Electronics > Projects, Designs, and Technical Stuff
Measuring the parasitics of my Siglent 1204X-E
arildj78:
I've been measuring a LC tank circuit and calculating both the unknown inductance and the unknown capacitance of the circuit. As part of figuring out this I would like to measure the input of the scope to check if it is actually 15pf / 1MΩ.
This is my test setup.
9V battery with a 10MΩ resistor in series with the positive supply wire. I wrapped the wires close to minimize the inductive reactance of the wires. The wires are then connected to the scope via MiniGrabber probes.
I got a standard C charge graph that leveled off at 804mV, indicating aproximately 8.8V at the battery. When I zoom in at the start of the rise I see a short spike that comes down again to 80mV before the standard C charge. Can anyone explain this behaviour? Am I seeing some inductive parasitic and if so, how can this be modeled?
Attached is a screenshot from the scope and a picture of the MiniGrabber probes.
Arild
magic:
The step is probably caused by capacitance of the resistor, which charges up and dumps some charge into the scope's input within nanoseconds.
The spike may be an overshoot caused by that sudden rush of current flowing through inductive wires. If you zoom in, there may even be some decaying ringing there.
You could perform your measurement by replacing the resistor with a known capacitor (a few pF) and measuring the resulting voltage division ratio (the height of the initial step).
arildj78:
I’be tried to model this in LTspice but I can’t figure out the circuit. I do get the spike by inserting some inductance, but I’m not getting the step.
When zooming in in the real circuit I see ringing on the step (186MHz).
Can you suggest an equivalent circuit to test in Spice?
magic:
Something like that:
9V → a few nH of cable inductance → 10M in parallel with 1pF → scope input (1M in parallel with 15pF)
arildj78:
I think that was it! I was fiddling with the extra capacitance and inductance inside the scope and not in the resistor in my test setup. I will post a picture as soon as I have tweaked the values to match reality. :)
Arild
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