Johnson,
It is also important to know that the probe can load the circuit too. Depending
on which kind of probe you have, you may or may not be loading the clock
signal. Something with high input impedance in the frequency of measurement is
important. In general, the other responses are right so far... even with good
termination, probing along the line will give you a different waveform than at
the end points. I suggest doing some simple simulations with ideal transmission
lines to get a good feel for the positional effects of probing.
It may also be the case that the differential termination is correct but there
are common mode reflections off the receiver that are affecting the signal
through mode conversion, so make sure you rule that out.
Also, make sure the receiver is not being over driven, in the case where the
input protection diodes get turned on as the signal transitions, you will get
some fairly strange, nonlinear behavior on the line.
Regards,
Josiah Bartlett
Principal Engineer
Tektronix, Inc.
T 503.627.2946
http://tek.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of john h
Sent: Thursday, June 16, 2016 8:35 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] clock signal non-monotonic nature
Hi SI Gurus,
I am new to hardware and I am thankful for your inputs on this .
I am probing a clock signal on the board. The clock is 125MHz differential and
I am probing it at the receiver end. The receiver is of BGA package and I am
probing at the BGA end (via )at the bottom side of the board. ( IC is placed
on the Top side)
The clock pins at the receiver are terminated with 100E internally.
The clock waveform is seen to be of non-monotonic nature. A dip is seen on the
rising and falling edge approximately at 80% of signal level. At that point
during ramp , the signal dips down a bit and then rise again to 100% of the
signal level.
I could not exactly understand on the issue.
To debug further, I removed the AC caps at the receiver (thereby isolating the
receiver) and placed a parallel 100E .And I probed across the 100E.
the signal looks to be completely fine. monotonic and no dips are seen on the
edge.
I could not exactly understand on this. Is the dip seen on the first case is
due to the reflection from the receiver. but the receiver is internally
terminated as per the datasheet. Appreciate your inputs on this.
Thank you,
Johnson
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