Characterizing high-speed oscilloscopes

A technique for measuring the risetimes of the fastest of these instruments, which will allow more accurate measurement of the fastest pulses, is described. The sampling process used by these oscilloscopes is essentially a brief switch closure of a few picoseconds between the signal to be measured at the oscilloscope input and a holding capacitor in the detection circuit. During this switch closure, sampling current flows from the input and is collected or integrated in a capacitor into a finite charge proportional to the input signal. The collected charge is then measured with an analog-to-digital converter. The sudden flow of sampling current during the switch closure creates an impulselike electric disturbance at the oscilloscope input. The characterization technique exploits the fact that this kickback response is a signature of the sampling process.<>