adaptive equalizers for LVDS data over CAT5

[Winfield]

What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

 

[John Larkin]

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave, modulated by
triggers or other useful packets of data; all slaves would phaselock
their clocks to the master's, and decode global triggers and such. I'm
thinking distances like, say, 50 to 100 feet would be enough; if we
ever had to go really far, or had a bad emi environment, we'd probably
cut over to fiberoptics.

One economical way to do the interconnects would be to use LVDS signal
levels and CAT5 cabling. Long runs will be trashed by cable losses
(how long? gotta find out) but equalized receivers are available, and
some are automatic adaptive equalizers.

I'm just starting to think about this, and will post as I learn.
Anyhow, it's better than talking about climate change or how fat and
stupid Americans are.

Has anybody done anything like this, sending fast clock/data like
this? Do the equalizers work in real life? What about errors?

But this weekend's project is to measure the harmonic distortion of an
analog output stage that can swing as much as +-10 volts behind 50
ohms, up to at least 32 MHz [2]. That's tricky. Our other project is
to pick up a load of compost at the local sewage treatment plant, not
as much fun.

John


[1] I understand that some organizations have stopped using "master"
and "slave" out of cultural sensitivity, but I don't recall what the
pc substitutes are.

[2] It's interesting what rotten THD specs many modern high-end RF
signal generators and arbs have. I see specs like -30 dBc all over the
place. Old tube generators were a lot better. I guess that's what
happens when you have one VCO or whatever and you follow it with a
string of pin diode attenuators and modulators and levelers and
low-headroom wideband amps.

 

[MooseFET]

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,

I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

 

[Vladimir Vassilevsky]

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave, modulated by
triggers or other useful packets of data; all slaves would phaselock
their clocks to the master's, and decode global triggers and such. I'm
thinking distances like, say, 50 to 100 feet would be enough; if we
ever had to go really far, or had a bad emi environment, we'd probably
cut over to fiberoptics.

An old good RS-232 would do. Synchronize the slave clock PLLs to the time of
arrival of bytes. Of course, the Ethernet can be used for that purpose also
unless somebody will stuck the hub on the way.

One economical way to do the interconnects would be to use LVDS signal
levels and CAT5 cabling.

You don't really have to transmit the high speed data unless there is a need
for the fast external master trigger.

Long runs will be trashed by cable losses
(how long? gotta find out) but equalized receivers are available, and
some are automatic adaptive equalizers.

Adaptive Equalizer implies the unpredictable amount of delay.

I'm just starting to think about this, and will post as I learn.
Anyhow, it's better than talking about climate change or how fat and
stupid Americans are.

If it would be only the Americans. How great I am and how fat, stupid and
unfair is the whole rest of the world.

Has anybody done anything like this, sending fast clock/data like
this? Do the equalizers work in real life? What about errors?

I know what the classic equalizer is; however I don't know much about the
modern CAT5 equalizers.

But this weekend's project is to measure the harmonic distortion of an
analog output stage that can swing as much as +-10 volts behind 50
ohms, up to at least 32 MHz [2]. That's tricky.

How much of distortion? A spectum analyser with a notch filter on the
fundamental frequency will probably do.

Our other project is
to pick up a load of compost at the local sewage treatment plant, not
as much fun.

How to get rid of the extensive algae groth in the swimming pool? The double
doze of algaecide + oxidizer didn't help.

[1] I understand that some organizations have stopped using "master"
and "slave" out of cultural sensitivity, but I don't recall what the
pc substitutes are.

Would it be politically correct to paint the master unit in the black and
the slave unit in the white colors?

[2] It's interesting what rotten THD specs many modern high-end RF
signal generators and arbs have. I see specs like -30 dBc all over the
place. Old tube generators were a lot better. I guess that's what
happens when you have one VCO or whatever and you follow it with a
string of pin diode attenuators and modulators and levelers and
low-headroom wideband amps.

You may have to add a bandpass filter to get the clean signal out of the
generator.

VLV

 

[Bob]

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave, modulated by
triggers or other useful packets of data; all slaves would phaselock
their clocks to the master's, and decode global triggers and such. I'm
thinking distances like, say, 50 to 100 feet would be enough; if we
ever had to go really far, or had a bad emi environment, we'd probably
cut over to fiberoptics.

