Re: [SI-LIST] : troubleshoot differential clock

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From: Perry Qu (perry.qu@alcatel.com)
Date: Wed May 02 2001 - 13:26:58 PDT


Cosmin:

Each line was terminated at receiver side with 2 resistors:
61.9 ohm to gnd and 249 ohm to 2.5 V, which gives us 50 ohm total resistance.
The termination is a bit far from the receiver (around 0.8 ").

Thanks

Perry

Cosmin Iorga wrote:

> Hi Perry,
>
> can you describe in more details the termination scheme?
>
> Cosmin
>
> > Date: Wed, 02 May 2001 15:30:57 -0400
> > From: "Perry Qu" <perry.qu@alcatel.com>
> > X-Accept-Language: en
> > MIME-Version: 1.0
> > To: "si-list@silab.eng.sun.com" <si-list@silab.eng.sun.com>
> > Subject: [SI-LIST] : troubleshoot differential clock
> >
> > Hi! Everyone:
> >
> > I have a case which need help from you guys:
> >
> > On one card, there is a LVPECL differential pair running from a buffer
> > to the receiver. The total length of the pair is about 8 inches, with 50
> > ohm termination at the receiver side. (2 resistors terminated to pwr/gnd
> > for biasing also). The routing was not done properly, e.g., long
> > parallel run with other differential pairs; unsymmetrical routing
> > between the plus and minus, etc.
> >
> > In the lab, the signal measured at reciever side shows strong
> > reflections, with non-monotonic points on the rising and falling edge,
> > which is bad for clocks. The measurement is well repeatable and smells
> > like some discontinueties problems. We did a simulation in XTK and we
> > saw some reflections (not as bad). By adding a series termination at
> > driver side (the driver is a really fast one, < 200 ps rise time), we
> > manage to eliminate the reflections almost completely. However, we did
> > not observe the same effect in the lab.
> >
> > The next thing we tried is to do some TDR measurement on the bare board.
> > Since one of the TDR channel is not working properly, we can only
> > measure the single-ended impedance. There is some dips here and there on
> > both lines but none of them is bad enough to cause such bad signals at
> > receiver side. We plan to do the the differential TDR once we have the
> > sampling head repaired. But I doubt that is the reason.
> >
> > Really appreciate your comments/suggestions on this.
> >
> > Regards
> >
> > Perry Qu

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