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

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teddychn:

--- Quote from: mawyatt on March 21, 2021, 02:14:43 pm ---Rob,

Thanks for the data, here's what we came up with as a quick estimate model of the LA. The Digital Input pulse was manipulated to try and create what we think the actual pulse to the LA looks like, and show the offset as shown on your DSO measured trace above.

The Digital Probe wire self inductance is based upon 200 & 150mm of #26 AWG wire, the 1100mm coaxial cable (shown as twisted pair in schematic) is modeled as having a Zo of 45 ohms and delay of 6.1ns based upon a velocity factor of 60%. Later we'll need to see how the twisted pair cable (and ribbon) rather than the coaxial cable effects these simulations.


--- End quote ---

Hi Mike,

This thread is informative. I'm trying to adapt my existing 54622D logic probe to SDS2104X HD and conduct some simulations concurrently. However, I'm not sure what R1 and C2 in your simulation represent. Have you already known the impedance model of the SDS2000X Plus/HD logic port?

Thanks,
Ted

mawyatt:

--- Quote from: teddychn on July 21, 2023, 04:20:45 am ---
--- Quote from: mawyatt on March 21, 2021, 02:14:43 pm ---Rob,

Thanks for the data, here's what we came up with as a quick estimate model of the LA. The Digital Input pulse was manipulated to try and create what we think the actual pulse to the LA looks like, and show the offset as shown on your DSO measured trace above.

The Digital Probe wire self inductance is based upon 200 & 150mm of #26 AWG wire, the 1100mm coaxial cable (shown as twisted pair in schematic) is modeled as having a Zo of 45 ohms and delay of 6.1ns based upon a velocity factor of 60%. Later we'll need to see how the twisted pair cable (and ribbon) rather than the coaxial cable effects these simulations.


--- End quote ---

Hi Mike,

This thread is informative. I'm trying to adapt my existing 54622D logic probe to SDS2104X HD and conduct some simulations concurrently. However, I'm not sure what R1 and C2 in your simulation represent. Have you already known the impedance model of the SDS2000X Plus/HD logic port?

Thanks,
Ted

--- End quote ---

Hi Ted,

These (R1 and C1) were used to attempt to mimic the Siglent response tautech had shown earlier, check the thread prior to the one this schematic originated from. You can see the simulated response and compare to the one tautech posted.

We used twisted pair for the signal lines and later just ribbon cable, recall both worked well for our needs, memory's not that good tho  ???

Anyway, at one time we were going to "tweak" the component values for optimum response but never needed to do such, as it worked well enough for our needs right from the start as shown, so we've never revisited this since.

Best,

teddychn:
Thanks Mike,

After some trials, I found that the cable length has a significant impact because the transmission line impedance doesn't perfectly match the LA's input impedance. I'm starting by inserting a 75-ohm resistor at the front end of the microstrip line of the adapter. I feel the front end resistor of the LA adapter in your picture seems to be ferrite beads, am I right?

mawyatt:
No ferrite beads were used, just R and Cs I recall.

Best,

KungFuJosh:
I bought one of the DIY type LA probes on eBay for $40 shipped. It worked great in the Batronix MSO board tests. I assume the seller is a member of this site.

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