?

I guess I need a pulse transformer that is specified for the correct bandwidth, right?

[Richard Crowley]

Sure, you could use a single transformer. But finding a single transformer with the proper characteristics for 10 Base-T will be extraordinarily difficult and expensive. Because nobody needs just a single transformer. They use dual transformers like that because 99.9999999% of all applications are for Ethernet interfaces where you need two transformers.  I'm just trying to be practical here so you can get on with the project instead of searching for parts for months. 

[SeanB]

Just use the plain socket and a standard transformer unit, and do not use the other half of the transformer.  Transmit side you could use the same transformer to drive 2 sockets, and those are available in both a dual and quad arrangement. Saves both looking for the right magnetics and is cheap, otherwise use the one with integrated magnetics, and use all 4 pairs for power and ground, probably with a supply to the receive devices around 5V higher than required to cope with the resistance in the cabling. Then at the receive end use a LDO regulator for both supplies to get the right regulated rail. You do not need excessive capacitance there, just something of 100-470uF per rail at most, and remember the correct capacitors on the LDO outputs.

[Chaos_Klaus]

Hm ok. You guys are right. Unfortunately the page you linked does not ship to germany.

I found this one:

http://eu.mouser.com/Search/ProductDetail.aspx?R=749013011Avirtualkey51110000virtualkey710-749013011A

It's basically two identical units in one package. Rated for 10Base-T 1MHz-1000MHz. Should work.

[tggzzz]

The Signal is an ADAT digital audio signal. 12.188Mbit/s is the bitrate. I have an optical TOSLINK input in the transmitting device. It gives me an output signal between 0,5V (Low) and 4,5V (High). I want to take the output of this optical reciever, make it differential and send it to another device that has an ADAT decoder/receiver.

Why not just transmit the optical signal to the other end?

[Chaos_Klaus]

Why not just transmit the optical signal to the other end?

Because then I'd need extra cables for power. I want both power and ADAT stream in a single cable.

I sadly don't recall the ADAT specification but I *think* it may be a NRZ (non return to zero) based single ended digital TDM representation.  If you have a NRZ TDM signal that isn't guaranteed to have good short term one to zero level frequency balance then there can be low frequences, maybe even DC represented in the single ended waveform.  Differential encoding by itself does not fix the DC / low frequency content of a signal, so that signal may not be suitable to be transformer coupled depending on the lowest frequency which the transformers can carry.

Hmmm. Wikipedia sounds like you are almost right. ADAT uses non-return-to-zero-inverted (NRZI) coding. That could complicates things, I guess.

EDIT: Here is some petter info on ADAT streams. It seems that it sends a 1 after every 4 databits to ensure that there are enough HIGH-LOW transitions in the NRZI encoded signal. So even if all channels are transmitting silence and are full of zeros, at least after five bits the signal changes. Wouldn't that set a lower limit to the bandwidth, eliminating DC components in the signal?

[Richard Crowley]

If you don't identify WHERE you are (by completing your user profile), then you can't really complain about suggestions you get from an international forum.

Even though it likely costs much less than copper to actually manufacture, optical fiber cable still sells for premium prices.

I think a ADAT/TOSLINK/optical -> 10 Base-T -> optical would be a very useful gadget to sell to people who are still using the ADAT protocol, but need longer distances than those cheap plastic extruded TOSLINK "cables".  I am very interested in the progress of this project.  Please keep us informed.

[Chaos_Klaus]

If you don't identify WHERE you are (by completing your user profile), then you can't really complain about suggestions you get from an international forum.

Sorry. I didn't mean that to sound like a complaint.

Even though it likely costs much less than copper to actually manufacture, optical fiber cable still sells for premium prices.

Yeah ... and I just hate optical cable. The way it feels ... it doesn't bend well. I don't like it.

Yes, you're right, the lower limit of the bandwidth should be above 1MHz given a 10 bit maximum "cycle time" and 12MHz UIs.  So I suspect that the 10-Base-T ethernet transformers would work though I don't recall their specific bandwidth limits it seems unlikely that such a short cable run would have problems with the bitstream.

The ethernet transformer I found is specified 1MHz to 1000MHz. I guess the Bandwidth doesn't just end completely at 1MHz. I'll just have to try it and see if it works.

[tggzzz]

Why not just transmit the optical signal to the other end?

Because then I'd need extra cables for power. I want both power and ADAT stream in a single cable.

Not a unique requirement... The first google result of many is

CX-Series - Deployable/Composite Copper/Fiber Cables
Applications:
    Used in outdoor applications that require both optical fiber and copper wire elements for communication and power
    Copper wire can power remote electronics used in fiber optic communications
    Copper wire can also be used for low data rate data transmission
    Deployable cables have been used in network and private broadcast applications around the world
    Cables can be designed for your custom applications
    Cable is designed for use with United States National Electric Code (NEC) class 2 power sources
http://www.occfiber.com/main/index.php?m=1&p=2&l=en&i=304

[Richard Crowley]

Why not just transmit the optical signal to the other end?

