An apparatus for generating a monocycle comprises an input signal source (76) for providing an input signal, and a step recovery diode (SRD) (80) for receiving the input signal and producing an impulse. A shunt inductor (102) is provided to act as a first differentiator and a capacitor (92) connected in series to the output of the step recovery diode acts as a second differentiator. The first and second differentiators are arranged to double differentiate the impulse to produce a monocycle.