## Measuring Differential Skew

The total differential skew at points *x*(*t*) and *y*(*t*) in Figure
1 equals the delay in the top path minus the delay in the bottom
path, *t*_{skew}=*t*_{1}+*t*_{3}+*t*_{5}–(*t*_{2}+*t*_{4}+*t*_{6}).
Rewriting this equation to group common terms produces *t*_{skew}=(*t*_{1}_{}–*t*_{2})+(*t*_{3}_{}–*t*_{4})+(*t*_{5}–*t*_{6}),
a number dependent on both the device under test and the matching of the test
cables or probes.

Many types of equipment allow you to automatically calibrate out, or *deskew*, the effect of test cables. To deskew your setup, first connect
the two cables directly from b to c and from x to y, shunting around the device
under test. Should delay *t*_{1}+*t*_{5} exceed delay *t*_{2}+*t*_{6}, your equipment automatically inserts
a compensating delay in the measuring circuit. This procedure doesn't eliminate
skew; it just inserts a permanent compensating delay, *Δt*=*t*_{1}+*t*_{5}_{}–(*t*_{2}+*t*_{6}),
in series with pathway *t*_{6}. Assuming the two pathways within
the device under test operate in an independent, uncoupled manner, further
measurements of skew with the device under test in place should always return
the correct (deskewed) answer.

What happens if you crisscross the connections at points c and y? Then the
signal *x*(*t*) comes from the inverted side of the generator, passing
through delays *t*_{2}, *t*_{4}, the crisscrossed
connection at c_{}–y, and finally *t*_{5}. A new measurement of skew
now gives you *t*_{crossed}=(*t*_{2}_{}–*t*_{1})+(*t*_{4}_{}–*t*_{3})+(*t*_{5}_{}–*t*_{6}).
One-half of the sum (*t*_{skew}+*t*_{crossed}) extracts the skew *following* point c_{}–y, which includes the last cable
section plus any artificial skew correction embedded within the receiver.
One-half the difference (*t*_{skew}_{}–*t*_{crossed}) extracts the skew *preceding* point c. By crossing the connections at
several points and retaking the data each time, you can determine the amount of
skew within each individual section of the setup.

Next, while the connections at c–y remain crisscrossed, make a second
crisscross at b and x. The average of this new measurement, *t*_{doublecross}, with the original, *t*_{skew},
isolates just the skew in the test setup,
(*t*_{1}–*t*_{2})+(*t*_{5}–*t*_{6}).
Half of (*t*_{skew}–*t*_{doublecross}) extracts just
the skew within the device under test. Crisscrossing is the basic technique for
investigating the accumulation of skew in highly cascaded systems.

A
highly coupled device under test, such as a transformer, turns up some serious
flaws in the deskewing procedure. Assume in Figure
1 that *t*_{2}, *t*_{5}, and *t*_{6} equal zero, but *t*_{1} doesn't. Place an
amount of compensating skew (*Δt*=*t*_{1}) in series with *t*_{6} to automatically balance out (deskew) *t*_{1}.
With the cables directly connected from b to c and x to y, the equipment reads
zero skew. Now insert a perfect, zero-skew transformer into the
device-under-test position. A perfectly balanced transformer passes differential
signals un-changed but blocks common-mode signals. The signal at point c_{}–y
therefore by definition has zero common-mode content and *zero differential
skew* (Reference 1). In this case, even
though the transformer is perfect, an automatically deskewed equipment setup
displays a skew equal to the negative of its internal compensation,
(–*Δt*). That's not right, illustrating perfectly my point that you can't
depend on automatic deskewing when measuring skew in tightly coupled
differential systems.

In a tightly coupled system, you must *separately* match both
transmitting and receiving cables for zero skew before you start measuring. You
know your setup is right when you can crisscross the instrument cables at any
point without altering the results.

### Reference

**[1]** Johnson, Howard, "Common-mode
analysis of skew," *EDN*, Jan 22, 2004