Products > Test Equipment
DIY Logic Analyzer Probe and Pods for Siglent (and LeCroy) scopes
mawyatt:
Please review the sketch again, the design uses two PODs but only one for bit 0-7 is shown. The other POD would be for bit 8-15, but not shown. The PODs have 16 pin header connectors for the Probes leads and the interface cable which can be a regular 16 wire ribbon cable with GSGSGSGSGSGSGSGS, or 8 twisted pairs, or 8 coaxial cables and interchangeable with different lengths. The base unit interfaces with the scope and has two 16 pin headers connectors for the 0-7 and 8-15 separate cable interfaces. Also note that you can forgo the use of the PODs and directly connect the Probe wires to the base if you want, but this doesn't have the network for the probe lead interface with the high value series resistor, so over voltage protection would be compromised.
I'm not too concerned about the header connectors impedance levels (using your idea of end PCB mount) and mismatch adding much additional signal corruption, but this needs to be addressed with some measurements and maybe simulations to verify.
In summery you have two PODS and two interface cables to the Base, all have connectors for added flexibility in cable types, length and placement.
Best,
oz2cpu:
thanks for the correction Mike, your are right.
another advantage to make it this way : is most likely normal of the shelf IDC cables could be used,
how much is the special twisted types better, over straight flat normal, at this speed, and short length
rf-loop:
--- Quote from: oz2cpu on March 18, 2021, 01:18:45 pm --- at this speed,
--- End quote ---
can you specify it.
Fundamental square freq what ever is not important but what are edges speed and not even what is probe/interface speed. Meaningful is this signal speed where probe is connected. It makes most of bad things - if any, where cross talk is one of very serious problem what need take seriously. Even when signal pulse stream is 1 or 10MHz but if DUT risetime is in ns class it is easy cross talking even over SGSGSG... flat ribbon. It need think, scope LA probes are connected to unknown signal where can be "what ever" except over voltage. Cross talks are very extremely annoying when they exist and can rotten measurements. Naturally one solution is reject edge speeds what can go to cable. Older SDS1000X+ model have ribbon LA probe cable SGSGSGSG... and it cross talk easy with fast edges. I think it is same SPL1016 probe what is today for SDS1000X-E what have external probe interface to scope but probe itself is same.
mawyatt:
--- Quote from: oz2cpu on March 18, 2021, 01:18:45 pm ---thanks for the correction Mike, your are right.
another advantage to make it this way : is most likely normal of the shelf IDC cables could be used,
how much is the special twisted types better, over straight flat normal, at this speed, and short length
--- End quote ---
The ribbon cable will be the worst, then IDC, with ribbon twisted pair next and the coaxial cable the best I suspect. Having this type setup makes it relatively easy to do a comparison of the different cable types. We've ordered a couple different types of ribbon twisted pair, and the ribbon, but don't have a low cost source for the coaxial cables. Another type of ribbon twisted pair has a shield around the entire ribbon, this should be better than the unshielded ribbon twisted pair I believe. Haven't found a low cost source for these either.
https://www.digikey.com/en/products/detail/3m/1785%2F60%2520100SF/1190424?utm_adgroup=Flat%20Ribbon%20Cables&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Product_Cables%2C%20Wires&utm_term=&utm_content=Flat%20Ribbon%20Cables&gclid=CjwKCAjw9MuCBhBUEiwAbDZ-7lfmaQmWYfxxI9q57kl3Y86O2jPrS2SEoI_EODoIM_kYkceup0tjxRoCGrEQAvD_BwE
We need to do some 3D EM modeling of the end feed header & connector to get an idea of what this looks like electrically. A little Red Neck Engineering [term for us folks born is Southern USA that like beer ;D ] and without any proper 3D EM tools, gives a very crude estimate of the end feed header and connector as a small section of transmission line of ~180 ohms, with a delay of ~150ps. Here's a crude estimate of the waveforms as "seen" by the Scope, the Probe, and Ideal Signal Source.
As a grad student we would do anything for a beer, never outgrew this limitation :o
Best,
Edit: Added 2nd plot which shows the effects of the input Probe lead as ~270nH based upon a 26AWG wire of ~200mm long, inductance estimated based upon Grover's Single Wire Equation. Scope waveform shows ~3.8ns mid-point delay.
oz2cpu:
mike ? did you look into the math about twisted pair cables ?
https://www.allaboutcircuits.com/tools/twisted-pair-impedance-calculator/
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