One economical way to do the interconnects would be to use LVDS signal
levels and CAT5 cabling. Long runs will be trashed by cable losses
(how long? gotta find out) but equalized receivers are available, and
some are automatic adaptive equalizers.

I'm just starting to think about this, and will post as I learn.
Anyhow, it's better than talking about climate change or how fat and
stupid Americans are.

Has anybody done anything like this, sending fast clock/data like
this? Do the equalizers work in real life? What about errors?

But this weekend's project is to measure the harmonic distortion of an
analog output stage that can swing as much as +-10 volts behind 50
ohms, up to at least 32 MHz [2]. That's tricky. Our other project is
to pick up a load of compost at the local sewage treatment plant, not
as much fun.

John


[1] I understand that some organizations have stopped using "master"
and "slave" out of cultural sensitivity, but I don't recall what the
pc substitutes are.

[2] It's interesting what rotten THD specs many modern high-end RF
signal generators and arbs have. I see specs like -30 dBc all over the
place. Old tube generators were a lot better. I guess that's what
happens when you have one VCO or whatever and you follow it with a
string of pin diode attenuators and modulators and levelers and
low-headroom wideband amps.

John,

You should look into how 10GBaseT is done. It's 10Gbps Ethernet over four
pairs (2.5Gbps full duplex per pair). The 'full duplex' aspect is what
really amazes me (obviously some pretty robust eq and echo cancellers).

It'll do 100 meters with CAT7 cable, but CAT5 is not officially included in
the spec (iirc). In the work I did with 10GBaseT, we were able to get about
30 meters with CAT5 at <10^-12 bit error rate.

http://www.ethernetalliance.org/technology/white_papers/An_Overview_of_10GBaseT.pdf

Bob

 

[John Larkin]

John Larkin wrote:
What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,

I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

National, TI, and Maxim seem to be the main guys doing LVDS
equalizers. What little I've seen so far is fairly vague as to how
they do adaptive receive equalization, and whether the signal rate and
data pattern influence equalizer training. We could send 8b/10b frames
or something to spread things out, I suppose.

Here's one intro:

http://www.national.com/appinfo/lvds/files/Analogedge9.pdf


Boosting up a tiny, ugly signal like this seems scairy to me.

As I said, I've got to do more reading, then experimenting.

John

 

[Bob]

John Larkin wrote:
What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,

I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

National, TI, and Maxim seem to be the main guys doing LVDS
equalizers. What little I've seen so far is fairly vague as to how
they do adaptive receive equalization, and whether the signal rate and
data pattern influence equalizer training. We could send 8b/10b frames
or something to spread things out, I suppose.

Here's one intro:

http://www.national.com/appinfo/lvds/files/Analogedge9.pdf


Boosting up a tiny, ugly signal like this seems scairy to me.

As I said, I've got to do more reading, then experimenting.

John

You should also look at how 10G ethernet CX4 version of XAUI is done...

http://www.commsdesign.com/showArticle.jhtml?articleID=29105954

....and look at what Broadcom calls their Eye Opener adaptive equalizer.

http://www.broadcom.com/products/Enterprise-Networking/10-Gigabit-Ethernet-Transceivers/BCM8011


Bob

 

[Martin Griffith]

On Sep 15, 8:58 am, John Larkin
John Larkin wrote:
What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,

I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

National, TI, and Maxim seem to be the main guys doing LVDS
equalizers. What little I've seen so far is fairly vague as to how
they do adaptive receive equalization, and whether the signal rate and
data pattern influence equalizer training. We could send 8b/10b frames
or something to spread things out, I suppose.

Here's one intro:

http://www.national.com/appinfo/lvds/files/Analogedge9.pdf


Boosting up a tiny, ugly signal like this seems scairy to me.

As I said, I've got to do more reading, then experimenting.

John

You should also look at how 10G ethernet CX4 version of XAUI is done...

http://www.commsdesign.com/showArticle.jhtml?articleID=29105954

...and look at what Broadcom calls their Eye Opener adaptive equalizer.

http://www.broadcom.com/products/Enterprise-Networking/10-Gigabit-Ethernet-Transceivers/BCM8011


Bob

the stuff at www.gennum.com looks interesting, although I only do
video, sort of low speed these days, any comments on gennum stuff?


