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DIY Logic Analyzer Probe and Pods for Siglent (and LeCroy) scopes

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Johnny B Good:

--- Quote from: 2N3055 on May 23, 2021, 07:21:41 pm ---
--- Quote from: Johnny B Good on May 23, 2021, 04:58:39 pm --- I'm quite impressed with mine but I do have a couple of minor criticisms. The most significant being the 46 seconds boot up delay versus the 16 seconds of the SDS1202X-E and the not quite so significant yet much higher than expected increased power consumption (54W versus the 22W of the 1202).

 In view of the almost 2 1/2 times greater energy consumption, suggesting the use of a more power hungry faster processor, that significantly protracted boot up time is a rather surprising shortcoming. I can only assume the extra channels and larger feature set have added to the initialisation burden with additional time constraints that no amount of extra CPU grunt can mitigate - the price of progress in this case, perhaps?  :-//

 I hope this isn't the start of a trend leading us back to the days of those Tektronix 500W space heaters so cunningly disguised as test and measurement kit labelled "545A Oscilloscope".  >:D

--- End quote ---

Keysight MSOX-3104T boots 1 minute, 45 seconds.. That's 105 seconds... |O
 Enjoy your very fast booting scope... :-DD

--- End quote ---

 Oh, don't worry about that. Now, thanks to both oz2cpu's and your contributions, I surely will!  :)

mawyatt:
Enjoying these SDS2XXX+ scopes indeed!!

Just finished using the superb Bode Plot feature to help evaluate a few HV Op Amps, then leveraging of Thomas's excellent DIY LA Probe to trace down a bug in the Analog Devices software/DAC hardware test PCB.

For the bug tracing this required finishing up the version of the DIY LA Probe that utilizes a 3M controlled impedance twisted pair cable with connectors since using standard ribbon cable could create false "glitches" with the SCI signals and wanted to make sure we were tracing the real bug/glitch rather than a cable induced glitch. This LA Probe allowed us to find that the AD software wasn't issuing a correct LDAC command and why the DAC output voltage wasn't being updated.


Best,

oz2cpu:
nice with a little bit LA user-case-stories

please people, do tell when you use LA, and what it did for your project
post pictures, and screen shoots :-)
we love to hear about when the Logic Analyzer feature is so fantastic.

mawyatt:
Ok, here's a few images of the setup and the LA Probe using the twisted pair cables with connectors on each end. The component values for the POD are input R of 250 ohms followed by 90.9K in parallel with 8.2pF, the BASE is 360 ohms series R to scope. The shunt elements in the base are not installed and these are for "tuning" the response for maximum BW and good pulse shape, but this must wait until time permits.

The cases are 3D printed but still learning this process, so not as nice as could be ::)

Anyway, works nicely with our "enhanced" SDS2102X+ and really like the display and SPI serial bus decoding. Agree with Rob at tautech, having a few more analog traces along with the 16 bit digital traces of the LA should prove valuable when we get to the multiple DAC outputs of the present project.

This is my first use of a LA and must say hats off to Thomas and Siglent for making this experience worthwhile and useful :-+

Best,

Slartibartfast:

--- 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

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