FreeLists / si-list / [SI-LIST] Re: differential pairs on backplane connectors

["Bert Simonovich" <lambert@xxxxxxxxxx>]

The explanation is these new connectors are specifically designed for
impedance control for edge coupled 100 ohm differential impedance and
minimum Xtalk. Using a 40 pair per inch Zd connector example, pins A,C,E
and G have a shorter path through the rt angle connector by approx 10ps
to their respective mates B,D,F and H. This intra-pair skew is then
compensated for by the PCB etch.=20

Since propagation delay through a medium is a function of dielectric
constant Er, the amount of trace compensation can vary depending on the
value of Er. Using FR4 & Er of 4.3 for example, the propagation delay is
176ps/inch. In etch length, this is approx. 1.5mm. Since the Zd
footprint intra-pair pin to pin spacing is 1.5mm  the etch breakout from
the connector will pretty much self compensate so long as the respective
pairs are length matched to up to pins B,D,F and H.

Using the 2mm hard metric connector example, there are 5 pins per
column; A,B,C and D. They have the pins routed in a close coupled
fashion as well. This would suggest the pairs be assigned as AB and DE
with pin C as a ground between the pairs. This arrangement would offer
better Xtalk control. I would caution against the assignment of the
pairs as you have suggested unless the overall link budget shows you
have margin.

Regards,

Bert Simonovich,


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Stephen Greenhalgh
Sent: Thursday, January 05, 2006 6:13 AM
To: Si-List (E-mail)
Subject: [SI-LIST] differential pairs on backplane connectors


When using standard pin matrix backplane connectors (for example, 2mm
hard metric), the conventional wisdom was to assign two adjacent pins in
the same row for a differential pair. This meant that the right angle
(daughter board) connections were of equal length and geometry. However,
the newer connectors, designed specifically for differential pairs (for
example, FCI Airmax VS, Amphenol GcX or ERNI ERmet ZD) use pairs of pins
in the same column. This seems surprising, since it appears to mean that
there is an inherent imbalance between the two connections. Perhaps this
is taken into account in the connector design? Can anyone provide an
explanation or clarification, please?

Stephen Greenhalgh

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