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Differential Pairs Routing: Uncoupled Length DRC error

federico.massimi , 03-21-2022, 06:54 AM
Hi, I have a problem with differential tracks. I created an impedance profile for 100 ohm differential and set a rule to use it.

But in the DRC I always have this error:
Differential Pairs Routing: Between Net TR0_P And Net TR0_N [Uncoupled Length = 28.946mm], [Maximum Uncoupled Length = 12.7mm]
Differential Pairs Routing: Between Net TR1_P And Net TR1_N [Uncoupled Length = 31.047mm], [Maximum Uncoupled Length = 12.7mm]
Differential Pairs Routing: Between Net TR2_P And Net TR2_N [Uncoupled Length = 29.227mm], [Maximum Uncoupled Length = 12.7mm]
Differential Pairs Routing: Between Net TR3_P And Net TR3_N [Uncoupled Length = 28.73mm], [Maximum Uncoupled Length = 12.7mm]

These are the tracks I'm talking about, how can I fix?

These traces are all about 30 mm long; therefore, Altium is saying that there is an Uncoupled Length for practically the entire length of the track.

Can anyone help me?
Thanks in advance
Lakshamana Balakrishnan , 03-21-2022, 10:07 PM
@federico.massimi If the gap of the differential pair is outside of the tolerance of the primary gap, then the pair is uncoupled.
If the amount of uncoupled routing is greater than the maximum uncoupled length constraint, then a DRC will be generated.
WhoKnewKnows , 03-23-2022, 07:11 PM
It looks like you have the min, preferred, and maximum gap set to 0.127mm. What is the actual gap when you use Reports>Measure? You should be able to zoom way in and measure the actual gap. Did you use Altium's differential routing tool or did you place these tracks next to each other so neatly as shown in the photo?
federico.massimi , 03-24-2022, 04:30 AM
@@Lakshamana What is the primary gap that you are talking about?

@@WhoKnewKnows The distance between the tracks is exactly 0.127mm. I used Altium's differential routing tool.
WhoKnewKnows , 03-24-2022, 04:55 AM
It looks like you have 3 differential pair routing rules. Is it possible that one rule is being followed during routing, while the DRC is failing against another rule?
WhoKnewKnows , 03-24-2022, 05:07 AM
In case the max uncoupled length is the same for all 3 rules, try setting the max uncoupled length of each rule to something slightly different, EG 12.701, 12.702, and 12.703,. This will telegraph exactly which rule is being broken. @robertferanec , What's the pro way to do this? Rules & violations panel?
federico.massimi , 03-25-2022, 04:57 AM
@ @WhoKnewKnows - I followed your advice and saw that the rule that is being violated is this, the one for generic differential pairs.

At this point it was enough for me to lower the min Gap value to remove the violation. Easier than expected.
A thousand thanks.
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