Differential Crosstalk

HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 3 Issue 20

*---------------------(QUESTION)---------------------*

DIFFERENTIAL CROSSTALK

Alex Leyn writes:

I have a number of high-speed differential PECL signals that I need to route in parallel on the PCB. Routing space is an issue, so ideally, minimal trace separation is desirable. The board is 10 layers with the following stackup:

       1 components and signal    
       2 --------------------- plane      
       3        signal      
       4 --------------------- plane      
       5     +A+   +B+   +C+      
       6     -A-   -B-   -C-      
       7 --------------------- plane      
       8        signal      
       9 --------------------- plane      
      10 components and signal
   

+A+ and -A- (and B and C) are the differential pair traces (one on top of the other). The + to - plane distance is 8.5mils. The top plane to + and bottom plane to - distance is 6 mils. The worst case situation is to assume that A and C are the aggressors on B the victim.

If A, B, and C were not differential, the "3 times rule" would imply a trace spacing of 3*6=18mils to achieve a crosstalk factor of 0.1 (using K=1/[1+(D/H)^2] calculation) from A to B and from C to B. However, I believe that since the aggressors are differential, most of the E field will be cancelled and the crosstalk factor would increase much more rapidly with trace separation D.

Do you agree? What would the K calculation be for this topology?

Thank you very much.

*----------------(DR. JOHNSON REPLIES)---------------*

Thanks for your interest in High-Speed Digital Design.

While it's true that two widely separated differential trace pairs on a PCB will have very little crosstalk, the differential approach is practically useless when the traces must be closely spaced. Here's why:

Crosstalk falls off very rapidly with distance. For closely spaced traces, the crosstalk from +A+ to +B+ is MUCH stronger than crosstalk from -A- to -B-.

To give you a sense of what might work in your application, I ran your configuration through a 2- dimensional field calculator from Hyperlynx and came up with the following numbers. All cases assume the use of source-terminated, 3.3-V differential drivers and receivers. The + to - plane distance is 8.5 mils, the top plane to + and bottom plane to - distance is 6 mils, and the trace width is 7 mils.

Trace pitch	Crosstalk
(mil)	(mV)
13	220
15	124
17	69
19	38
21	22
23	12
25	7

If you are working with a bus, remember to add crosstalk from your two nearest neighbors, plus a little from the next two traces over.

If you want to compute these answers yourself, look into the software tools available from Mentor, Cadence, Veribest, and Hyperlynx (among others). They have done a lot of work on differential trace impedance lately and all have ways to directly compute the crosstalk for your configuration.

Oh, as a little bonus, I calculated one other configuration. In this design, the + to - plane distance is 12.5 mils, the top plane to + and bottom plane to - distance is 4 mils, and the trace width is 5 mils. The general idea here is that pressing the traces closer to the planes will give you a little less crosstalk, but more difficulties with skin effect losses on long traces.

Trace pitch	Crosstalk (new layer stack)
(mil)	(mV)
13	143
15	80
17	45
19	25
21	14
23	8
25	4.5

Best Regards,
Dr. Howard Johnson

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