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The use of a phase-locked loop for the synchronisation of a varactor-controlled pulsed microwave solid-state source is described. The application is to radar systems and some forms of digital communication where locking must be established repetitively. Of particular importance is the speed of lock-up, and major attention is paid to the design of low group delay loop amplifiers so that maximum lock-in range is obtained with first-order characteristics. The locking performance of a 3 GHz source tunable over more than 10% bandwidth is described and it is shown that lock-up times in the region of 20 ns may be obtained. It is also shown that loop lock-up performance may be substantially influenced by exceedingly low-level leakage signals which can prime the source phase during the build-up of the pulse and either aid or hinder locking.