Because then I'd need extra cables for power. I want both power and ADAT stream in a single cable.

Not a unique requirement... The first google result of many is

CX-Series - Deployable/Composite Copper/Fiber Cables
Applications:
    Used in outdoor applications that require both optical fiber and copper wire elements for communication and power
    Copper wire can power remote electronics used in fiber optic communications
    Copper wire can also be used for low data rate data transmission
    Deployable cables have been used in network and private broadcast applications around the world
    Cables can be designed for your custom applications
    Cable is designed for use with United States National Electric Code (NEC) class 2 power sources
http://www.occfiber.com/main/index.php?m=1&p=2&l=en&i=304

Yeah, good luck finding anything remotely resembling that that doesn't cost the better part of $10 per foot.
Even install-grade fiber is remarkably expensive. And you don't want to know what tactical-grade fiber cabling costs.

[tggzzz]

Why not just transmit the optical signal to the other end?

Because then I'd need extra cables for power. I want both power and ADAT stream in a single cable.

Not a unique requirement... The first google result of many is

CX-Series - Deployable/Composite Copper/Fiber Cables
Applications:
    Used in outdoor applications that require both optical fiber and copper wire elements for communication and power
    Copper wire can power remote electronics used in fiber optic communications
    Copper wire can also be used for low data rate data transmission
    Deployable cables have been used in network and private broadcast applications around the world
    Cables can be designed for your custom applications
    Cable is designed for use with United States National Electric Code (NEC) class 2 power sources
http://www.occfiber.com/main/index.php?m=1&p=2&l=en&i=304

Yeah, good luck finding anything remotely resembling that that doesn't cost the better part of $10 per foot.
Even install-grade fiber is remarkably expensive. And you don't want to know what tactical-grade fiber cabling costs.

Quite probably! I've no idea where to buy it because I'm not intertested and it isn't my problem.

My point was only that the reason given for not using optical transmission was invalid.

It is a classic time-vs-cost tradeoff. If someone simply wants the job done and dusted, it is probably simpler to grit teeth and buy it. If someone wants a "learning experience" (especially with rework) and the cost of their time is zero, then there's no reason why not to roll-your-own.

[Richard Crowley]

Quite probably! I've no idea where to buy it because I'm not intertested and it isn't my problem.

My point was only that the reason given for not using optical transmission was invalid.

It is a classic time-vs-cost tradeoff. If someone simply wants the job done and dusted, it is probably simpler to grit teeth and buy it. If someone wants a "learning experience" (especially with rework) and the cost of their time is zero, then there's no reason why not to roll-your-own.

Even if @Chaos_Klaus were designing a commercial product for sale, choice of REASONABLY PRICED cable is a PRIME factor in acceptance of the product.  Cat5 UTP would be my first choice of cabling for a product like this. It is ubiquitous, dirt-cheap and has probably the best bandwidth/cost of any commonly available multi-conductor cable (perhaps exceeded only by RG6 in the "commonly available" category).

Only TV production people can afford tactical-grade combined optical and power cable when they drive up with a truck holding $20M worth of gear and the better part of $1M just in cables.

[Chaos_Klaus]

Well, I'm not really reinventing the wheel here. The whole project is studio monitoring system with individual mixers for each musician. There are several commercial products that do just that. Many of them use CAT5 network cable with power. However, these things are quite expensive and hard to get in Germany. Lots of shipping cost and taxes on top of that. The things are bulky, or have way too many buttons.

I just wanted do too it myself. Because: "How hard can it be?" ... ... and I wanted to learn about digital audio.

My first idea was to use regular TOSLink connections and simply daisychain as many devices as I need for a certain recording. However that would have meant extra wall-warts for every device. That's really inconvenient in a studio environment. So by now I probably encountered the same realities as those commercial companies. In consequence, I ended up with a similar concept. A single distribution unit and several endpoints. Connections with cheap network cable.

Yeah. TV people ... it's really ridiculous what kind of stuff they use. Money doesn't seem to be too important in that sector.

I really try using cost effective components that are easy to aquire. I think that's a more elegant way to do it.


EDIT: How do I find out at what signal level my transformer cores will saturate? (Apart from just comparing primary and secondary side with an oscilloscope.) I can't find anything in the datasheet of the transformer. They probably assume that it's used with Ethernet signals at their typical levels.

EDIT2: The Wikipedia article on the Ethernet physical layer states that the signal in the twisted pairs is +-2,5V. On the recieving side I need 3.75V for HIGH level. So when I feed in a 5V signal on the transmitting side, I get -1.1dB insertion loss on each transformer (see datasheet) and some loss in the cable. I think that might cut it a little short. When possible, I'd rather drive the cable a little hotter and leave out an extra transciever at the receiving side. The input pin on the decoder is high impedance, I think.

[SeanB]

You probably will just want to use a digital receiver at the end, and terminate the line properly then use the squared up signal. Must be a few coax receiver chips that are usable in digital audio, and this will be a similar signal. Transmit side you just use line drivers and series termination.