From: Christopher R. Johnson (firstname.lastname@example.org)
Date: Tue Jan 02 2001 - 17:47:07 PST
It seems that the biggest down side to using twisted pair for the resistive probe is the lack of the coaxial shield for noise reduction and impedance stabilization.
The actual application is for a very low cost relatively high performance logic analyzer (500 MHz sample rate), so there must be 16 probes. The cables must be very flexible to allow easy probing without pulling off the probe clips due to the cables stiffness. I like the idea of the resistive probes because other low cost analyzers have probe capacitances in the 10pf range, which negates their high input impedance at high frequencies.
The biggest reason that I like the higher impedance cable and resulting higher signal level is that the cable directly drives a high speed comparator. The biggest signal integrity problem is probably the on board noise level that the comparator sees, relative to the incoming signal level. With a 100 ohm cable, you can get a 10:1 attenuation with a ~1K probe resistance, and that means that you are looking for your comparator to resolve 10mv to be accurate to 0.1V logic threshold setting. This is borderline. In reality, 20mv / 0.2V would probably be acceptable.
As far as cost, does anyone know of a good source of low cost very flexible high impedance micro coax? The RG-180 (95 ohms) that I have seen is a little too stiff. I have looked at the Gore Speedline micro-coax assemblies that are normally used for routing critical signals on PC boards, but the cost is prohibitive. I suppose there is no reason that the PC board can't be designed to have either coax or twist and flat cable soldered to it, and charge extra for the coax version.
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