The application of pulse modulation to signal generators involves difficulties with regard to external modulation and output-level monitoring. Direct anode- or grid-modulation proved unsatisfactory, but a method using a pulsing diode on the oscillator grid is reasonably satisfactory, and details are given of a unit which was widely used.An improved method switches the generator output with a gate valve, the oscillator operating at a constant level throughout. Starting delay and variation of pulse length with carrier frequency are avoided, but the l.f. modulating pulse is also present in the output, and has to be removed by filters. Two practical applications of this system are described. A suggested improvement uses a pair of grounded-grid triodes in a balanced switching arrangement.