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LaPOD: Low cost Logic Analyzer probe for MSO5k, DHO900 and more!
ebastler:
--- Quote from: Mechatrommer on November 08, 2023, 03:02:14 pm ---10 pages is still easy to handle ;) and usually like all other threads, the first few pages are the most usefull one anyway, can ignore the rest. ymmv. cheers.
--- End quote ---
Well, in this particular thread the first 6 pages focus on a low-cost probe with limited capabilities (TTL/CMOS level only, fixed threshold). Towards the end of page 6, oliv3r presents his project which supports different flavors of probes -- either using the same low-cost TTL/CMOS driver, or using the proper (but expensive) comparators and 10:1 divider front-ends as used in the original Rigol probe.
That might make the overall thread a bit hard to follow; and for the second part one could argue that the thread title is a bit misleading. Also, it would be nice to get the DHO900 mentioned in the thread title, since that new scope uses the same logic probes as the MSO5000.
oliv3r:
--- Quote from: Mechatrommer on November 08, 2023, 03:02:14 pm ---
--- Quote from: oliv3r on November 08, 2023, 01:04:00 pm ---
--- Quote from: Mechatrommer on November 08, 2023, 03:19:14 am ---subscribing...
--- End quote ---
I asked the admin to split of the thread, but no reply yet :(
--- End quote ---
10 pages is still easy to handle ;) and usually like all other threads, the first few pages are the most usefull one anyway, can ignore the rest. ymmv. cheers.
--- End quote ---
Problem is, that this has absolutly no relation to easyeda anymore :) and I really just hijacked the thread, which isn't nice :p
oliv3r:
So I ran into one little bit of an issue, the 497 resistor doesn't seem to exist ... I know that the original drawing is reverseengineered and 'measured'. But then I also don't understand the original/intended design (at all) to see what is a good fit replacement.
So for now, I'm going with 499Ohm, which does exist, which is 'close enough', and also fits in the E96 list, so not unlogical to have been used. Same for the 90k8, which will become a 90k9
ebastler:
--- Quote from: oliv3r on November 08, 2023, 04:21:54 pm ---So I ran into one little bit of an issue, the 497 resistor doesn't seem to exist ... I know that the original drawing is reverseengineered and 'measured'. But then I also don't understand the original/intended design (at all) to see what is a good fit replacement.
So for now, I'm going with 499Ohm, which does exist, which is 'close enough', and also fits in the E96 list, so not unlogical to have been used. Same for the 90k8, which will become a 90k9
--- End quote ---
Sure, neither of these should be highly critical, and especially the 49x Ohm resistor is not.
The 90kx resistor forms a 10:1 voltage divider together with the 10k resistor, so the "perfect" value would be 90k0. But who cares about a 1% error in the divider ratio?
The 49x resistor in series with the 3.3pF capacitor forms a bypass for high frequencies. I.e. they will not be attenuated by 10:1 but much less; presumably to preserve exact timing of the signal edges. But I don't see why you couldn't even be 10% off here without creating a problem. (The capacitor will have at least a few % tolerance anyway, so your RC time constant won't be that well-defined.)
oliv3r:
--- Quote from: ebastler on November 08, 2023, 03:17:13 pm ---Towards the end of page 6, oliv3r presents his project which supports different flavors of probes -- either using the same low-cost TTL/CMOS driver, or using the proper (but expensive) comparators and 10:1 divider front-ends as used in the original Rigol probe.
--- End quote ---
Well that's also part of the reason I went with quad pods, to lower the cost of the probe, not often you need 16 * expensive parts :)
the quad vs dual LMH's is a cost/availability/performance/original thing. We'd want to have 1 pod that's next to identical of the original, and LMH7324's that might be cheaper/but maybe hotter? Who knows why rigol did what they did :)
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