Martin

 

[Nico Coesel]

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy

Has anybody done anything like this, sending fast clock/data like
this? Do the equalizers work in real life? What about errors?

I did something similar a few years ago with much slower clocks (2MHz
with a frame sync encoded in it) using RS485 transceivers to
synchronise E1 trunks. It worked very well except in an environment
where they had electric trains passing by while pulling out of a
railway station (engines at full power).

Besides, most gigabit ethernet connections are using multiple pairs
and/or multi-level encoding schemes which essentially pack more bits
in a clock period.

 

[Fred Bloggs]

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave, modulated by
triggers or other useful packets of data; all slaves would phaselock
their clocks to the master's, and decode global triggers and such. I'm
thinking distances like, say, 50 to 100 feet would be enough; if we
ever had to go really far, or had a bad emi environment, we'd probably
cut over to fiberoptics.

Doesn't CAT5 have intentionally different propagation delays for the
pairs? Anything can be equalized, there seems to be a ton of papers on
various more or less successful schemes, and you're not constrained the
same way IC designers are.

 

[Fred Bloggs]

[1] I understand that some organizations have stopped using "master"
and "slave" out of cultural sensitivity, but I don't recall what the
pc substitutes are.

How about dominant and subservient then...

 

[PeteS]

John Larkin wrote:
What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?
That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.
We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,

I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

National, TI, and Maxim seem to be the main guys doing LVDS
equalizers. What little I've seen so far is fairly vague as to how
they do adaptive receive equalization, and whether the signal rate and
data pattern influence equalizer training. We could send 8b/10b frames
or something to spread things out, I suppose.

Here's one intro:

http://www.national.com/appinfo/lvds/files/Analogedge9.pdf


Boosting up a tiny, ugly signal like this seems scairy to me.

As I said, I've got to do more reading, then experimenting.

John

I've had equalisers in the cable for 2.5Gb/s signalling and for a single
pair on really nice cable we went about 50 feet before the eye closed
out - that was a 8b/10b link. At 5Gb/s getting that range wasn't quite
feasible though. I don't think CAT5 would have given that performance,
though.

Using a line code (like 8b/10b) helps a lot because it minimises the
bandwidth necessary for the equaliser (it doesn't have to work down to
DC), although the equalisers I see advertised don't usually put such
requirements on it. I seem to recall that the maximum run length in
8b/10b is 5, incidentally.
Equaliser training is sensitive to the data pattern because of the
bandwidth of the patterns; the wider the signalling bandwidth, the
tougher it is for the equaliser - that seems obvious, but it's amazing
how many times it seems to get forgotten

The most common applications I know of for LVDS signalling are FPD-Link
(video data, DVI (more video and a horrendous EMI generator) and general
medium speed interconnects. I used LVDS for MPEG2 data about 10 years
ago, and each link was running at about 3.3Mb/s, not fast by today's
standards, but we were sending the data quite a long way and with up to
1024 links coming off the board we needed to manage crosstalk

I've been using LVDS recently for fairly short run interconnects (up to
20 feet) for link rates ~700Mb/s. I use pre-emphasis on those links
(some call it de-emphasis, confusingly) to get the range without a
receive equaliser (because I'm sending the data to someone else's
equipment and I can't do anything outside my own box).

For this app, you'd need to get a nice amplitude and phase v. frequency
plot for the cable you want to use - the better mfrs have them
available. That would help determine what a given eye would look like
after a particular run so you could choose an appropriate equaliser.

Note that limiting the driver rise/fall times can help minimise EMI
problems and ISI.

Aside: when I was working on an older piece of equipment (designed
mid-70s) that was running a 90Mb/s link in free space (4.5 - 6.25GHz
band) we had discrete adaptive equalisers for the IF with a dynamic
range of 50dB

Cheers

PeteS

 

[MooseFET]

[....]

I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

They obviously must. They need to know or at least be able to guess
at the amplitude and phase across the entire band that is used for
signalling. Unless you do something dumb, the curves will be smooth
ones but they won't be simple single pole cases.

