Cross Talk in Differential Signals
A trace near a differential signal can generate cross talk. The trace generates a crosstalk that has same direction for not the traces of the signal but have different magnitute. The trace that is nearer to the agressor trace has higher amount of the crosstalk than the trace that is farther from it. As a result there is a differential cross talk on the differential trace.
If the differential traces are routed tightly coupled then the amount of the crosstalk on the two traces have less difference. If the traces are loosely coupled, the amount of the noise generated on the two differential traces are of different amounts. This leads to the greater differential noise on the differential traces.
If the aggressor trace is far from the differential trace then the amount of the coupled noise is small for the both the differential traces. Also the magnitude difference of the noise on the two differential trace gets smaller.
Here are two simple rules that you should keep in mind if you want to reduce the amount of the crosstalk on a differential signal.
1. Keep the aggressor trace as far as possible from the differential trace.
2. Keep the differential trace as tightly coupled as possible.
Of course if you are interested in the quantitative figure, you may like to run some simulation and see what the ball park numbers are.