JasonC SBB wrote:The noise you see is typical in a noisy environment like a car.
Where did you ground the scope probe, physically? How far from the TPS?
Are you powering the scope from the mains, from an inverter powered by the car, or from a battery?
Probe was grounded to the battery -ve which is about 2' as the crow flies from the TPS, and I'd guess about 6' of copper by the time the electron makes its way into the cabin and back to the battery. Scope was powered from mains.
JasonC SBB wrote:
Try this experiment. Take the scope probe and place it on the same point as where the probe's ground clip is connected. i.e. the tip is shorted. You would think it should read zero, right? You may need to turn up the gain on the scope (make it read mV/div - think about the A/D resolution being 5 mV for 1 bit)
If it still reads some spikiness, the spikey noise is "common mode noise" getting through the scope's front end amplifier. Repeat with the scope grounded to the TPS ground pin.
If you get no noise, for fun, disconnect the probe, clip the ground clip onto the tip, then touch various ground points on the engine. (Do this with no other probes connected). On the valve cover near the ignition coils, near the alternator, etc.
Try it again with a 2nd probe whose ground clip is connected elsewhere.
I would like to better understand oscilloscopes, but I'm afraid mine is not much fun to use at the moment. I did do the first test and the trace was pretty much zero, just a slight flutter at highest frequency, lowest voltage range, so not much sign of common mode noise there.
I have previously seen how readily the scope can pick up apparently random voltages (often when the probe slipped off) and have learnt to be wary of signals that don't make sense (as far as any of these deep mysteries make any sense to me). It usually comes down to something being disconnected, or some scope control set in the wrong range. I do understand, in a theoretical sense, how magnetic fields abound and can induce voltages all over the place, and higher frequencies can be much weirder to deal with than lower ones, and almost any bit of wire can act like an antenna; but this understanding doesn't get me terribly far, except to feel pretty happy that I ended up with a career in software. Anyhow, when I got the above trace I had verified that the TPS trace did go up and down with the throttle and everything did make reasonable sense, noisy as it was.
So if this noise is fairly typical, other people who have probed their TPS will see something like the above trace?
Assuming yes, was I right in what I asked before? These values surely aren't what the MS CPU's A2D unit is seeing? In order to get suitable real-world input for testing the software filters, I need to get the output of the hardware filter don't I? Or perhaps feed the figures from my trace into a filter function to simulate what the hardware is doing? Or maybe this is a bit of a fool's errand?