Tape decks and hard disks have to solve parts of the same problem as
running signals over long cables. The encoding methods used restrict
the bandwidth of the signal to not include either low or high
frequencies. You may want to look at some of the methods they use.

 

[John Larkin]

On Sep 15, 8:58 am, John Larkin
John Larkin wrote:
What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?
That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.
We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,
I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

National, TI, and Maxim seem to be the main guys doing LVDS
equalizers. What little I've seen so far is fairly vague as to how
they do adaptive receive equalization, and whether the signal rate and
data pattern influence equalizer training. We could send 8b/10b frames
or something to spread things out, I suppose.

Here's one intro:

http://www.national.com/appinfo/lvds/files/Analogedge9.pdf


Boosting up a tiny, ugly signal like this seems scairy to me.

As I said, I've got to do more reading, then experimenting.

John

I've had equalisers in the cable for 2.5Gb/s signalling and for a single
pair on really nice cable we went about 50 feet before the eye closed
out - that was a 8b/10b link. At 5Gb/s getting that range wasn't quite
feasible though. I don't think CAT5 would have given that performance,
though.

Using a line code (like 8b/10b) helps a lot because it minimises the
bandwidth necessary for the equaliser (it doesn't have to work down to
DC), although the equalisers I see advertised don't usually put such
requirements on it. I seem to recall that the maximum run length in
8b/10b is 5, incidentally.
Equaliser training is sensitive to the data pattern because of the
bandwidth of the patterns; the wider the signalling bandwidth, the
tougher it is for the equaliser - that seems obvious, but it's amazing
how many times it seems to get forgotten

The most common applications I know of for LVDS signalling are FPD-Link
(video data, DVI (more video and a horrendous EMI generator) and general
medium speed interconnects. I used LVDS for MPEG2 data about 10 years
ago, and each link was running at about 3.3Mb/s, not fast by today's
standards, but we were sending the data quite a long way and with up to
1024 links coming off the board we needed to manage crosstalk

I've been using LVDS recently for fairly short run interconnects (up to
20 feet) for link rates ~700Mb/s. I use pre-emphasis on those links
(some call it de-emphasis, confusingly) to get the range without a
receive equaliser (because I'm sending the data to someone else's
equipment and I can't do anything outside my own box).

What sort of cabling? Your 20 feet could easily become my 50 feet at
200 MHz (ie, 400 mb/s), so just pre-emphasizing might work. I like
that, because it puts more energy into the line and should have better
s/n than receive equalization.

I could also maybe use some ghastly-high-power driver like an EL89,
add some homebrew pre-emphasis network, and still blast 3x LVDS levels
down the line! Sounds like some interesting experiments.

For this app, you'd need to get a nice amplitude and phase v. frequency
plot for the cable you want to use - the better mfrs have them
available. That would help determine what a given eye would look like
after a particular run so you could choose an appropriate equaliser.

I'm thinking shielded Cat5e, which is plenty available in pre-made
assemblies.

Note that limiting the driver rise/fall times can help minimise EMI
problems and ISI.

If you mean *other peoples* EMI problems, that's not my problem!


Good stuff: thanks.

John

 

[John Larkin]

John Larkin wrote:

What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave, modulated by
triggers or other useful packets of data; all slaves would phaselock
their clocks to the master's, and decode global triggers and such. I'm
thinking distances like, say, 50 to 100 feet would be enough; if we
ever had to go really far, or had a bad emi environment, we'd probably
cut over to fiberoptics.

Doesn't CAT5 have intentionally different propagation delays for the
pairs? Anything can be equalized, there seems to be a ton of papers on
various more or less successful schemes, and you're not constrained the
same way IC designers are.

We really only need one pair. We're already doing this at 32 MHz,
synchronizing VME waveform generator modules within one crate, at
essentially TTL levels. We send a constant 32 MHz square wave, which
slaves use to phaselock their oscillators, and occasionally invert a
cycle or so to convey global triggers and such. We want to extend the
concept to higher speeds and longer distances, like between racks.

Sending the clock and triggers on a single pair ensures that we won't
have any trigger timing (relative to clock) ambiguity. If we used
another pair for triggers, then we'd have to investigate pair skew.

