Please help an ignoramus understand oscilloscope inputs

Thread Starter


Joined Apr 10, 2019
So, I've got this old Tektronix 2445 analog scope with 1Mohm/15pF at the inputs. I've known this happens for a long time and despite searching around, never found an explanation simple enough for me to understand.

***If I take a signal generator making a 5V amplitude 50 kHz square wave and attach the probe, I see a square wave with sharply-defined edges.
***If I take that same 50 kHz square wave output and run it through a 47 kohm resistor before attaching the scope probe, the output looks like a square wave run through a LP filter.

Supposing the 47k makes a 1st order LP with the input capacitance, the cutoff would be around 225kHz...HOWEVER...if I use a 50kHz 5Vp-p sine wave instead of a square wave, the output from the same 47k resistor is another sine wave, about 1Vp-p, phase-shifted about 80 degrees, so it seems that something else is going on. Please explain to this ignoramus what's happening here.