LVPECL differential pair routing - Page 1
Forum section: electronics / projects

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 20, 2015, 10:18:30 pm
content: As a learning exercise, I'm building a 4 GHz frequency counter, based on MC100EP series logic. I plan on counting directly, without any prescaling. Reference will be a GPSDO.Anyway, I'm reading up on termination of the signal lines and am getting a little confused.Some examples use single ended signals, and specify Z0=50R. No problem, I can do that.Others use differential pairs, and specify Zdiff = 100R. Makes sense, but... What about Z0 in this case? Does it still have to be 50R?Thanks,Thomas

author: nctnico
author_rank: Super Contributor
timestamp: January 20, 2015, 10:36:33 pm
content: Yes. Z0 has to be 50 Ohm. There are online calculators for PCB trace widths and distances.

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 20, 2015, 11:14:05 pm
content: Excellent, thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2015, 03:03:17 am
content: If the lines are routed willy-nilly, but still controlled 50 ohm impedance... yep, exactly the same. But match the lengths, of course!If you route them as pairs, you still must be concerned about their normal mode impedance (even traces very close together have a lower impedance against ground than against each other). In that case, the differential impedance would be controlled to 100 ohms, which means the normal mode will be somewhat higher than 50 ohms per, because the trace-to-trace impedance acts in parallel with it.You can draw the triangle of trace1-trace2, trace1-GND and trace2-GND impedances and see how this works out. The differential impedance is from trace1 to trace2, while the normal impedance (which is also how the impedance of a lone, unpaired, trace is measured) is trace to GND.Also, it can be worthwhile terminating said common mode impedance. Because, you'd normally run ECL as point-to-point with a terminator to VREF or whatever. While you could probably get away with a differential terminator and no connection to VREF, if you still terminate the traces to VREF independently, you automatically terminate (absorb) any energy carried in the common mode path, which will give your design better confidence and stability.Basically, what it comes down to is: for unrelated signals, you want to keep them relatively distant, to avoid cross-talk. For complementary signals, you can route them closer together, because you don't have to worry about cross-talk (they're already crossed, I guess you could say). You still can't route pairs closer to unrelated pairs, because of the same original reason.Tim

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 21, 2015, 04:23:36 pm
content: Quote from: T3sl4co1l on January 21, 2015, 03:03:17 amThat's the part that gets me...I have been playing with the various calculators, and it seems impossible to get both.Using Er = 4.3 and 0.2mm / 8 mil dielectric thickness, I get 13 mil traces for Z0 ~50 ohm.But! to get 100 ohm Zdiff the distance must be >27 mil, which will result in more or less two uncoupled 50 ohm lines.If I bring them closer together, I can get Zdiff ~100 ohm with 8 mil traces and 7 mil distance. This results in a Z0 of 63 ohm.

author: c4757p
author_rank: Super Contributor
timestamp: January 21, 2015, 04:37:55 pm
content: Itisimpossible to get both. Work out the math for the impedances, and you see that Z0approaches1/2 Zdiffas you get closer to ideal. It's a tradeoff. I usually prefer to have Zdiffaccurate than Z0, which seems like it'd be best for signal integrity in a properly routed system with truly differential inputs and outputs.If a large component of the signal current returns to ground rather than to the other signal, then Z0becomes more important. This depends on the topology of the receiver circuit.

author: jahonen
author_rank: Super Contributor
timestamp: January 21, 2015, 06:12:09 pm
content: Only way to get Z0 of 50 ohm and Zdiff as 100 ohm is to route them as two single ended traces, i.e. put them far away from each other so there is no coupling between pair halves. That would get Zdiff of 100 ohms and Zcm of 25 ohms.This works perfectly fine unless you have specification for both differential and common-mode impedances, like in USB case where spec says that differential impedance should be 90 ohms and common-mode impedance 30 ohms. Of course, routing differential pair traces as two single-ended traces is usually physically difficult, so one must accept higher Zcm than Z0/2 and lower common mode impedance than Z0*2 through the coupling.Having a termination which correctly terminates both differential and common-mode signals is always preferred to the simple differential termination.Regards,Janne

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2015, 08:15:29 pm
content: Quote from: ThomasDK on January 21, 2015, 04:23:36 pmExactly, they have to get thinner as they get closer because the coupled impedance reduces the total.Tim