XILINX RocketIO / MGT signal quality problems

[Michael Mustermann]

Hi anyone out there,

we have designed a board with XILINX Virtex II Pro using the
RocketIO / MGT serial high speed transceivers.
Recently we experienced a problem with bit error rate (BER) and
measured the signal quality of the MGT serial links with a 20Gsample
scope and 5GHz differential probe. We found a very poor signal
waveform and got an almost closed eye diagram.
We analyzed this phenomenon and now we assume, that the signal
degradation is caused by high reflections on the line. The overshoot
and undershoot amounts to 50% of the singal swing. It seemed, that
the MGT receiver's input termination does not work properly.
Then we tried a TDR ("time domain reflectometer") measurement
to check the impedance characteristics of our board even into
the MGT's termination. The board traces are fine. Some impedance
mismatches are to be seen at vias, AC-coupling capacitors and the
Virtex II Pro package. But we think, these are not too bad, the
mismatch is in the range of 20%.

Does anyone have experience with Virtex II Pro RocketIO?
Did anyone measure signal quality or eye diagrams on such a link?
May the impedance mismatches cause the high ringing we found?
Can anyone imagine the reason for the reflections though the signal
path's impedance seems to be not so bad?

At the moment I don't have a clue.
Thank you for any hint!

Michael

 

[Austin Lesea]

Michael,

At the speeds of the MGT signals, just about anything can be a 'bump in
the road', and cause reflections.

No, the termination in the receiver is not perfect, (nothing is
perfect), but it is just fine regardless. Thousands of customers have
pcb's working at 3.125 Gbs error free, so it is more likely that you
have a pcb issue in your board.

I suggest you immediately contact your local FAE, and arrange to go
visit one of our RocketLabs locations, where we have all of the
equipment to troubleshoot just such an issue, and the FAEs associated
with the RocketLab are all trained and familiar with the equipment, and
how to address the issues.

One of the most common mistakes made in measuring the input impedance,
or return loss of the 100 ohm differential receiver, is that they
measure it single ended (50 ohm) and fail to take into account that a
differential return loss measurement is not a trivial or simple thing to
characterize accurately. For example, two single ended 50 ohm traces
are NOT 100 ohms differential (they are less if they are routed together
as they should be to be differential). Bad mismatch right there!

This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the Tek
CSA8000 is the only true differential TDR scope that I know of, although
I think Agilent now has one as well -- check! does it send two impulses
(or steps) at the same time of opposite voltages? If not it isn't
differential).

As well, the time resolution fo the TDR may be much faster than the rise
time of the MGT signal, and may be showing issues that do not affect the
MGT operation (ie a mis-match at 20 GHz is not an issue, as the signal
has no energy at 20 GHz).

Save yourself time and money, and go use the services that we provide to
get your problems solved. After all, we wnat to sell you chips, not
have you endlessly troubleshooting.....

Austin

Michael Mustermann wrote:

Hi anyone out there,

we have designed a board with XILINX Virtex II Pro using the
RocketIO / MGT serial high speed transceivers.
Recently we experienced a problem with bit error rate (BER) and
measured the signal quality of the MGT serial links with a 20Gsample
scope and 5GHz differential probe. We found a very poor signal
waveform and got an almost closed eye diagram.
We analyzed this phenomenon and now we assume, that the signal
degradation is caused by high reflections on the line. The overshoot
and undershoot amounts to 50% of the singal swing. It seemed, that
the MGT receiver's input termination does not work properly.
Then we tried a TDR ("time domain reflectometer") measurement
to check the impedance characteristics of our board even into
the MGT's termination. The board traces are fine. Some impedance
mismatches are to be seen at vias, AC-coupling capacitors and the
Virtex II Pro package. But we think, these are not too bad, the
mismatch is in the range of 20%.

Does anyone have experience with Virtex II Pro RocketIO?
Did anyone measure signal quality or eye diagrams on such a link?
May the impedance mismatches cause the high ringing we found?
Can anyone imagine the reason for the reflections though the signal
path's impedance seems to be not so bad?

At the moment I don't have a clue.
Thank you for any hint!

Michael

 

[Allan Herriman]

On Fri, 23 Jul 2004 08:03:05 -0700, Austin Lesea <austin@xilinx.com>
wrote:

Michael,

At the speeds of the MGT signals, just about anything can be a 'bump in
the road', and cause reflections.

