Since the even and odd modes have different velocities, what comes out at the end will get really interesting. As in "impossible to untangle."
The modes will have different attenuation vs frequency curves, even more fun.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
It's not clear from your description whether you plan to encode your clock into your data or have something like a clock on one pair and your multi-level coded data on another pair of the Cat5 (or similar) cable.
If you plan on sending a separate clock and data you will need to take into account the different lengths (i.e. propagation delay) of the four pairs in the Cat 5 (or similar) cable. I spent quite a while debugging someone's design that worked like this a few years ago. This sent clock and approx
100Mb/s data in each direction along a Cat5 cable, 2 pairs in each direction. The thing worked with certain cable lengths, but not with others and there didn't seem to be any obvious pattern in the working/not working cases.
Eventually I realised that Cat5 cables have (intentionally) different "twists per inch" pairs, so the skew between clock and data depended on the cable length, and which pairs were longer and shorter in each type of cable (there's no fixed standard for this I think). At the receiving FPGA the setup and hold times were not satisfied in certain cases, leading to extreme flakiness.
Things were made even worse by the system's built-in cable length calibration mechanism. This sent a "training" pattern at link startup, to measure the round-trip delay/cable length and then advance or delay the transmitted data stream to compensate (this was all part of a more or less synchronous telecomms system where everything had to be in phase across several racks of equipment). Unfortunately the delay/advance mechanism was implemented using some combinational logic at the serial output of the (Altera) FPGA. The intention was that this would advance or delay the Tx data by various numbers of 100MHz clock periods, but unfortunately the combinational logic used gave variable delay on top of the wanted 10ns steps, further mucking up the clock relative to transmitted data timing. At the receiving end, with the variable cable-added skew, there was little chance of reliably receiving reliable data.
Long story short... it took quite a bit of fixing!
CATx unshielded (UTP) or foil-shielded (F/UTP) tends to have radically different twists on different pairs. Expect skew in the ballpark of 45 ns/100m between the fastest and slowest pairs.
*Some* cable with individual pair shielding (U/FTP or whatever) may have identical pairs, and might get down to maybe 5 ns/100m.
With baseband digital data, waveforms look like hell, risetimes in the
10+ ns range at 50m, increasing as the square of lenght. That varies a lot, too.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
Evidently 5-level signaling is standard on 1000-Base-T. Being at the "I use it, not build the hardware" end of that, news to me since I never really cared how it worked so long as it worked. Might mean you can repurpose GbE drivers for your work, or just use GbE to do your work, if a gigabit will do you. Any new Cat5 you can find today is probably Cat5e, though if you really want Cat5 I can part with some, slightly used.
However, I'd suggest a long hard look at optical fiber (bend insensitive unless you care to be fiddly with it) for anything you are building from scratch where high-speed data transmission is a big deal. Depending on exactly how special your setup is, you might be able to design around existing plug-in module parts (either dirt cheap used and 1-4Gb/s or more expensive and faster) to keep from having to do any fiddly optical work.
4Gb/sec fiber channel SFPs were down to $3.49 shipped last time I checked on sleazebay. Naturally you can pay more, but I've been using 30 or so of those clean IBM takeout SFPs for a couple of years (running GbE) now without a hitch. Faster ones can be had by applying more money.
Sleazebay or FIS, et al, can supply you with pre-terminated optical cable assemblies, so obtainability is not difficult at all.
--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.
I'm sure you will find that optical fibre is much easier to use than you imagine. Multi-mode (850nm) SFP transceivers are indeed very cheap on eBay. So are ready made duplex patch cables up to 30m or sometimes 50m long with LC connectors. It is easy to join several patch cables in series with cheap adapters.
Although the interface looks a bit complicated with i2c as well as differential data connections, you can completely ignore the i2c pins and everything will "just work". There are various types of SFP transceiver around, including Gbit Ethernet and various speeds of Fibre Channel. These are fundamentally the same, they just have different maximum signalling speeds. Some have a bandwidth control pin which adjusts the low-pass filter in the receiver to suit the data rate. Any SFP will give a low-pass corner at 1.5GHz or higher, regardless of the setting (so long as you avoid the old and rare
100Mbit ones).
These devices are completely protocol agnostic, so long as the data is coded with an equal number of high and low bits to prevent the detector dc working point from drifting around. Data rates below
1GBit/s should work fine, but avoid going too low in frequency in case the laser objects to driving at full power for too long. The internal dc blocking capacitors are normally 10nF in each 50 Ohm line of the differential pair. The laser is amplitude modulated between "just on" and "full brightness". Nearly all receivers include a comparator, so don't try driving multiple levels to get even more bandwidth!
Any variant is likely to work fine from a few hundred Mbit/s to just over 1Gb/s. Don't worry about matching brands at each end - the only significant difference is the manufacturer name stored in the i2c eeprom.
Avoid single mode devices (usually 1310nm) and cable unless you want to spend a bit more or need a range greater than a few hundred metres.
Just lurking... I'm not sure what you mean by even and odd modes. Can I just think of a vibrating string? even modes have a node on each end, odd modes have an anti-node at one end?
On a twisted pair, you can tie the wires together, in parallel, and drive them together as a transmission line, working against ground or a shield. The e-field of one wire bootstraps the environment of the other. The impedance of each wire in that case is the "even mode" impedance. The pair looks like that value over 2.
