Hi group,
I have been doing measurements lately to see fast rise-time (50ps)
"digital" signals on diffpairs. The question occured to me: what is
the scope displaying?
I am using an Infiniium (13GHz BW and 40GSa/s) and a 1169A 12GHz
probe.
The probe has no local ground. I just mash the two bits of wire on
exposed signal traces. There is no handy local ground nearby on the
PCB anyways. Apparently the probe looks like a 50k resistor with a .
25pF cap across it.
So is the scope displaying some kind of average of each line's rise
time?
What if one side is rising at 20ps (these are 10-90% figures) and the
other falling at 30ps?
I can't easily make the single-ended measurements I am more
comfortable with.
Sure we can make tons of measurements relating to bit error rates and
eye openings, but none of these relate easily (for me) to a rise-time,
and more generally what a differential rise-time means.
I am still trying to debug the disastrous loss of speed on our board.
I've seen a FR-4 board run at 6Gbps with better specs than my Rogers
monster. There is a big problem somewhere and I want to make sure I
understand the basics first. Cuz I think I don't.
TIA gang.
I have been doing measurements lately to see fast rise-time (50ps)
"digital" signals on diffpairs. The question occured to me: what is
the scope displaying?
I am using an Infiniium (13GHz BW and 40GSa/s) and a 1169A 12GHz
probe.
The probe has no local ground. I just mash the two bits of wire on
exposed signal traces. There is no handy local ground nearby on the
PCB anyways. Apparently the probe looks like a 50k resistor with a .
25pF cap across it.
So is the scope displaying some kind of average of each line's rise
time?
What if one side is rising at 20ps (these are 10-90% figures) and the
other falling at 30ps?
I can't easily make the single-ended measurements I am more
comfortable with.
Sure we can make tons of measurements relating to bit error rates and
eye openings, but none of these relate easily (for me) to a rise-time,
and more generally what a differential rise-time means.
I am still trying to debug the disastrous loss of speed on our board.
I've seen a FR-4 board run at 6Gbps with better specs than my Rogers
monster. There is a big problem somewhere and I want to make sure I
understand the basics first. Cuz I think I don't.
TIA gang.