Abstract: A frequency synthesizer for scanning a number of predetermined frequencies using radiated electromagnetic energy detected at an array of electromagnetic energy transducers for providing a frequency determinative code. The frequency code is obtained by insertion of a programmed data mask between a source of electromagnetic energy and the array of electromagnetic energy transducers. The frequency code is connected to the input of a programmable divider which also receives the output from a voltage controlled oscillator. The programmable divider provides an output frequency which is a quotient of the voltage controlled oscillator output frequency. The quotient output frequency is compared with a reference frequency for producing a control voltage for the voltage controlled oscillator. The voltage controlled oscillator is thereby driven to produce an output frequency determined by the coded input to the programmable divider.