Re: Differential Pair Skew

Andy Ingraham (ingraham@wrksys.ENET.dec.com)
Tue, 30 Jul 96 08:11:18 EDT

Hi Don,

Here are a few more comments to go along with your discussion/questions
about Differential Pair Skew.

You wrote:
> I've put together a little Spice simulation to help me
> visualize the circuit principles and various termination techniques.

Just be careful that SPICE models the ideal world; and unless you have
modeled things very carefully and thoroughly, they do not necessarily
reflect reality to the extent that may be necessary.

The same should be said of idealized, hand-drawn waveforms, like the
ones below.

You wrote:
> Question/Comment - The bottom waveform would be the output of a
> differential amplifier (ignoring amplitude). The output is skewed from
> the input, but original bit cell time is preserved. If I did a BER eye
> diagram using a differential amplifier and a scope the skew in this
> case would have no effect on the eye. The only effects on the eye
> would come from differences in rise/fall times, differences in Voh and
> Vol between drivers, and jitter (ignoring loss).
>
>
> Differential Pair with skew
> + ------\ /-----------------------------\ /---------
> \ / \ /
> X \ /
> / \ \ /
> / \ \/
> / \ / \
> - ______/ \________________________/ \_________________
> ^ ^
> | |
> | |
> Output of Differential Receiver
> /---------------------------------\
> / \
> / \
> ____________/ \______________

For the signals in question, how close is the above to the actual
waveshape? Do you really have nice clean pulses with relatively short
rise/falltimes and long rest periods in between? In many communications
signals for which the channel bandwidth is efficiently utilized, the
signal has barely risen when it is already falling again.

Consider the following case, without differential skew:

+ ------\ /----------\ /----\ /----------------
\ / \ - - / \ /
\ / \ / \ / \ / \ /
X X X X X
/ \ / \ / \ / \ / \
/ \ / - - \ / \
- ______/ \__________/ \____/ \_________________

Note the activity in the middle, where the signals don't go far beyond
crossing one another before they switch again; yet the eyes are open.

Now add differential skew:

+ ------\ /----------\ /----\ /---------------
\ / \ - - / \ /
\ / \/ \ / \/ \ /
\/ /\ \/ /\ \/
/\ / \ /\ / \ /\
/ \ / - - \ / \
- _______/ \__________/ \____/ \_________________

... and a bit more:

+ ------\ /----------\ /----\ /--------------
\ / \ - - / \ /
\ / X \ / X \ /
\ / / \ \ / / \ \ /
X / \ X / \ X
/ \ / - - \ / \
- ________/ \__________/ \____/ \_________________

Look what's happened to the eyes. They've shrunk considerably. Yet the
skew was much less than one risetime of the full amplitude signal.

But even this is a simplistic case that doesn't take into account things
like impedance mismatch and signal reflections, different propagation
velocities for the differential-mode and the common-mode signals, etc.
All of these things also shrink the eye.

You wrote:
> Question/Comment - The output risetime of a differential receiver is
> determined by the gain, output buffer, and output loading
> characteristics. Of the three, only gain relates to the differential
> input. Therefore the slew rate of the differential inputs effects the
> rise time of the differential receiver output. Loss of energy in the
> harmonics will result in decreased slew rate. Skew will not effect
> differential receiver output edge rate.

Real signal edges are often rounded so that the greatest slew rate
happens near the middle. Like sine waves. Differential skew forces
them to cross one another away from their middles, where the slew rate
can be much lower.

In the extreme case where the skew exceeds one risetime, one signal will
have risen fully before the other one starts falling, or vice-versa.
The differential input would be near zero for some length of time. Ugh.

The simple addition of a common-mode AC signal can affect the receiver's
output, unless the receiver has infinite common-mode rejection at high
frequencies. None do.

Regards,
Andy Ingraham