No, the termination in the receiver is not perfect, (nothing is
perfect), but it is just fine regardless. Thousands of customers have
pcb's working at 3.125 Gbs error free, so it is more likely that you
have a pcb issue in your board.

I suggest you immediately contact your local FAE, and arrange to go
visit one of our RocketLabs locations, where we have all of the
equipment to troubleshoot just such an issue, and the FAEs associated
with the RocketLab are all trained and familiar with the equipment, and
how to address the issues.

One of the most common mistakes made in measuring the input impedance,
or return loss of the 100 ohm differential receiver, is that they
measure it single ended (50 ohm) and fail to take into account that a
differential return loss measurement is not a trivial or simple thing to
characterize accurately. For example, two single ended 50 ohm traces
are NOT 100 ohms differential (they are less if they are routed together
as they should be to be differential). Bad mismatch right there!

This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the Tek
CSA8000 is the only true differential TDR scope that I know of, although
I think Agilent now has one as well -- check! does it send two impulses
(or steps) at the same time of opposite voltages? If not it isn't
differential).

I've used an Agilent 54754A dual 18.4GHz TDR plugin in an 86100A scope
for testing 10Gbps connections. I *think* it does a true differential
measurement. [ I don't have the documentation handy. ]

As well, the time resolution fo the TDR may be much faster than the rise
time of the MGT signal, and may be showing issues that do not affect the
MGT operation (ie a mis-match at 20 GHz is not an issue, as the signal
has no energy at 20 GHz).

Yes, but better time resolution means better spatial resolution,
allowing you to work out what went wrong with your board design.

(I found this out the hard way.)

Regards,
Allan.

 

[Michael Mustermann]

Hi Allan, hi Austin,

thank you for you reply, but it won't help.
Of course, at MGT's speeds everything becomes just esoteric.
But we know this, we've designed serial gigabit board for several
years. We are familiar to single ended and differential impedance
and know how to lay out a board for these signals.

We've carried out _true_ TDR measurement with Tek CSA8000
series scope which is able to send a positive and a negative pulse
exactly at the same time. Both the board and the MGT receiver
don't seem to be the issue, as I wrote earlier (see below).

When I cut off the signal path by lifting the AC-coupling capacitors
100mil before the MGT receiver and short the differential pair by
a simple 100 Ohms termination resistor, then I get wonderful waveform
and eye diagram!!! So it is proven, that I do not have a problem
along the traces, but together with MGT receiver.

We use such links in the opposite direction as well, with the MGT as
the transmitter and a competitor's chip as the receiver. All other details
are absolutely identical - same trace geometry, same connectors, same
distances - just different direction. Those links work _excellent_ and the
eyes look as pretty as in a schoolbook!!! So what do you thing?
Do I have a problem with my pcb???

We have contacted the local FAE and he had no idea except the noise
at the MGT supply voltage. We were advised to measure at the
regulator and found app. 70mVpp noise. But even a lower noise supply
with 30mV hardly provided signal improvement.

After 15" FR4 and two special high speed connector (AMP HM-Zd)
we get an 65mV eye opening at the MGT receiver package pin, but an
650mV overall signal swing! App. 90% is degraded by reflections!
I could double the transmitter voltage swing, but I wonder, if this is the
right approch...

I'm out of ideas now.
Thank you for your assistance!

Regards.

Michael




Allan Herriman wrote:

On Fri, 23 Jul 2004 08:03:05 -0700, Austin Lesea <austin@xilinx.com
wrote:



Michael,

At the speeds of the MGT signals, just about anything can be a 'bump in
the road', and cause reflections.

No, the termination in the receiver is not perfect, (nothing is
perfect), but it is just fine regardless. Thousands of customers have
pcb's working at 3.125 Gbs error free, so it is more likely that you
have a pcb issue in your board.

I suggest you immediately contact your local FAE, and arrange to go
visit one of our RocketLabs locations, where we have all of the
equipment to troubleshoot just such an issue, and the FAEs associated
with the RocketLab are all trained and familiar with the equipment, and
how to address the issues.

