My questions are: what is causing the initial charge to go negative?
AND: In this case, is the pulse application the wrong point for X1?
The wiggling up and down oscillations you see on the yellow trace of the oscilloscope can be from two sources:
- 1. software artifacts from sin(x)/x interpolation (specific to digital oscilloscopes only, but these become visible only when the oscilloscope is pushed close to its limits in terms of acquisition speed - so you also have to tell which model of oscilloscope you are using)
- 2. and this is more probable, the capacitor you measure, together with the probe leads, make a resonant LC circuit. If you used the alligator GND clip of the probe to connect to ground, that alligator clip has a 2 inch or so piece of wire that is also a coil (any piece of wire has also a small parasitic inductance (L), a small parasitic capacitance (C), and a small parasitic resistance (R))
Please post a picture with the physical setup showing how you connected the probe, to see what else might be the reason.
When measuring high speed signals, you have to remove the parrot-like hook from the tip of the probe, and instead of the GND aligator clip, use the tiny small spring that came with the probes in a separate ziplock plastic bag. That spring fits the metal ring visible at the top of the probe after the removal of the parrot hook tip.
This video tells the problems of measuring with an oscilloscope probe, in general:
#9: Basic 1X and 10X Oscilloscope Probe tutorial
w2aew
https://youtube.com/watch?v=SX4HGNWBe5MThis one tells about the ground lead alligator, and how to connect the probes when measuring high speed (signals with fast edges) or high frequency:
#68: Oscilloscope Probe Ground lead length effects on signal quality
w2aew
https://youtube.com/watch?v=zodpCuxwn_oOr, you can make a high speed socket for your probe, socket that you solder on the PCB then insert the probe tip in it:
#111: How to make a high performance oscilloscope probe socket
w2aew
https://youtube.com/watch?v=-4q8geE5ef8