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| 16 channel digital probe for HP54645D MSO? |
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| carl_lab:
--- Quote from: timb on September 29, 2016, 05:48:27 pm ---This led to "round" cables being commercially produced; they're not actually twisted pair in the true sense (though the entire bundle may be twisted together). --- End quote --- I don't think so, this one looks to me like a perfect multiple twisted pair cable. Each pair of wires is twisted separately, not only the whole bundle... Anyway, now I don't think twisted pair cable will have any benefit, because the patent paper's sketches show, the special cable is NOT twisted pair. Another point: The linked patent paper also says: "... is constructed from wire and fabric woven together to form a flat ribbon cable, having resistance wires for signals alternating with standard lossless wires for ground to form a lossy transmission line." But standard ribbon cable uses high conductible copper wire... This will cause reflections as mentioned and reduce bandwidth. I don't know, if I need the full bandwidth so using a standard cable maybe OK. Does anybody know, if there will be a signal level problem at lower frequencies, when you use a standard copper ribbon cable instead of the original resistance wire cable? What is the resistance over one signal wire of the original cable? |
| JFJ:
--- Quote from: carl_lab on September 29, 2016, 06:03:53 pm ---What is the resistance over one signal wire of the original cable? --- End quote --- There's only direct access to signal wires at the probe end of an original cable (the plastic shell at the scope end is riveted). The resistance between scope connector and the other end of one signal wire is 190 ohm: --- Quote from: carl_lab on September 29, 2016, 06:03:53 pm ---Anyway, now I don't think twisted pair cable will have any benefit, because the patent paper's sketches show, the special cable is NOT twisted pair. --- End quote --- Another problem with using an 80-conductor PATA cable is that one of signal wires used by the HP54645D, for channel 9 (I think), would be internally shorted to ground, by the connector: It would probably be easier to modify a used woven cable from a different HP Agilent logic analyzer - e.g. a 16510-61601 or 16510-61602 16-channel cable (they usually sell for a tiny fraction of the price of a used 54620-61601 cable). |
| carl_lab:
I tried to make a simulation by LTSpice a) using a line resistance of 190 Ohms (original cable) b) using a line resistance of 0 Ohms (standard ribbon cable) Ri calculation: Ri = (100k+370-90.9k-250-190) Ohm = 9030 Ohm Ci is roughly estimated 10 pF. |
| carl_lab:
The amplitude response does not look very flat in both diagrams... I'm sure my model is not absolutely correct (I'm neither a RF specialist nor a LTSpice specialist)... How can I simulate the distributed resistance and distributed capacitance of the woven special cable more realistic? |
| tautech:
IMO you're over thinking it. Digital channels probes do not offer the same response and accuracy as a analogue scope probe or need to. Keep it as simple as possible and give a DIY probe a try. In a real use situation you're only looking at timing correlations between channels and an indication of amplitude. Even if decoding waveforms can look gawd awful and the scope will still decode. Real bus line or logic performance is best left to a normal or active scope probe. |
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