One of the most common mistakes made in measuring the input impedance,
or return loss of the 100 ohm differential receiver, is that they
measure it single ended (50 ohm) and fail to take into account that a
differential return loss measurement is not a trivial or simple thing to
characterize accurately. For example, two single ended 50 ohm traces
are NOT 100 ohms differential (they are less if they are routed together
as they should be to be differential). Bad mismatch right there!

This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the Tek
CSA8000 is the only true differential TDR scope that I know of, although
I think Agilent now has one as well -- check! does it send two impulses
(or steps) at the same time of opposite voltages? If not it isn't
differential).



I've used an Agilent 54754A dual 18.4GHz TDR plugin in an 86100A scope
for testing 10Gbps connections. I *think* it does a true differential
measurement. [ I don't have the documentation handy. ]



As well, the time resolution fo the TDR may be much faster than the rise
time of the MGT signal, and may be showing issues that do not affect the
MGT operation (ie a mis-match at 20 GHz is not an issue, as the signal
has no energy at 20 GHz).



Yes, but better time resolution means better spatial resolution,
allowing you to work out what went wrong with your board design.

(I found this out the hard way.)

Regards,
Allan.


Hi anyone out there,

we have designed a board with XILINX Virtex II Pro using the
RocketIO / MGT serial high speed transceivers.
Recently we experienced a problem with bit error rate (BER) and
measured the signal quality of the MGT serial links with a 20Gsample
scope and 5GHz differential probe. We found a very poor signal
waveform and got an almost closed eye diagram.
We analyzed this phenomenon and now we assume, that the signal
degradation is caused by high reflections on the line. The overshoot
and undershoot amounts to 50% of the singal swing. It seemed, that
the MGT receiver's input termination does not work properly.
Then we tried a TDR ("time domain reflectometer") measurement
to check the impedance characteristics of our board even into
the MGT's termination. The board traces are fine. Some impedance
mismatches are to be seen at vias, AC-coupling capacitors and the
Virtex II Pro package. But we think, these are not too bad, the
mismatch is in the range of 20%.

Does anyone have experience with Virtex II Pro RocketIO?
Did anyone measure signal quality or eye diagrams on such a link?
May the impedance mismatches cause the high ringing we found?
Can anyone imagine the reason for the reflections though the signal
path's impedance seems to be not so bad?

At the moment I don't have a clue.
Thank you for any hint!

Michael

 

[Allan Herriman]

On Sat, 24 Jul 2004 07:11:29 +1000, Allan Herriman
<allan.herriman.hates.spam@ctam.com.au.invalid> wrote:

On Fri, 23 Jul 2004 08:03:05 -0700, Austin Lesea <austin@xilinx.com
wrote:
[snip]
This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the Tek
CSA8000 is the only true differential TDR scope that I know of, although
I think Agilent now has one as well -- check! does it send two impulses
(or steps) at the same time of opposite voltages? If not it isn't
differential).

I've used an Agilent 54754A dual 18.4GHz TDR plugin in an 86100A scope
for testing 10Gbps connections. I *think* it does a true differential
measurement. [ I don't have the documentation handy. ]

I checked the 54754A. It claims to be a true differential tester.

Regards,
Allan.

 

[Austin Lesea]

Michael,

I apologize if I seemed to imply that you were new at this.

Series capacitors? Why? When you AC couple, now you have the impedance
bump due to the layout from the caps, and their own ESL/ESR.

Has anyone extracted the pcb layout and simulated the pcb traces using a
3D field solver? Have you looked at the impedance mismatch created by
the series caps?

Again, I strongly suggest getting to a RocektLab which is equipped to
handle this. A quick call to a disti FAE (or even a Xilinx FAE) that
has no real knowledge of your situation is not really going to get you
the help you need.

Is there a case on this? Who is the CAE?

Working with our Hotline is the best, fastest, and most effective way to
solve any issue. This forum here is probably the worst from a
time/accuracy/solution point of view for a specific customer issue. It
is very useful to ping others in the community.

Austin

Michael Mustermann wrote:

Hi Allan, hi Austin,

thank you for you reply, but it won't help.
Of course, at MGT's speeds everything becomes just esoteric.
But we know this, we've designed serial gigabit board for several
years. We are familiar to single ended and differential impedance
and know how to lay out a board for these signals.

We've carried out _true_ TDR measurement with Tek CSA8000
series scope which is able to send a positive and a negative pulse
exactly at the same time. Both the board and the MGT receiver
don't seem to be the issue, as I wrote earlier (see below).

