The design and development of a broadcast FM modulator with a RDS (radio data system) encoder is presented. The system is fully coherent with the modulated RF frequency synthesized in the 875-108 MHz frequency band with 100 kHz frequency steps. This feature has been obtained by a feedback loop where a PLL acts as a frequency demodulator, thus providing control capability on the phase noise of the modulated signal without any restrictions in the modulation bandwidth, thereby reducing the adjustments when a frequency change is made. The design of the RDS encoder has been done according to the CCIR regulations, avoiding the BPSK approach, by means of current electronic circuits plus a switch-capacitor filter in a simplified implementation derived from the RDS standard and suitable for an eventual ASIC design. The developed system gives a full capability as a FM stereo broadcast modulator according to the CCIR regulations, in order to be used in a university broadcast station for testing and developing all the features of the RDS system, specially the use of the transparent data channel as a communication method between the university community.