On 3/31/12 12:24 PM, Achim Vollhardt wrote:
Dear John and all,
I do work in high energy physics and we use LEMO and other standards.
For some years now, I have started to advertise against LEMO (in
particular the LEMO 00 size), as it is VERY sensitive to mechanical
defects and partial connection (yes, you can ..). We have found very
often defective LEMO connections, which could only be detected via time
domain reflectometry. Or LEMO interfaces, which changed impedance
significantly when rotating them in the fully mated position.. this list
could be extended a lot more. And this famous click when mating is
inaudible in typical high energy physics electronics barracks.. too much
fan noise.
In addition, for the very fast signals of modern DAQ systems, LEMO 00 is
just not up to speed (literally) anymore. If size is not of ultimate
importance, we switched to SMA, or SMC if size matters.
On bad connectors.. "K" connectors are horrible.. they can intermate
with SMA, but they don't tolerate the range of SMA mechanical variation,
so it's possible to damage a K jack by mating an SMA plug with it. Not
all SMA plugs, but some percentage of them. And it's not obvious that
the jack has been damaged unless you measure it. You can mate a K or
SMA (properly) with it, it will work, mostly, but now, there's a tiny
gap in the center conductor, sometimes, depending on the connector flex,
etc.
We have a dreadful connector at JPL which is a mini twinax (used for
balanced pairs, like MIL-STD-1553). The key is a stamped metal bump so
small that it's easy to mismate in the 180 degree reversed position.
There's a much better version which is hermaphroditic (each side has one
male and one female pin) and it can only mate one way. But that other
connector hangs on, and on, because you get the "device A uses connector
type X", so you build test equipment with mating Connector Type X, then
the next device B has that connector, so we can re-use the test
harnesses. The next batch of test equipment is made to be compatible
with device B, and so it goes. The last people I know (not at JPL) who
bought those connectors called up the mfr and was quoted a spectacularly
long lead time (6 months or a year), so they asked who else had bought
them, maybe they could buy the half dozen they needed. Naturally, JPL
had bought the last batch, something like 15 or 20 years ago, and of
course, we had dozens of them sitting in bonded stores.
I was at an IEEE conference a few years ago on high speed interconnects
(10 Gbps is slow for those guys), and a bunch of the folks were talking
about how too much time is being spent on trying to get impedance
controlled connectors, instead of reliable connectors. Their point was
that in any real high speed (which would, for time nuts be "high timing
precision") system, you already have multiple transitions: die to IC
package pin to PWB trace to connector pin to connector, before you worry
about the connector's potential mismatch. These other ones are even
harder to control the impedance on, and trying to do so makes the boards
hard to route, etc, so why not bite the bullet and just implement a good
adaptive equalizer in your high speed interface. Then you can use a
mechanically robust connector that has a positive mating, can't be
mismated or partially mated, and in fact, the equalizer can tell you if
the cabling is screwed up.
From a cost standpoint, it might actually be cheaper. Implementing the
equalizers in a standard high speed SERDES doesn't cost much more in
silicon (yes, the IP development costs money, so the selling price is
higher), and you save a lot in the rest of the system.
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