When I cut off the signal path by lifting the AC-coupling capacitors
100mil before the MGT receiver and short the differential pair by
a simple 100 Ohms termination resistor, then I get wonderful waveform
and eye diagram!!! So it is proven, that I do not have a problem
along the traces, but together with MGT receiver.

We use such links in the opposite direction as well, with the MGT as
the transmitter and a competitor's chip as the receiver. All other details
are absolutely identical - same trace geometry, same connectors, same
distances - just different direction. Those links work _excellent_ and the
eyes look as pretty as in a schoolbook!!! So what do you thing?
Do I have a problem with my pcb???

We have contacted the local FAE and he had no idea except the noise
at the MGT supply voltage. We were advised to measure at the
regulator and found app. 70mVpp noise. But even a lower noise supply
with 30mV hardly provided signal improvement.

After 15" FR4 and two special high speed connector (AMP HM-Zd)
we get an 65mV eye opening at the MGT receiver package pin, but an
650mV overall signal swing! App. 90% is degraded by reflections!
I could double the transmitter voltage swing, but I wonder, if this is the
right approch...

I'm out of ideas now.
Thank you for your assistance!

Regards.

Michael




Allan Herriman wrote:

On Fri, 23 Jul 2004 08:03:05 -0700, Austin Lesea <austin@xilinx.com
wrote:



Michael,

At the speeds of the MGT signals, just about anything can be a 'bump
in the road', and cause reflections.

No, the termination in the receiver is not perfect, (nothing is
perfect), but it is just fine regardless. Thousands of customers
have pcb's working at 3.125 Gbs error free, so it is more likely that
you have a pcb issue in your board.

I suggest you immediately contact your local FAE, and arrange to go
visit one of our RocketLabs locations, where we have all of the
equipment to troubleshoot just such an issue, and the FAEs associated
with the RocketLab are all trained and familiar with the equipment,
and how to address the issues.

One of the most common mistakes made in measuring the input
impedance, or return loss of the 100 ohm differential receiver, is
that they measure it single ended (50 ohm) and fail to take into
account that a differential return loss measurement is not a trivial
or simple thing to characterize accurately. For example, two single
ended 50 ohm traces are NOT 100 ohms differential (they are less if
they are routed together as they should be to be differential). Bad
mismatch right there!

This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the
Tek CSA8000 is the only true differential TDR scope that I know of,
although I think Agilent now has one as well -- check! does it send
two impulses (or steps) at the same time of opposite voltages? If
not it isn't differential).



I've used an Agilent 54754A dual 18.4GHz TDR plugin in an 86100A scope
for testing 10Gbps connections. I *think* it does a true differential
measurement. [ I don't have the documentation handy. ]



As well, the time resolution fo the TDR may be much faster than the
rise time of the MGT signal, and may be showing issues that do not
affect the MGT operation (ie a mis-match at 20 GHz is not an issue,
as the signal has no energy at 20 GHz).



Yes, but better time resolution means better spatial resolution,
allowing you to work out what went wrong with your board design.

(I found this out the hard way.)

Regards,
Allan.


Hi anyone out there,

we have designed a board with XILINX Virtex II Pro using the
RocketIO / MGT serial high speed transceivers.
Recently we experienced a problem with bit error rate (BER) and
measured the signal quality of the MGT serial links with a 20Gsample
scope and 5GHz differential probe. We found a very poor signal
waveform and got an almost closed eye diagram.
We analyzed this phenomenon and now we assume, that the signal
degradation is caused by high reflections on the line. The overshoot
and undershoot amounts to 50% of the singal swing. It seemed, that
the MGT receiver's input termination does not work properly.
Then we tried a TDR ("time domain reflectometer") measurement
to check the impedance characteristics of our board even into
the MGT's termination. The board traces are fine. Some impedance
mismatches are to be seen at vias, AC-coupling capacitors and the
Virtex II Pro package. But we think, these are not too bad, the
mismatch is in the range of 20%.

Does anyone have experience with Virtex II Pro RocketIO?
Did anyone measure signal quality or eye diagrams on such a link?
May the impedance mismatches cause the high ringing we found?
Can anyone imagine the reason for the reflections though the signal
path's impedance seems to be not so bad?

At the moment I don't have a clue.
Thank you for any hint!