Anybody got guesses as to Cat5e pair skew?

Maybe we could use the other pairs for some sort of slow signalling,
RS485 maybe, if we come up with a use for that.

John

 

[Bob]

John Larkin wrote:

What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.


We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave, modulated by
triggers or other useful packets of data; all slaves would phaselock
their clocks to the master's, and decode global triggers and such. I'm
thinking distances like, say, 50 to 100 feet would be enough; if we
ever had to go really far, or had a bad emi environment, we'd probably
cut over to fiberoptics.

Doesn't CAT5 have intentionally different propagation delays for the
pairs? Anything can be equalized, there seems to be a ton of papers on
various more or less successful schemes, and you're not constrained the
same way IC designers are.

We really only need one pair. We're already doing this at 32 MHz,
synchronizing VME waveform generator modules within one crate, at
essentially TTL levels. We send a constant 32 MHz square wave, which
slaves use to phaselock their oscillators, and occasionally invert a
cycle or so to convey global triggers and such. We want to extend the
concept to higher speeds and longer distances, like between racks.

Sending the clock and triggers on a single pair ensures that we won't
have any trigger timing (relative to clock) ambiguity. If we used
another pair for triggers, then we'd have to investigate pair skew.

Anybody got guesses as to Cat5e pair skew?

Maybe we could use the other pairs for some sort of slow signalling,
RS485 maybe, if we come up with a use for that.

John

If you get to the point that you need to worry about the pair-to-pair skew
then you might need to do something like what XAUI does.

XAUI uses the concept of "pair bonding" where the pair-to-pair skew is
relatively unimportant because each pair's data is fifo'd and then resynced.
It's more complex than you probably need and you do incur some latency, but
as a last resort it will work.

Bob

 

[Rene Tschaggelar]

That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.

Whoever,
why not have a slower clock to distribute and PLL
it up locally ? Targetting low phase noise I guess.
Nevertheless the skew stays the same. I'd counter
that with a MC100EP196, a digital delay chip to stay
flexible. On the other hand Onsemi has clock
distribution solutions that are thought to counter
clock skew.

Rene

 

[John Larkin]

Whoever,
why not have a slower clock to distribute and PLL
it up locally ? Targetting low phase noise I guess.
Nevertheless the skew stays the same. I'd counter
that with a MC100EP196, a digital delay chip to stay
flexible. On the other hand Onsemi has clock
distribution solutions that are thought to counter
clock skew.

We could do that, but we still need the triggers to be unambiguous as
to which 400 MHz cycle they fire on. So if we did distribute something
slower, the edge rates would still need to be ballpark 1ns to make
sure we don't slip a cycle. Given the parts available nowadays, it
seems pretty simple to just pump 200 or 400 MHz around.

It's a more general issue of how well one can pump logic-level signals
around, between racks maybe, using cheap, available parts, with one
candidate being LVDS over Cat5 cables and connectors.

John

 

[PeteS]

John Larkin wrote:

Top posting this comment: Inline responses

On Sep 15, 8:58 am, John Larkin
John Larkin wrote:
What we need here is a thread on adaptive equalizers for
LVDS data over CAT5. Anybody working with that?
That would be a great topic. How about the efforts made to
equalize TV or color-monitor signals sent over CAT5, that
might be a good start, or is it likely their bandwidth would
have been inadequate? Tell us your specs.
We're thinking about building a rackmount signal generator that has a
basic internal clock, ballpark 400 MHz maybe. It will have, say, 8
channels of output, analog or digital, with all channels synchronized
in time. If a customer wanted to expand to more channels, he'd buy
another box, and we'd interconnect them somehow. One box would be the
"master", and any number of additional boxes would be "slaves" [1].
The master would send out a 200 or 400 MHz square wave,
I don't think you want the idle state to be a square wave. With a
square wave you only get information about the workings of the cable
at a small collection of frequencies. When you send data, many
frequencies you couldn't learn about are used.

Do the equalizers acquire additional information from wider-bandwidth
data patterns?

