INTRODUCTION Digital Synchronizer TF2171 is designed for use with Signal Generator TF2015, to which it can be fitted as shown in Fig. 1.1. Together, the instruments provide a 10 to 520 MHz signal generator system with high discrimination tuning and very good frequency stability. Warm-up drift is eliminated, so measurements can be commenced almost immediately after switching on. Incremental frequency control can be obtained digitally by adjusting the frequency switches of the synchronizer in increments of 100 Hz up to a maximum of 0.5% of the initially set frequency without any readjustment of the generator. The synchronizer can also be switched to provide a 1 MHz standard frequency or to accept an external 1 MHz frequency standard. The setting up procedure is extremely simple and calls only for the positioning of the synchronizer frequency switches and the tuning of the signal generator to obtain a 'locked-on' condition. All the features of Signal Generator TF 2015 are fully maintained including the a.m. and f.m. facilities and the synchronizer can be switched to allow the generator to be used independently i.e. in the unlocked mode.