Michael

 

[Joseph H Allen]

In article <ce8p5u$r8c3@cliff.xsj.xilinx.com>,
Austin Lesea <austin@xilinx.com> wrote:

Michael,

Series capacitors? Why? When you AC couple, now you have the impedance
bump due to the layout from the caps, and their own ESL/ESR.

Solder some extras in parallel with them, but I doubt this is his issue.
We've sent 2.5 Gb/sec SONET through capacitors and jumpers (stubs when open)
without problems (the FR-4 helps you by reducing the high frequencies).

Here's a low tech long shot: maybe a ball is opening when the chip heats up.
It wouldn't be always open, otherwise the TDR would have picked it up. So
is it broken in one instance (one particular board, one location) or
in several?

Rule out other low tech problems as well, like incorrect supply voltage.

--
/* jhallen@world.std.com (192.74.137.5) */ /* Joseph H. Allen */
int a[1817];main(z,p,q,r){for(p=80;q+p-80;p-=2*a[p])for(z=9;z--q=3&(r=time(0)
+r*57)/7,q=q?q-1?q-2?1-p%79?-1:0%79-77?1:0<1659?79:0>158?-79:0,q?!a[p+q*2
]?a[p+=a[p+=q]=q]=q:0:0;for(;q++-1817printf(q%79?"%c":"%c\n"," #"[!a[q-1]]);}

 

[Michael Mustermann]

Austin Lesea wrote:

Michael,

I apologize if I seemed to imply that you were new at this.

Series capacitors? Why? When you AC couple, now you have the
impedance bump due to the layout from the caps, and their own ESL/ESR.

We had to manage different bias voltages for transmitter and receiver.
Thus we separated both by AC-coupling capacitors.

Has anyone extracted the pcb layout and simulated the pcb traces using
a 3D field solver? Have you looked at the impedance mismatch created
by the series caps?

No, we didn't exctract and simulate pcb traces. But TDR measurement
showed an
impedance bump from 100 Ohms down to 80 Ohms diff. And behind the
AC-coupling
we saw app. 115 Ohms which represented the MGT receiver.

Again, I strongly suggest getting to a RocektLab which is equipped to
handle this. A quick call to a disti FAE (or even a Xilinx FAE) that
has no real knowledge of your situation is not really going to get you
the help you need.

We are in steady contact with Xilinx representative and FAEs. They have
been to our lab.
But there has not been any progress for a few weeks and no solution to
our problem !
That's why I turned to this forum and asked for assistance.

Is there a case on this? Who is the CAE?

What means "case on this" and what is a CAE?

Working with our Hotline is the best, fastest, and most effective way
to solve any issue. This forum here is probably the worst from a
time/accuracy/solution point of view for a specific customer issue.
It is very useful to ping others in the community.

I'm sorry for the bad public relation. Xilinx claims to have thousands
of customers who
have boards at 3.125Gig working fine. But until this very moment, nobody
(even in this
forum) could tell me: "I truly measured a beautiful eye at my board's
MGT receiver! Here
is the screen shot..." Actually, can _SOMEONE_ give me such a
confirmation? Please!

The whole thing is weird and for my personal feeling quite nebulous!

Allen, sorry for my honesty! I hope you don't feel offended!

Regards.

Michael

Austin

Michael Mustermann wrote:

Hi Allan, hi Austin,

thank you for you reply, but it won't help.
Of course, at MGT's speeds everything becomes just esoteric.
But we know this, we've designed serial gigabit board for several
years. We are familiar to single ended and differential impedance
and know how to lay out a board for these signals.

We've carried out _true_ TDR measurement with Tek CSA8000
series scope which is able to send a positive and a negative pulse
exactly at the same time. Both the board and the MGT receiver
don't seem to be the issue, as I wrote earlier (see below).

When I cut off the signal path by lifting the AC-coupling capacitors
100mil before the MGT receiver and short the differential pair by
a simple 100 Ohms termination resistor, then I get wonderful waveform
and eye diagram!!! So it is proven, that I do not have a problem
along the traces, but together with MGT receiver.

We use such links in the opposite direction as well, with the MGT as
the transmitter and a competitor's chip as the receiver. All other
details
are absolutely identical - same trace geometry, same connectors, same
distances - just different direction. Those links work _excellent_
and the
eyes look as pretty as in a schoolbook!!! So what do you thing?
Do I have a problem with my pcb???

