A system and method for calibrating a duration of a pulse or a delay. A reference clock signal includes a sequence of reference pulses, and controls a switch in a first charge pump that is configured to charge a first capacitor. Each of a sequence of test pulses controls a switch in a second charge pump that is configured to charge a second capacitor. At the end of each charging cycle, the respective capacitor voltages are compared and the duration of the test pulses is adjusted, by a feedback circuit, in a direction tending to make the capacitor voltages equal. When the capacitor voltages are equal, the ratio of the lengths of the reference pulses and test pulses equals the ratio of the capacitances, if the charge pumps deliver the same current when switched on.