Digital clock through a length of cable - 50 ohm matching? - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 12:39:04 pm
content: Hi!I need to run a few clock signals through a length of a 50ohm coax cable. The clock source is a standard 3.3V CMOS level output and the receiver should be also a 3.3V CMOS input. How should I match the outputs and inputs to the cable run? The frequencies are in a broad range from few MHz up to 100MHz or so.How should I do the impedance matching, both for the output and input side?My idea was to buffer the clock output with a suitable gate with low enough output impedance (probably few gates in parallel?) and then match that to 50ohm with a series resistor.However on the reception side, due to the double termination, I have only half (or less) amplitude. How should I amplify back? Probably an AC coupled gate linearized with a feedback resistor, to make a sort of amplifier?What gates should I use for the clock buffering, to cover the range up to 100MHz and obtain low enough output impedance? Are my ideas correct or would you suggest using completely another technique?Thank you for help,Yan

author: Kalvin
author_rank: Super Contributor
timestamp: June 09, 2016, 12:42:24 pm
content: Match the driver side to 50 ohm and leave the receiving end unterminated. Use a separate driver for each signal ie. do not try to use one driver to drive multiple coax lines. Select the driver so that it can drive a 50 ohm coax. If you want to go on a budget, use one CMOS chip for each coax and connect the chip's logic gates in parallel increasing the drive capability.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 12:50:00 pm
content: Thank you for fast response. Shouldn't the open end of the transmission line cause problems with reflections distorting the signal?What gate would you use on the driver side?

author: Kalvin
author_rank: Super Contributor
timestamp: June 09, 2016, 01:02:42 pm
content: Quote from: Yansi on June 09, 2016, 12:50:00 pmNo, when the signal path is a point-to-point transmission line terminated with a high-z CMOS input, the signal quality will be just fine. Any reflections will decay in the transmitting side as the transmitter is matched to the 50 ohms. You can check that easily with a signal generator and a coaxial line.Sorry, cannot give any specific recommendations on the driver chip because I do not know what you are really after, what is acceptable signal quality, and I cannot validate the design by measurements. It the coaxial line is longer than a few meters, or you are expecting any EMI or noise issues, I would start looking for a proper high-speed coaxial driver IC and a proper receiver IC, and terminate the coaxial at the both ends.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 01:28:55 pm
content: It will be for some lab experimenting only, so no longer than a meter or so (just enough to connect few instruments or boards together on the bench).Do you think such gate as LVC245 might be sufficient to drive the coax? I'd probably use 3 outputs parallel per cable.

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2016, 01:43:58 pm
content: Quote from: Yansi on June 09, 2016, 12:50:00 pmIt actually restores the signal amplitude. Halved at the sender, doubled due to the reflection.Going from the datasheet the output impedance of LVC logic for TI seems ~15 Ohm driving low and ~25 Ohm driving high (upto ~20 mA drive). So somewhere between 40 and 45 Ohm series resistor with 3 paralleled outputs.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 02:53:20 pm
content: Which datasheet have you used? I couldn't find anything about output impedance (Rdson of the P/N mosfets) anywhere in the DS I have downloaded.Yes, open end should produce twice the amplitude, but I wasn't sure.

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2016, 03:08:28 pm
content: Look for an LVDS driver and receiver! Some shielded twisted pair cable will do the rest.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 03:37:57 pm
content: That would be nice, however I need 50ohm unbalanced cabling to be able to interface already existing stuff.

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2016, 06:12:48 pm
content: Quote from: Yansi on June 09, 2016, 02:53:20 pmYou can derive it from theOutput Drive Currentcurves, R = dU/dI.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 06:36:34 pm
content: Piece of cake of course, if you come over a datasheet with this info inside. Most of them don't include these characteristics. (And sure enough, both datasheets I've been looking at didn't include this).Thanks.Will try to build a prototype soon (until monday, maybe) and test it.

author: Kalvin
author_rank: Super Contributor
timestamp: June 09, 2016, 07:06:08 pm
content: I have used a trimpotentiometer to find the proper terminating resistor value. When the trimpot is adjusted to the correct value, I desolder the trimpot, measure the resistance and replace it with a real resistor. Works well enough.

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2016, 07:07:44 pm
content: Quote from: Yansi on June 09, 2016, 03:37:57 pmThen use a balun (=small transformer). That also breaks ground loops if it is a real transformer.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 08:05:25 pm
content: That might be good idea, I might have some suitable transformers here...https://www.minicircuits.com/pdfs/TC1-1T+.pdfThank you for suggestion, I will consider that possibility.

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2016, 08:56:04 pm
content: These kind of transformers are also sold by catalog distributors like Mouser, Digikey, Farnell, etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2016, 10:01:05 pm
content: It's perhaps regrettable that CMOS doesn't have a tighter input threshold range. But not to worry, there are plenty of ways to address that.First of all:Topology----------If you're only using point-to-point links, and the transmitter and receiver are well defined, then: use a source terminated configuration. The reflected wave piles up on itself at the receiver end, so the signal is always full amplitude (equal to the source) -- well, minus line losses, but this will still be good enough relative to logic thresholds unless the line is very long (>10m?). Use several logic outputs in parallel to reduce the uncertainty in source resistance, then use series resistors (one per pin, which helps current sharing too) to increase the total to a well defined match to the line. The total source resistance should be slightly less than the line Zo (maybe by 0-10%), so that you get slight overshoot to compensate for line attenuation.If you need multi-drop (linear) links, you need termination at the far end, and as little loading as possible from each intermediate node. Source does not matter, but probably needs to be terminated for best results (i.e., if it's a low impedance voltage source, you'll have problems with accidental situations: resonance under unterminated conditions, and fault current under short-circuit conditions).Avoid star configurations altogether. If you must, use buffer/fanout amplifiers to provide multiple linear or p2p links.Thresholds-------------CMOS isn't good, because the threshold is poorly controlled, typically 30-70% of VDD. That said... if you choose a family that is well-controlled, you're fine! TTL or LVCMOS compatible families (e.g., 74HCT) will interface very nicely with source-load terminated 5V CMOS.Otherwise, consider using a more complicated device, like an RS-422 or LVDS line receiver (strap the -IN to a voltage divider to set the desired threshold), or a comparator. They're cheap, and excellent insurance!Differential-------------You don't need to use differential signaling, and the addition of one more wire simply invites one more degree of freedom you need to control for: the common mode.As it is, you need to control common mode for the two-wire (one plus shield) cable. But this is usually handled by the fact that it's a solid hunk of metal on the outside, and that hunk of metal is physically bonded into the solid metal enclosure you are presumably putting the project inside. It's hard to care about common mode, if it washes effortlessly over your system!However, if you're not using solid grounding (e.g., a PCB mount BNC connector, especially one of the horrible cheap kinds where the ground is just a wire tacked on the ass end of the thing), do give it some consideration. You may want to treat the coax cable as a differential source itself, so that your receiver or comparator is sensing the difference at the connector (or as close to it as possible), without dropping voltage across the ground return path (which is where common mode gets in).Tim

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2016, 10:32:40 pm
content: Thank you for detailed reply Tim. The first solution with the parallel gates was the first I came across. However the receiver side was sort-of mystery.The link will be only point-to-point, no longer than a 1 meter or so, maybe quite shorter. I'll try the simplest possible, which is the unterminated receiver side. Should be enough. Would it help (or is it sensible) to use a schmitt input gate to restore the proper levels?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 10, 2016, 02:27:29 am
content: Quote from: Yansi on June 09, 2016, 10:32:40 pmSure!Tim