Modulator for generating a frequency-shift-keyed RF signal

To generate a frequency-shift-keyed RF signal (F1 modulation) with a narrow spectrum, a numerically controlled oscillator is controlled by a bivalent modulation signal with the intermediate connection of a counter and a read-only memory. When the modulation signal is shift-keyed, the counter runs up or down through its counting range and stops at the counter reading until the next shift keying. The counter output signal addresses the read-only memory which reads out from a stored table an oscillator control value assigned to the address value with which it controls the oscillator. The spectral width can be significantly reduced through non-linear allocation of the control value to the address value. A plurality of different selectable tables are preferably stored in the read-only memory.