Products > Test Equipment
DIY Logic Analyzer Probe and Pods for Siglent (and LeCroy) scopes
Slartibartfast:
Mike,
I really like your concept with the detachable pods. I'd envision that I'd more often use the LA with the less than 8, rather than more, channels. Having only one cable lying around would be useful.
Maybe I missed it, but did you post your design files?
Cheers Peter
ElectronicsHobbyist:
Hi Peter
All design files are on thingiverse (See first post on this topic).
Im about to order them right now by the way. Thanks very much to oz2cpu and all the other developers for developing and publishing this great piece of equipment. :-+
mawyatt:
--- Quote from: Slartibartfast on July 13, 2021, 05:06:44 pm ---
--- Quote from: mawyatt on May 06, 2021, 02:18:01 pm ---
However, we can verify that Thomas (oz2cpu) LA Probe works well with standard 10 line ribbon cable (8 active, 2 ground), the twisted pair cable we have is a little too wide so used ribbon cable instead. Don't have a high speed pulse generator, but did verify that using a sine wave input from a AWG the Probe works to ~100MHz with ribbon. You do "see" a small amount of crosstalk, but that's expected, with twisted pair the crosstalk should be eliminated.
--- End quote ---
I wonder ...
At RF, inductive, capacitive and even radiative coupling are so much worse than at AF (audio frequency), that RF guys frequently neglect thinking about galvanic coupling. Usually that can be justified if a massive, low-impedance ground plane can be used. Here, we do not have such a ground plane. In fact, the two concepts discussed in this thread have TWO separate ground planes (one in the pod, one in the scope adapter), connected by a set of ground lines. I'm not sure whether that concept is a smart idea, I think a significant part of the crosstalk may be due to galvanic coupling.
What happens if a logic signal edge travels along signal line, say, #3? Some current will flow in the line, and some return current will flow in the ground line. Which ground line? After all, all ground lines of a pod connect the same two ground planes. Well, the ground line that is associated with the signal line, i.e. ground #3, will carry the majority. The reason is that this ground line out of the eight forms the lowest inductance loop with the signal line. But in the same way as a 10k resistor parallel to a 1k resistor also carries a smaller part of the current, the other ground lines will also carry a part of the return current that ought to travel via ground #3. I'd expect that part of the return current to cause crosstalk to the signals on lines #2 and #4, and, to a lesser degree, to the other signals.
For this reason I think it may be smarter to keep the grounds of the various signals separate inside the pod. IMHO this would reduce the crosstalk that you measured. The final purpose of a logic analyser however would in most cases have the grounds connected inside the DUT anyway, so I'm not sure how much difference it makes in terms of the real-world application of the device.
Cheers Peter
--- End quote ---
Don't think this is a galvanic coupling issue, it's simply signal edge coupling to adjacent lines due to dynamic fields. With twisted pair or coaxial lines there is little EM field outside the pair or coaxial line, and thus little coupling to adjacent lines. A few simple tests proved this once the twisted pair LA was assembled and tested, no coupling whatsoever to any lines under any signal conditions we could generate.
We are using this twisted pair LA now without any issues, coupling or otherwise. It works beautifully with either the SDS2102X Plus or SDS2104X Plus (both enhanced) without issue :-+
Best,
mawyatt:
--- Quote from: Slartibartfast on July 13, 2021, 05:33:21 pm ---Mike,
I really like your concept with the detachable pods. I'd envision that I'd more often use the LA with the less than 8, rather than more, channels. Having only one cable lying around would be useful.
Maybe I missed it, but did you post your design files?
Cheers Peter
--- End quote ---
I didn't't post my design files, they are different than Thomas's files. I use the single 8 bit input mostly, in fact have yet to find a need for the full 16 bits use, and could even use the single bit lines now since I'm sensing SPI lines.
Best,
Slartibartfast:
--- Quote from: mawyatt on July 13, 2021, 11:43:04 pm ---I use the single 8 bit input mostly, in fact have yet to find a need for the full 16 bits use, and could even use the single bit lines now since I'm sensing SPI lines.
--- End quote ---
That is exactly what I anticipate for my usage scenarios too, hence my preference for detachable pods and wiring.
So I'll do my own design then. Should not be much work given all the information on the circuitry you have provided in this thread.
You have correctly pointed out that oz2cpu's in line resistors at the base do not terminate, but it seems in your design you also only use in-line resistors, instead of parallel-connected terminators. Is that work left for the future, or is there some background to it that you did not explain in the thread?
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