connector for LVDS

[John Larkin]

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector? Something
like a BNC-type twinax, but smaller. Or a differential LEMO, but
cheaper. Two pins and a shield would be best, so an RJ-type thing
might not be so good. Two SMB or MCX coaxal connectors would work but
seems klunky.

John

 

[Joerg]

Hello John,

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector? Something
like a BNC-type twinax, but smaller. Or a differential LEMO, but
cheaper. Two pins and a shield would be best, so an RJ-type thing
might not be so good. Two SMB or MCX coaxal connectors would work but
seems klunky.

3.5mm stereo phone plugs? ...... SCNR

USB plugs come to mind, being cheap and plentiful. Of course that
carries the risk of plugging into the wrong system. But you may be able
to make sure electrically that nothing will fry if someone does that.
Then you'd have four contacts plus shield. The host sides are a bit wide
and clunky but the other end is a smaller square. If you'd have the
small squares only then nobody could plug into a laptop or PC.

Regards, Joerg

 

[[email protected]]

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector? Something
like a BNC-type twinax, but smaller. Or a differential LEMO, but
cheaper. Two pins and a shield would be best, so an RJ-type thing
might not be so good. Two SMB or MCX coaxal connectors would work but
seems klunky.

Have a look at one of the smaller D-type hybrid shells,

http://rswww.com/cgi-bin/bv/rswww/subRangeAction.do?catoid=-1600754777&cacheID=ukie

The coaxial inserts are essentially SMB - I think we got away with
using a standard SMB crimp tool to crimp them - and they are both
robust and tidy. The standard D-type pins are a bonus - you could use
them for power supplies or extra shield connections.

I don't know of a stocking distributor in the US, but there ought to be
at least one. RS Components aren't my favourite distributor. but they
are the only source of these parts I know of in Europe.

 

[Winfield Hill]

[email protected] wrote...

Have a look at one of the smaller D-type hybrid shells,
http://rswww.com/cgi-bin/bv/rswww/subRangeAction.do?catoid=-1600754777&cacheID=ukie

The coaxial inserts are essentially SMB - I think we got away with
using a standard SMB crimp tool to crimp them - and they are both
robust and tidy. The standard D-type pins are a bonus - you could
use them for power supplies or extra shield connections.

Those have always seemed a bit too specialized and complex for my
taste, surely there's something better suited for this new common
problem. One doesn't need 3GHz bandwidth for most LVDS channels.
John, what about differential LEMOs? Maybe they aren't so bad?

 

[John Larkin]

[email protected] wrote...

Those have always seemed a bit too specialized and complex for my
taste, surely there's something better suited for this new common
problem. One doesn't need 3GHz bandwidth for most LVDS channels.
John, what about differential LEMOs? Maybe they aren't so bad?

No, they're beautiful connectors, just expensive.

John

 

[[email protected]]

[email protected] wrote...

Those have always seemed a bit too specialized and complex for my
taste, surely there's something better suited for this new common
problem. One doesn't need 3GHz bandwidth for most LVDS channels.
John, what about differential LEMOs? Maybe they aren't so bad?

I first ran into this problem back in the late 1980s. Within a rack,
the mixed signal DIN41612 connectors worked extremely well, letting us
plug in all the coaxial connections at once.

The hybrid D-types use the same coaxial inserts, but they come with
back-shells, screw locks and all the rest of the paraphenalia that you
need to make a reasonably tidy and secure connection between two
free-standing boxes.

They were much cheaper than LEMO connectors, and available off the
shelf, in small quantities - LEMO stuff (though quite beautiful) can be
hard to get hold of.

At that time we got all our hybrid connectors from the Quadrant
Connector company, who looked after Cambridge Instruments very well
indeed. They still exist but I don't know anything about what they sell
today, or how they sell it.

http://www.quadrant-ltd.co.uk/English/Products.html

The temptation to use multiple BNC or SMA/SMB/SMC connectors should be
resisted. No matter how carefully you label the cables, some idiot
graduate student/technician will plug them in in the wrong order, then
give you a hard time about your unreliable design.

The pleasure one gets from pointing out that the cables have been
plugged into the wrong sockets is no compensation for having to spend
half an hour walking around to the equipment and persuading the
offenders to look at what they've done wrong.

The first time we ran into the problem, we threaded all the half-dozen
coax cables involved through strategically placed holes in a perspex
plate (labelled the "monkey frame" by the technician who did the work)
before fitting the coaxial connectors.

We then bound the cables together close to the monkey frame and the
connectors so that there wasn't enough slack to let anybody plug the
cables into the wrong sockets. It worked fine, but wasn't elegant.
Since the machine was minimally improved copy of a Siemens-physicists'
development machine, which Cambridge Instruments then sold to
Thopmson-CSF, this wasn't the only inelegance. The production version
was quite a lot better, and the fully digitised version (which got
canned before we could get it into production) was an absolute dream

 

[Tim Shoppa]

Something like a BNC-type twinax, but smaller.

