RocketIO connexion to an optical transceiver

Hi,

I have forgotten to mention that the traces between the FPGA and the transceiver are routed as very loosely coupled traces where each trace has a 50 ohm characteristic impedance. Each trace is about 5 cm long.

Best regards,

JF

Hi,

R2 is inside the transceiver so I do not know how to answer this question. All i know is that normally everything inside the transceiver is taken care of so I do not have to worry about it (anyway I hope so !!!).

Best regards,

JF

Symon a =E9crit :

Howdy JF,

Did you measure this differentially, or just one leg at a time? If you don't have a differential probe, get one. In the mean time, you can get an approximation by using two probes, placed at the transceiver, and attempt to verify that the edges of each leg transistion at exactly the same time. If the scope has a "Difference mode", where it subtracts one probe from another, it can come in handy here.

In your other message, you mentioned that the nets "are routed as very loosely coupled traces where each trace has a 50 ohm characteristic impedance". I'd think you'd want a fairly tightly coupled 100 ohm differential pair. How loosely are they coupled? Are the _p and _n legs close to the same length?

Do you have an optical scope so you can look at the eye diagram?

Good luck,

Marc

Hi,

I have measured differentially and single ended. The single ended signal is somehow poorer than the differential one but still the results are satisfying. I did look at the optical eye diagram and it is quite closed because of the low ER of the transceiver on my board. BTW, th _p and _m are closely matched in terms of length but they are quite far away from each other compared to the height above the reference planes.

Best regards,

JF

Marc Randolph a =E9crit :

same length?

ive

Hi Marc, The coupling between them doesn't matter if the individual lines are 50 ohms and the propagation time. How can the electricity know the difference between that and a 100 ohm diff pair, or indeed, all the hybrid combinations in between?

JF, You should check the datasheet or call the manufacturer to make sure you don't need the put any DC bias on the optics' rx CML port. Normally it's something like Vicm, input common mode voltage.

Cheers, Syms.

Whoops, typo!

Howdy Symon,

The main point of tigher coupling is so that anything that affects one leg will also affect the other leg - allowing it to be cancelled out by the differential receiver. I don't know exactly what his definite of loosely coupled is, but if it is "loose" enough that an aggressor signal could affect one leg of the pair much more than the other, it throws out the main benefit of using differential pairs in the first place.

If he's using the more common internally AC-coupled transcievers (which I got the impression from the first post that he is), any external DC-biasing is not going to have an affect on his signals. They do make DC-coupled transceivers though, so it is worth checking to make sure that JF didn't end up with some.

I realize the chances of it are very low, but I assume JF has ruled out the possibility this is just a bad transciever.

Regards,

Marc

But most of the aggressors are likely to be other PCB traces, and they are *not* going to couple symmetrically onto the pair. My understanding is that you just can't couple PCB traces that well.

The main benefit of using differential pairs for signals is off-board using twisted pair cabling. So long as your PCB traces match in delay (not length), you've got what you can out of them.

Cheers, Martin

Howdy Martin,

I think that tight vs. loose coupling discussions come up every year or so on the SI-list. Each has its supporters, for different reasons (including debating how well the _p and _n legs couple when routed tightly). But I don't believe there is any real debate that aggressors can and will affect nearby signals - in fact, we've had it happen on a few different prototypes in our lab (admittedly, we run pretty dense boards). It's all a function of distance between the traces.

BTW, I don't think I ever answered JF's original query - we use RocketIO's exactly the same way he has it set-up, even down to using Finisar (internally AC-coupled) optics. It works like a hose.

Regards,

Marc

Hi Marc, This is lazy I know, but could you post a link to point me at the SI-list so I can hunt down the stuff you mention? I have to say that with the devices and PCB technologies I use, edge-coupled pairs are the only ones that make sense and the coupling between the traces is

Symon wrote on 05.08.2005 11:45:

cu, Sean