We have contacted the local FAE and he had no idea except the noise
at the MGT supply voltage. We were advised to measure at the
regulator and found app. 70mVpp noise. But even a lower noise supply
with 30mV hardly provided signal improvement.

After 15" FR4 and two special high speed connector (AMP HM-Zd)
we get an 65mV eye opening at the MGT receiver package pin, but an
650mV overall signal swing! App. 90% is degraded by reflections!
I could double the transmitter voltage swing, but I wonder, if this
is the
right approch...

I'm out of ideas now.
Thank you for your assistance!

Regards.

Michael




Allan Herriman wrote:

On Fri, 23 Jul 2004 08:03:05 -0700, Austin Lesea <austin@xilinx.com
wrote:



Michael,

At the speeds of the MGT signals, just about anything can be a
'bump in the road', and cause reflections.

No, the termination in the receiver is not perfect, (nothing is
perfect), but it is just fine regardless. Thousands of customers
have pcb's working at 3.125 Gbs error free, so it is more likely
that you have a pcb issue in your board.

I suggest you immediately contact your local FAE, and arrange to go
visit one of our RocketLabs locations, where we have all of the
equipment to troubleshoot just such an issue, and the FAEs
associated with the RocketLab are all trained and familiar with the
equipment, and how to address the issues.

One of the most common mistakes made in measuring the input
impedance, or return loss of the 100 ohm differential receiver, is
that they measure it single ended (50 ohm) and fail to take into
account that a differential return loss measurement is not a
trivial or simple thing to characterize accurately. For example,
two single ended 50 ohm traces are NOT 100 ohms differential (they
are less if they are routed together as they should be to be
differential). Bad mismatch right there!

This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the
Tek CSA8000 is the only true differential TDR scope that I know of,
although I think Agilent now has one as well -- check! does it
send two impulses (or steps) at the same time of opposite
voltages? If not it isn't differential).




I've used an Agilent 54754A dual 18.4GHz TDR plugin in an 86100A scope
for testing 10Gbps connections. I *think* it does a true differential
measurement. [ I don't have the documentation handy. ]



As well, the time resolution fo the TDR may be much faster than the
rise time of the MGT signal, and may be showing issues that do not
affect the MGT operation (ie a mis-match at 20 GHz is not an issue,
as the signal has no energy at 20 GHz).




Yes, but better time resolution means better spatial resolution,
allowing you to work out what went wrong with your board design.

(I found this out the hard way.)

Regards,
Allan.


Hi anyone out there,

we have designed a board with XILINX Virtex II Pro using the
RocketIO / MGT serial high speed transceivers.
Recently we experienced a problem with bit error rate (BER) and
measured the signal quality of the MGT serial links with a
20Gsample
scope and 5GHz differential probe. We found a very poor signal
waveform and got an almost closed eye diagram.
We analyzed this phenomenon and now we assume, that the signal
degradation is caused by high reflections on the line. The
overshoot
and undershoot amounts to 50% of the singal swing. It seemed, that
the MGT receiver's input termination does not work properly.
Then we tried a TDR ("time domain reflectometer") measurement
to check the impedance characteristics of our board even into
the MGT's termination. The board traces are fine. Some impedance
mismatches are to be seen at vias, AC-coupling capacitors and the
Virtex II Pro package. But we think, these are not too bad, the
mismatch is in the range of 20%.

Does anyone have experience with Virtex II Pro RocketIO?
Did anyone measure signal quality or eye diagrams on such a link?
May the impedance mismatches cause the high ringing we found?
Can anyone imagine the reason for the reflections though the signal
path's impedance seems to be not so bad?

At the moment I don't have a clue.
Thank you for any hint!

Michael

 

[Michael Mustermann]

Austin Lesea wrote:

Hi again (I confused the names Allan und Austin in my previous posting),

Michael,

I apologize if I seemed to imply that you were new at this.

Series capacitors? Why? When you AC couple, now you have the
impedance bump due to the layout from the caps, and their own ESL/ESR.

We had to manage different bias voltages for transmitter and receiver.
Thus we separated both by AC-coupling capacitors.

Has anyone extracted the pcb layout and simulated the pcb traces using
a 3D field solver? Have you looked at the impedance mismatch created
by the series caps?