If you drive the pair diffferentially, the inter-wire capacitance is effectively doubled, as each wire sees the other slewing in the opposite direction, fighting it. So the impedance of each wire is lower than the even-mode impedance.
The modes have different impedances, velocities, and losses. The even mode is a better antenna, picking up and radiating.
As the pair of wires (or PCB traces) get farther apart, the impedances of the two modes will converge... the visibility of each wire to the other goes to zero as they get far apart.
There are calculators online for PCB pairs and wire twisted pairs. You can play with them to get feel for how the even/odd impedances change with geometry.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
I wish I could find single-mode SFPs at that price in the UK. I have been able to buy multi-mode devices for GBP0.99 plus minimal postage. Single-mode seldom sell for less than GBP10.00 plus postage. They are usually a lot more - I've been watching for a few months!
Prices in the USA do seem to be lower, but postage then becomes a problem.
I agree that new single-mode fibre is cheaper than multi-mode, but eBay prices don't seem to be very different for terminated patch cables.
Incwww.highlandtechnology.com jlarkin at highlandtechnology dot com
And, as Phil notes, there's Goofy Mode, where you use a twisted pair but ground one wire and use the other for signal. That has two simultaneous propagation modes, with different impedances and velocities and losses.
There must be analysies of that published somewhere. The math must be very messy.
It would be fun to try that on a sampling scope. I will if I have time.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
.highlandtechnology.com jlarkin at highlandtechnology dot com
Hmm you mean twisted pair inside a sheild. (Running twisted pair in the 'open' with one wire signal and the other ground seems normal, not goofy.... or is that still goofy?)
You could model the twisted pair as twin lead... So twin lead over a ground plane, maybe.
I must admit one part of me can see two different propigation modes/ impedances/ velocities. And the other part says, "ground is ground" and the extra ground wire may change the impedance and all, but doesn't give two modes.
(I suspect the "ground is ground" voice is wrong, but...?)
Say if you make stripline (one signal line between two ground planes) do you get two modes if the distance to each ground plane is different? I'm guessing that in this case the two ground planes are 'well connected' which makes all the difference.
Incwww.highlandtechnology.com jlarkin at highlandtechnology dot com
The pair could be shielded or not. Either way, there are two propagation modes. If it's unshielded, as Phil says, the even mode will radiate, too.
Parallel microstrip traces on a PCB, with opposite ground plane, is one case that TXline will analyze for odd and even modes.
Imagine wires A and B in a twisted pair. B is grounded, A is driven.
Suppose A steps 0 to +5 at the source and B stays at ground. Mathematically, that's equivalent to a common-mode voltage step of
+2.5 on both wires, summed with a differential step of 5 between them.
In reality, on a long pair, the signal starts off looking single-ended (5 volt step on A, ground on B, confirmed with a scope probe) and gets symmetrical at the middle of the run. At the far end, it's single-ended again, probe-able. Where the transition occurs, and how complete it is, is complex. A short run (relative to signal speed) will stay single-ended, and a long run will look differential over most of its length. Something like that. Messy.
A ferrite core (ie, a balun) can strip out the common-mode part.
I'm not sure. I'll look it up. I hate asymmetric stripline.
Parallel planes have a lot of capacitance. They are usually equipotential as far as a stripline is concerned.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Good point on the better skew characteristics of some U/FTP cables ... in my current job I recently had the need for some individually foil screened Cat
5 or better cable (for a military application with special EMC requirements).
I used a Datwyler cable (type 6002 - Cat 6). As you have pointed out, this has a delay skew of only 5ns/100m, i.e. pretty perfectly matched pairs. In my recent application (Gb Ethernet) skew wasn't an issue, but if I had had this cable type when I was sorting out the earlier problem with the 100Mb/s LVDS data plus clock, it would have simplified things greatly!
Either I'd misremembered what my supplier had dropped them to, or they raised it up 50 cents (currently $3.99) but they only ship the US anyway. I think they were $5.50 or so when I bought my pile, and I was happy not to be paying $35 each for new ones (best quote I had gotten for new, buying a lot of 30.)
All the low cost ones I've found are 4Gb/s fiber-channel takeouts - Fiber channel has left 4Gb dead at the side of the road, so they are as obsolete as punch cards or thickwire ethernet in that market. In most equipment (Cisco may be a notable exception from nasty rumors I've heard, but not confirmed since I don't use them) they work just peachy for gigabit ethernet.
Another (USA) seller has IBM 77P3338 for $1.49, + $7 shipping to USA (0.10 each additional - so if you buy three or more they are cheaper than the above) and does claim to ship worldwide, so you could see what they would charge - the things are quite small and light so they are pretty easy to ship cheaply...
There's also a vendor in Canada with a pile at a slightly higher price, but they may have cheaper shipping within the commonwealth? I don't know, but you could ask them.
All you can do is ask - some vendors will quote an absurd price, others will actually want to make a sale and may be able to give you a decent price on shipping, especially if you are not in a huge hurry to get things. First class mail small package cost from the post office is $5.46 for 4 ounces, which will fit 5-6 SFPs according to my scale. For $17, you can send an up to 4-lb priority mail box, perhaps 80-90 SFPs accounting for weight of box and packing. If you have use for a bunch, it might be worth inquiring about. Perhaps you can get a bulk shipment across and become the new low-price leader for the UK.
My own project got far beyond pre-terminated fibers, so the fiber cost factored into it for me. I agree that patch cords are often a wash.
--
Cats, coffee, chocolate...vices to live by
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