Derek, in addition to the triboelectric effect there is another mechanism that will create an AC voltage from a mechanically deformed cable. When a cable is deformed the capacitance changes. If the cable is charged to some DC voltage then there is a fixed amount of charge stored at that instant in time. The open-circuit voltage must change when the capacitance changes. Formula: Q = CV, V = Q/C. There are low-triboelectric cables that will reduce the triboelectric effect. The charge effect would be solved by limiting deformation of the cable.
However, it sounds like the original problem might be that the circuit has been inadvertently configured as a TLP (Transmission Line Pulser). To limit the effect to a single pulse (duration equal to twice the propagation through the cable) the switch can be configured to terminate the cable into it's characteristic impedance (50 to 100 ohms). Or, to generate a waveform that exponentially rises or falls, two resistors and a capacitor can be added. If you think this might be the problem I can send you some SPICE simulations. Dave Cuthbert Micron Technology From: [email protected] [mailto:[email protected]] Sent: Sunday, August 31, 2003 1:50 PM To: [email protected] Subject: ESD from wiring Hi all, well, I have found myself involved with an interesing problem. I'm testing an aircraft system, that only has a few wires... 8 or so, that interface with the outside world. 28 volt dc power, an programming loop, a manually operated switch and an indicator. Well, evey time the push button is pressed, I see a 10 to 50 nS pulse, very consistant. The problem first was noticed during RE testing, and I found it easier to hook my scope to the antenna rather than use the SA. I have eliminated the EUT operating, since I have disabled the trigger by removing the component that allows it to trigger. I did leave in place the 2 resistors that terminate the push button wire. I have a predictable set up. I remove either of the two reminating components, and the signal drops, it almost goes away. Anyway, after I'd removed the circuit board to make a change, I pushed the wire harness and saw a very similar transient! So I jiggled the harness and saw many... Cursing a loose connection, I checked, they were all sound. Not sure what was going on, I disconnected power, I could still got them. So I stripped down to my cotton shorts.. still got them. One section of the harness is sleeved with heatshrink sleeving, it holds the wires tighly. Jiggling that produced no events. So, I can only conclude, that the wire is generating a charge during the movement/jiggling, that when it discharges any one of a number of places causes my event. The parasitics of the harness set the ringing frequency. When I press my push botton, I believe I also kick the wiring with the impulse, again it rings very close to the frequency when I jiggle the cables. My questions are, am I off base with the charging supposition? The cable appears to be a teflon type insulation. If so, how can I get around it. I can't really blame the EUT for it. Opinions very welcome Thanks, Derek Walton