No, we didn't exctract and simulate pcb traces. But TDR measurement
showed an
impedance bump from 100 Ohms down to 80 Ohms diff. And behind the
AC-coupling
we saw app. 115 Ohms which represented the MGT receiver.

Again, I strongly suggest getting to a RocektLab which is equipped to
handle this. A quick call to a disti FAE (or even a Xilinx FAE) that
has no real knowledge of your situation is not really going to get you
the help you need.

We are in steady contact with Xilinx representative and FAEs. They have
been to our lab.
But there has not been any progress for a few weeks and no solution to
our problem !
That's why I turned to this forum and asked for assistance.

Is there a case on this? Who is the CAE?

What means "case on this" and what is a CAE?

Working with our Hotline is the best, fastest, and most effective way
to solve any issue. This forum here is probably the worst from a
time/accuracy/solution point of view for a specific customer issue.
It is very useful to ping others in the community.

I'm sorry for the bad public relation. Xilinx claims to have thousands

of customers who
have boards at 3.125Gig working fine. But until this very moment, nobody
(even in this
forum) could tell me: "I truly measured a beautiful eye at my board's
MGT receiver! Here
is the screen shot..." Actually, can _SOMEONE_ give me such a
confirmation? Please!

The whole thing is weird and for my personal feeling quite nebulous!

_Austin_, sorry for my honesty! I hope you don't feel offended!

Regards.

Michael

Austin

Michael Mustermann wrote:

Hi Allan, hi Austin,

thank you for you reply, but it won't help.
Of course, at MGT's speeds everything becomes just esoteric.
But we know this, we've designed serial gigabit board for several
years. We are familiar to single ended and differential impedance
and know how to lay out a board for these signals.

We've carried out _true_ TDR measurement with Tek CSA8000
series scope which is able to send a positive and a negative pulse
exactly at the same time. Both the board and the MGT receiver
don't seem to be the issue, as I wrote earlier (see below).

When I cut off the signal path by lifting the AC-coupling capacitors
100mil before the MGT receiver and short the differential pair by
a simple 100 Ohms termination resistor, then I get wonderful waveform
and eye diagram!!! So it is proven, that I do not have a problem
along the traces, but together with MGT receiver.

We use such links in the opposite direction as well, with the MGT as
the transmitter and a competitor's chip as the receiver. All other
details
are absolutely identical - same trace geometry, same connectors, same
distances - just different direction. Those links work _excellent_
and the
eyes look as pretty as in a schoolbook!!! So what do you thing?
Do I have a problem with my pcb???

We have contacted the local FAE and he had no idea except the noise
at the MGT supply voltage. We were advised to measure at the
regulator and found app. 70mVpp noise. But even a lower noise supply
with 30mV hardly provided signal improvement.

After 15" FR4 and two special high speed connector (AMP HM-Zd)
we get an 65mV eye opening at the MGT receiver package pin, but an
650mV overall signal swing! App. 90% is degraded by reflections!
I could double the transmitter voltage swing, but I wonder, if this
is the
right approch...

I'm out of ideas now.
Thank you for your assistance!

Regards.

Michael




Allan Herriman wrote:

On Fri, 23 Jul 2004 08:03:05 -0700, Austin Lesea <austin@xilinx.com
wrote:



Michael,

At the speeds of the MGT signals, just about anything can be a
'bump in the road', and cause reflections.

No, the termination in the receiver is not perfect, (nothing is
perfect), but it is just fine regardless. Thousands of customers
have pcb's working at 3.125 Gbs error free, so it is more likely
that you have a pcb issue in your board.

I suggest you immediately contact your local FAE, and arrange to go
visit one of our RocketLabs locations, where we have all of the
equipment to troubleshoot just such an issue, and the FAEs
associated with the RocketLab are all trained and familiar with the
equipment, and how to address the issues.

One of the most common mistakes made in measuring the input
impedance, or return loss of the 100 ohm differential receiver, is
that they measure it single ended (50 ohm) and fail to take into
account that a differential return loss measurement is not a
trivial or simple thing to characterize accurately. For example,
two single ended 50 ohm traces are NOT 100 ohms differential (they
are less if they are routed together as they should be to be
differential). Bad mismatch right there!

