Background
Consider the structure pictured alongside — an edgecoupled differential pair, minus the underlying ground plane.
From a modelling standpoint the structure is like a paired wire
transmission line. However the question arises — as this is a differential
structure how should you test the impedance?

Edgecoupled differential pair
without ground plane

Modelling
this structure
The
model above does not appear directly in the Si8000/9000, however
there is a simple adjustment you can make to one of the standard
Si8000/9000 structures to enable you to predict the finished
impedance.

Select
the "Differential Surface Coplanar Waveguide 1B" from the
list of Si8/9000 structures (i.e. the structure without a lower
ground plane) and set the ground strip separation dimension, D1, so
it is 20 x that of the Trace Separation dimension, S1.
This way the adjacent
coplanar grounds will be far enough away to have negligible
influence. Now you can model the above structure before putting it
into production.

Differential Surface
Coplanar Waveguide

Testing the structure
In the
structure to be tested all the signal current flows out through one
conductor and the return current comes back through the other. With
a signal of equal positive and negative going potential a virtual
ground exists midway between the two traces.
How do I connect probes to this?
Imagine
using a differential probe with + on one line and – on the
other —
you are left with nowhere to connect the ground. So in order to
test you need to think of this structure as a single ended
transmission line and consider connecting to it via a single
ended probe. The signal goes to one side of the transmission line and
the ground connects to the other.
Using
a single ended TDR and probe connection you can connect the probe
either way around and measure the impedance of the structure using
the system set for single ended measurements. The measurement
returned will represent the differential impedance of the transmission line.
For this structure this will equal Zo, the singleended impedance.

Footnote by Dr Alan Staniforth
The impedance of the
structure is the ratio of the voltage between, and the current in, the
conductors. The concept of driving the conductors as a differential
pair implies the presence of a zero voltage ground. The definition of the controlled
impedance for this configuration does not require a ground. Hence, without loss of generality, one
conductor can have zero voltage assigned to it.
Thus only a singleended measurement is required, even though one pin of the measurement probe is at ground potential. A three pin differential measurement probe requires a track configuration which already has a ground to which the ground pin is connected. A differential probe, in practice, measures the impedance between the two active pins and the ground pin. The differential impedance is calculated from these two impedances. If the ground pin is not connected, the impedance at the active pins will be incorrect and so will be the differential impedance.