National, TI, and Maxim seem to be the main guys doing LVDS
equalizers. What little I've seen so far is fairly vague as to how
they do adaptive receive equalization, and whether the signal rate and
data pattern influence equalizer training. We could send 8b/10b frames
or something to spread things out, I suppose.

Here's one intro:

http://www.national.com/appinfo/lvds/files/Analogedge9.pdf


Boosting up a tiny, ugly signal like this seems scairy to me.

As I said, I've got to do more reading, then experimenting.

John

I've had equalisers in the cable for 2.5Gb/s signalling and for a single
pair on really nice cable we went about 50 feet before the eye closed
out - that was a 8b/10b link. At 5Gb/s getting that range wasn't quite
feasible though. I don't think CAT5 would have given that performance,
though.

Using a line code (like 8b/10b) helps a lot because it minimises the
bandwidth necessary for the equaliser (it doesn't have to work down to
DC), although the equalisers I see advertised don't usually put such
requirements on it. I seem to recall that the maximum run length in
8b/10b is 5, incidentally.
Equaliser training is sensitive to the data pattern because of the
bandwidth of the patterns; the wider the signalling bandwidth, the
tougher it is for the equaliser - that seems obvious, but it's amazing
how many times it seems to get forgotten

The most common applications I know of for LVDS signalling are FPD-Link
(video data, DVI (more video and a horrendous EMI generator) and general
medium speed interconnects. I used LVDS for MPEG2 data about 10 years
ago, and each link was running at about 3.3Mb/s, not fast by today's
standards, but we were sending the data quite a long way and with up to
1024 links coming off the board we needed to manage crosstalk

I've been using LVDS recently for fairly short run interconnects (up to
20 feet) for link rates ~700Mb/s. I use pre-emphasis on those links
(some call it de-emphasis, confusingly) to get the range without a
receive equaliser (because I'm sending the data to someone else's
equipment and I can't do anything outside my own box).

What sort of cabling? Your 20 feet could easily become my 50 feet at
200 MHz (ie, 400 mb/s), so just pre-emphasizing might work. I like
that, because it puts more energy into the line and should have better
s/n than receive equalization.

That was cabling specifically designed for Infiniband / PCIe and the
like. Based on SpectraStrip, which is available from all the usual
suspects. (If you need a contact at AMP in Pa [which is where they make
their high speed cable assemblies], email me privately and I'll give you
a couple of names). There is usually some 'extra' hanging around for
experiments if there is a possible sale in the offing, as always.

Pre-emphasis is great for a wide range of issues (and it's not
pre-emphasis in the classic sense of boosting the edge, but limiting the
DC portion of the signal - the key is keeping the relative levels the
same). 6dB is pretty standard and available on most line drivers for
this sort of stuff. Look at the specs for the rocket IO on Xilinx parts
for a typical implementation of pre-emphasis in digital parts.

I could also maybe use some ghastly-high-power driver like an EL89,
add some homebrew pre-emphasis network, and still blast 3x LVDS levels
down the line! Sounds like some interesting experiments.

Oh my! Actually, bumping to 3x levels won't necessarily help a huge
amount - that's only 8dB (voltage wise) although at your speeds might
get you to where you want. Interestingly, the levels I worked with at in
InfiniBand (for up to 5Gb/s) were up to 1.6V drive.

I'm thinking shielded Cat5e, which is plenty available in pre-made
assemblies.

Good stuff, but get the datasheet anyway so you can predict the
performance

If you mean *other peoples* EMI problems, that's not my problem!

Actually, it's your problem - ISI is partly [mainly, imo] due to
deterministic jitter which is determined primarily by the
amplitude/phase v. frequency response of the cable. The EMI is caused by
the same issue and it's always nice to minimise emissions, because that
means more signal gets to the other end of the link.

Good stuff: thanks.

John

Note that 'pure' LVDS is a current mode output (CML) so there needs to
be a DC return path. On long runs, you want to AC couple for a number of
reasons, so put a terminator on the link at the source and then couple
to the cable through a nice cap.

Cheers

PeteS

 

[PeteS]

[1] I understand that some organizations have stopped using "master"
and "slave" out of cultural sensitivity, but I don't recall what the
pc substitutes are.

How about dominant and subservient then...

Well, CAN has dominant and recessive modes

Cheers

PeteS