This goes for TDR as well. Unless it is a true differential TDR
measurement, you are not measuring what you need to measure (eg the
Tek CSA8000 is the only true differential TDR scope that I know of,
although I think Agilent now has one as well -- check! does it
send two impulses (or steps) at the same time of opposite
voltages? If not it isn't differential).




I've used an Agilent 54754A dual 18.4GHz TDR plugin in an 86100A scope
for testing 10Gbps connections. I *think* it does a true differential
measurement. [ I don't have the documentation handy. ]



As well, the time resolution fo the TDR may be much faster than the
rise time of the MGT signal, and may be showing issues that do not
affect the MGT operation (ie a mis-match at 20 GHz is not an issue,
as the signal has no energy at 20 GHz).




Yes, but better time resolution means better spatial resolution,
allowing you to work out what went wrong with your board design.

(I found this out the hard way.)

Regards,
Allan.


Hi anyone out there,

we have designed a board with XILINX Virtex II Pro using the
RocketIO / MGT serial high speed transceivers.
Recently we experienced a problem with bit error rate (BER) and
measured the signal quality of the MGT serial links with a
20Gsample
scope and 5GHz differential probe. We found a very poor signal
waveform and got an almost closed eye diagram.
We analyzed this phenomenon and now we assume, that the signal
degradation is caused by high reflections on the line. The
overshoot
and undershoot amounts to 50% of the singal swing. It seemed, that
the MGT receiver's input termination does not work properly.
Then we tried a TDR ("time domain reflectometer") measurement
to check the impedance characteristics of our board even into
the MGT's termination. The board traces are fine. Some impedance
mismatches are to be seen at vias, AC-coupling capacitors and the
Virtex II Pro package. But we think, these are not too bad, the
mismatch is in the range of 20%.

Does anyone have experience with Virtex II Pro RocketIO?
Did anyone measure signal quality or eye diagrams on such a link?
May the impedance mismatches cause the high ringing we found?
Can anyone imagine the reason for the reflections though the signal
path's impedance seems to be not so bad?

At the moment I don't have a clue.
Thank you for any hint!

Michael

 

[Austin Lesea]

Michael,

Please email me directly with your contact information, FAE name,
distribution FAE, etc. so I may escalate your case. By entering a case
in the hotline, and communicating your expectations, we know how to
address the issue, and we can track it. It is is not resolved quickly,
we can escalate the issue to a team of experts. Not dealing with a
customer case is a serious issue for the support staff, and is dealt
with accordingly. Without a case number, and a contact, no one is
taking the issue seriously, as it is not in the system as a "problem."

FAEs may just consider this something that you may be 'playing' around
with, and does not require their attention.

I will personnaly find out why the field organization has not provided
you with the answer, but without a case open, I can tell you right now
that is a major factor!

Visiting a RocketLab(tm), you could see many eye patterns on real boards.

http://www.support.xilinx.com/products/xaw/hsd/simulator.htm

Is the on line tool that has been checked to show that the simulated eye
patterns are accurate, and represent the actual eye patterns on the
thousands of combinations.

I would suppose that others could send you their eye patterns, but that
is up to them to decide to take the time and effort to do that. As
well, posting graphics on this newsgroup is highly discouraged (as the
graphics will be removed -- text only) so they would have to respond to
you directly, and we do not know if the email address that appears in
your posting is valid (many are not due to spam problems).

My direct email is austin @ xilinx . com (without the spaces).

One last comment, if the driver is also not 100 ohms matched, then any
discontinuities upon reflection will re-reflect back to the receiver.
There are some devices out there with very low, or very high transmit
impedances (not ours) that have this problem. In any system or
standard, the SI engineering makes assumptions. The assumption here is
that both the Transmitter and the Receiver are to be as good a match as
possible, so as to minimize the distortion to the eye pattern from
'normal' (that is, unavoidable)impedance discontinuities from vias,
connectors, series AC blocking capacitors, package, and die variations.

A simulation of an improper impedance transmitter will demonstrate this
issue.

(rant on)

For all those who are reading this: do not let this happen to you! If
you have an issue, get a case number! Follow the case progress on-line
through our on-line tracking system. If at any poinht you feel progress
is not satisfactory, ask to escalate the case (bumps it up the system to
the next level of technical expertise). You are the customer: you are
always right (by definition). We do not have a choice. If you ask, we
must respond. And we do.

(rant off)

Austin