A device for producing a varying frequency sine wave synthesized so that frequency is swept linearly with time for use in a continuous transmission FM sonar. A crystal clock, divider and a 16 bit digital output sweep counter produce 2 16 counts for each sweep period T s . The 2 16 counts are applied to a 16 bit digital to analog converter to provide a highly linear voltage ramp (the smoothed result of 2 16 steps). The ramp is converted to a proportional frequency which is applied to a 5 bit digital counter to access a read only memory programmed to provide a digital output representing points along a sine wave. The memory output is applied to a digital to analog converter to produce a voltage output that is a 32 stepped approximation of a sinewave at a frequency which increases betweeen f 1 and f 2 with the linearity of the ramp voltage. A filter smoothes the output.