Mini-DIN's are smaller than a BNC twinax. And a lot more readily
available than LEMO's.

There is a connector called the "mini twinax" in the computer world but
I don't know where to buy the raw plugs/jacks.

Two pins and a shield would be best, so
an RJ-type thing might not be so good.

Pair-to-pair skew doesn't hurt you when you've only got one pair!
Don't knock the easy availability of CAT5 stuff.

Two SMB or MCX coaxal connectors would
work but seems klunky.

Amphenol Fakra's seem to be popping up everywhere these days, and they
seem to be exactly dual SMB's. My impression is that this is the GHz
range auto-RF connection being pushed these days.

Tim.

 

[[email protected]]

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector?

Circular DIN connector? XLR audio connector? On the other hand, if a
BNC is too big, those might be too big as well. Maybe a mini-DIN (PS/2
keyboard and mouse) connector?

Two SMB or MCX coaxal connectors would work but seems klunky.

Use one of each on each box and make cables to match - that way it's
harder to hook up the cables backwards.

Matt Roberds

 

[Fred Bloggs]

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector? Something
like a BNC-type twinax, but smaller. Or a differential LEMO, but
cheaper. Two pins and a shield would be best, so an RJ-type thing
might not be so good. Two SMB or MCX coaxal connectors would work but
seems klunky.

John

Amp always had those individual pin type things for coaxial termination-
you don't get smaller than that, but they hold on by friction. Then
there's Gore, not so expensive when dealing with stock items.

 

[Joerg]

Hello John,

No, they're beautiful connectors, just expensive.

How about the low-tech low cost path, red and white RCA? Then you could
use audio cables from Walmart and buy color coded jacks for your modules
almost anywhere and any style

Regards, Joerg

 

[John Larkin]

Hello John,

How about the low-tech low cost path, red and white RCA? Then you could
use audio cables from Walmart and buy color coded jacks for your modules
almost anywhere and any style

Regards, Joerg

Seems to me that using two separate coax'es for the two sides of the
LVDS is asking for trouble. If one has a different prop delay from the
other, then, after a transition, there will be a short time when both
signals are at the same level, which sure sounds nasty from a jitter
standpoint. If I went for two connectors, we could use SMB or MCX
types, both cheap and easy to get.

Actually, I guess a twisted-pair has a similar but less severe
problem: classicly there's an even mode and an odd mode, but it seems
to me there are actually three prop modes: line1 to ground, line2 to
ground, and differential. And the first two modes could have different
prop delays. So you could get silly looking waveforms at the receiver.

John

 

[KoKlust]

Speeds up to 240 MBit/s can be handed via cheap plain old-fashioned SUB-D
connectors and STP, IF you take some precautions:
- chose the pinning so that the LVDS pairs are on two adjacent pins
- all pins surrounding a pair must be connected to ground
- use a shielded twisted pair cable with matched differential and single
ended impedances
- keep the number of connectors in the line as low as possible
- once built, verify the LVDS signals for integrity / eye pattern.

Of course good HF connector is different.

 

[[email protected]]

Seems to me that using two separate coax'es for the two sides of the
LVDS is asking for trouble. If one has a different prop delay from the
other, then, after a transition, there will be a short time when both
signals are at the same level, which sure sounds nasty from a jitter
standpoint. If I went for two connectors, we could use SMB or MCX
types, both cheap and easy to get.

The rise and fall times for LVDS are 0.5nsec, which is four inches
(100mm) of propagation delay in normal coax. You shouldn't have much
trouble matching your coax cable lengths to better than that - even
half an inch (12.7mm) difference in length would be pretty obvious
after assembly.

 

[Joerg]

Hello John,

Seems to me that using two separate coax'es for the two sides of the
LVDS is asking for trouble. If one has a different prop delay from the
other, then, after a transition, there will be a short time when both
signals are at the same level, which sure sounds nasty from a jitter
standpoint.

True. I guess nowadays they are all machine made and molded but the
coaxes can come from two separate spools during the process. I doubt the
prop delays will be much different though because that's all in the
materials.

...If I went for two connectors, we could use SMB or MCX
types, both cheap and easy to get.

SMB cheap? Wow. Last time I heard that was from the mil spec world.

Actually, I guess a twisted-pair has a similar but less severe
problem: classicly there's an even mode and an odd mode, but it seems
to me there are actually three prop modes: line1 to ground, line2 to
ground, and differential. And the first two modes could have different
prop delays. So you could get silly looking waveforms at the receiver.

Twisted pair is surprisingly good in group delay flatness. It does come
shielded but it's not very flexible and looks ugly once somebody made a
sharp bend.

Regards, Joerg

 

[H]

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector? Something
like a BNC-type twinax, but smaller. Or a differential LEMO, but
cheaper. Two pins and a shield would be best, so an RJ-type thing
might not be so good. Two SMB or MCX coaxal connectors would work but
seems klunky.

