The broadside coupled trace is a common technique for routing differential pair signals.
The inductance of a broadside coupled trace is easy to calculate. Broadside coupled traces are on adjacent planes and the return is identical to the trace in width and length. It is important to note that in industry we can find that some differential pairs are routed this way for a strong coupling, but the actual return for the signal is on a power plane. This calculator is for the case where the signal return is on the adjacent trace. The geometry inputs are:
The inductance for a set of broadside coupled traces is defined as:
\frac{\mu _{o}\mu _{r}h}{w}
Where
(w>>h,h>t)