LVDS problem - Black magic anyone?

hi Jon,

Jon Elson <jmelson@wustl.edu> Wrote in message:
> Well, one other thing, is he terminating the LVDS with a resistor matching
> the characteristic impedance of the cable?  I'll bet he isn't, his results
> would VERY likely match what he describes.  We use 100 Ohm resistors, they
> are close enough.  For best results, tune between 110 and 120 Ohms for
> minimum reflection.

from the OP:

 I tried changing and even removing the termination resistors,
 this did not change the behavior at all. I have changed both
 boards, it is really the cable that makes the difference.

BTW, removing the terminators on an LVDS - single drop - line is
 *never* a good idea. LVDS drivers are current sources and the
 receiver may simply not tolerate the differential swing on it's
 big  input impedance.

I did not understand how the op solved his problem changing
 receiver equalization, he clearly mentioned some cables did work
 ok without the need to change equalization...

Al

Wow, quite some discussion going on here... (I have considered this myself =
"solved" and working on other stuff again.)

To give some comments:
Yes, we use 100 ohm termination, in fact on both sides of the cable. I trie=
d to remove the termination on both sides (but I don't think I have removed=
 both sides at the same time - would not really make sense...). I have also=
 checked with a multimeter and got the correct values (e.g. 50 ohm, when bo=
th are mounted and cable connected.)

My conclusion was, that with the original (quite strong) equalizer setting =
things were barely working (even with "good" cables I got an transmission e=
rror here and then while "bad" cables resulted in errors all over). I think=
 it was simply to strong for the short cable distance. With the lowest poss=
ible equalizer setting we have not seen any transmission errors anymore wit=
h any combination of boards and cables.

I had dismantled one cable and found that the (while they were really twist=
ed) the turn rate was very low, about 1 to 2 full turns about the complete =
length. This might be good enough for long ethernet cables, but I can imagi=
ne that this  short length make some of them almost behave like untwisted c=
ables with quite some cross talk. This is my explanation why touching the u=
ntwisted wires had some influence although I am still surprised that the ef=
fect was such a drastic improvement.

What I still cannot understand is why the (in my opinion) much more dramati=
c change of removing the termination resistors had no influence (not good n=
or bad) on this obviously barely working system while the touching had.

Regards,

Thomas

Hi Thomas,

thomas.entner99@gmail.com wrote:
[]
> To give some comments: Yes, we use 100 ohm termination, in fact on both 
> sides of the cable. I tried to remove the termination on both sides (but I 
> don't think I have removed both sides at the same time - would not really 
> make sense...). I have also checked with a multimeter and got the correct 
> values (e.g. 50 ohm, when both are mounted and cable connected.)

I do not understand why you need termination at both ends. There are two 
aspects when terminating an LVDS line:

1. line impedence matching.
2. provide a load for a current source.

In both cases it does not make too much sense to have a termination at the 
source.

> My conclusion was, that with the original (quite strong) equalizer setting 
> things were barely working (even with "good" cables I got an transmission 
> error here and then while "bad" cables resulted in errors all over). I think 
> it was simply to strong for the short cable distance. With the lowest 
> possible equalizer setting we have not seen any transmission errors anymore 
> with any combination of boards and cables.

Did you see your signal on the scope *after* the receiver for both a /good/ 
and /bad/ cable? Do you have an eye diagram?

> 
> I had dismantled one cable and found that the (while they were really 
> twisted) the turn rate was very low, about 1 to 2 full turns about the 
> complete length. This might be good enough for long ethernet cables, but I 
> can imagine that this short length make some of them almost behave like 
> untwisted cables with quite some cross talk. 

twist per meter is usually specified in the cable datasheet; considering the 
rate you're working at I'd seriously consider the cable as a fundamental part 
to be analyzed.

> This is my explanation why 
> touching the untwisted wires had some influence although I am still 
> surprised that the effect was such a drastic improvement.

I would not call 'improvement' something that you cannot justify or explain. 
Since you do not know what is going on, it is *very* possible the problem is 
simply hidden for a various combination of causes and will likely reappear 
later on.

> What I still cannot understand is why the (in my opinion) much more dramatic 
> change of removing the termination resistors had no influence (not good nor 
> bad) on this obviously barely working system while the touching had.

IMO your connection is far from being a proper one, with serious matching 
issues and a very scattered amount of information to analyze. With these basis 
any conclusion on 'working' or 'not working' are pointless at best.

If you don't have a model to explain why it works and why it doesn't, your 
likely exposed to see the issue again later and thanks to Murphy's law it 
would be right two days before shipping!

If you want to seriously remove your issues you need to be able to explain 
every detail, spot the cause and remove it. Otherwise it would just be another 
piece of guesswork.

Al

On 17/08/14 17:57, alb wrote:
> Hi Thomas,
>
> thomas.entner99@gmail.com wrote:
> []
>> To give some comments: Yes, we use 100 ohm termination, in fact on both
>> sides of the cable. I tried to remove the termination on both sides (but I
>> don't think I have removed both sides at the same time - would not really
>> make sense...). I have also checked with a multimeter and got the correct
>> values (e.g. 50 ohm, when both are mounted and cable connected.)
>
> I do not understand why you need termination at both ends. There are two
> aspects when terminating an LVDS line:
>
> 1. line impedence matching.
> 2. provide a load for a current source.
>
> In both cases it does not make too much sense to have a termination at the
> source.

See "LVDS Owner's Manual Design Guide , 4th Edition"
http://www.ti.com/lit/ml/snla187/snla187.pdf


> Did you see your signal on the scope *after* the receiver for both a /good/
> and /bad/ cable? Do you have an eye diagram?

Ensure the probes don't disturb the signal - which is easier said
than done.