I know you said you wanted small, but CAT 5/5e is damn cheap and very
suitable for differential use -- it might be perfect if you can afford
the larger RJ-45 jacks.

 

[[email protected]]

Hello John,




True. I guess nowadays they are all machine made and molded but the
coaxes can come from two separate spools during the process. I doubt the
prop delays will be much different though because that's all in the
materials.

If the cables were long enough for propagation delay differences to
become ostensibly significant (aproaching 0.5nsec), the degradation in
the pulse edge speed would probably be much more important - coax isn't
perfectly non-dispsersive.

SMB cheap? Wow. Last time I heard that was from the mil spec world.

SMA is the mil spec minature coax connector, though SMB doesn't seem to
be much cheaper.

 

[Mikko Kiviranta]

If one had a couple of small boxes and wanted some single-lane LVDS
connections between them, any idea of a suitable connector? Something

I don't have hands-on experience with LVDS, but wouldn't that
need a controlled transmission-line impedance for the differential
signal? Others have proposed guiding the lines through two separate
MCX/RCA/whatever connectors here in s.e.d.- wouldn't that introduce
a significant loop and thus lumped inductance to the signal path?
Hmmm, I suppose if the termination is w.r.t the common
ground (rather than between the diff wires) and the drive is
strictly symmetrical the loop won't pose a problem.

The reason I'm interested is that I need to feed a number
of twisted pairs through some sort of a multi-pin connector.
So far I've been using the 1.27mm pitch double-row headers,
using a pair of pins for the twisted pair and grounding the
neighbouring pin pairs. I'm sort-of happy with this,
even though opening the twist and feeding through the header
introduces ~10nH worth of lumped inductance, but the 1GHz
cutoff on the 60-ohm differential line due to this (and more
importantly the phase shift present at lower frequencies) is
tolerable.

That header is not suitable for John because there is no
shell. I would be glad to hear of a shelled connector for
multiple twisted pairs, and I suppose John could use those
also. To make the matter not-too-easy I'd prefer the connector
to be vacuum tight. One possibility would be to use Stycast
to make a leaky connector vacuum tight. Any suggestions?
(Microminiature D ? SCSI2 ? Others?)

Regards,
Mikko

 

[John Larkin]

If the cables were long enough for propagation delay differences to
become ostensibly significant (aproaching 0.5nsec), the degradation in
the pulse edge speed would probably be much more important - coax isn't
perfectly non-dispsersive.


SMA is the mil spec minature coax connector, though SMB doesn't seem to
be much cheaper.

We're paying $2.31 for the basic pcb-mount right-angle SMB. I hate
SMA's... you spend half your life screwing/unscrewing them.

MCXs are very nice, and there are tons of cheap cable assemblies on
ebay.

John

 

[John Larkin]

I don't have hands-on experience with LVDS, but wouldn't that
need a controlled transmission-line impedance for the differential
signal? Others have proposed guiding the lines through two separate
MCX/RCA/whatever connectors here in s.e.d.- wouldn't that introduce
a significant loop and thus lumped inductance to the signal path?
Hmmm, I suppose if the termination is w.r.t the common
ground (rather than between the diff wires) and the drive is
strictly symmetrical the loop won't pose a problem.

The reason I'm interested is that I need to feed a number
of twisted pairs through some sort of a multi-pin connector.
So far I've been using the 1.27mm pitch double-row headers,
using a pair of pins for the twisted pair and grounding the
neighbouring pin pairs. I'm sort-of happy with this,
even though opening the twist and feeding through the header
introduces ~10nH worth of lumped inductance, but the 1GHz
cutoff on the 60-ohm differential line due to this (and more
importantly the phase shift present at lower frequencies) is
tolerable.

That header is not suitable for John because there is no
shell. I would be glad to hear of a shelled connector for
multiple twisted pairs, and I suppose John could use those
also. To make the matter not-too-easy I'd prefer the connector
to be vacuum tight. One possibility would be to use Stycast
to make a leaky connector vacuum tight. Any suggestions?
(Microminiature D ? SCSI2 ? Others?)

Regards,
Mikko

I've used SCSI2 connectors and commercial cables for differential PECL
over short distances, but differential prop delays (one wire of the
pair relative to the other) added jitter, on the order of 30-40 ps rms
over a meter or two of cable.

John

 

[John Larkin]

Speeds up to 240 MBit/s can be handed via cheap plain old-fashioned SUB-D
connectors and STP, IF you take some precautions:
- chose the pinning so that the LVDS pairs are on two adjacent pins
- all pins surrounding a pair must be connected to ground
- use a shielded twisted pair cable with matched differential and single
ended impedances
- keep the number of connectors in the line as low as possible
- once built, verify the LVDS signals for integrity / eye pattern.

Of course good HF connector is different.

Yeah, it's sounding like a plain ole D9 would be the best pick for two
or three shielded lvds signals. At least they're easy for everybody to
get and wire to.

John