1. A method for modulating a F-N synthesizer in response to applied sequences of in-phase (I) and quadrature (Q) analog modulation signals, said F-N synthesizer comprising a loop divider having a variable fractional divisor, the method comprising comparing said sequence of I analog modulation signals with a first plurality of threshold values at successive time intervals, thereby to generate a sequence of I digital output signals, said I digital output signals corresponding to respective relationships between said I analog modulation signals and said plurality of thresholds at said successive time intervals, comparing said sequence of Q analog modulation signals with a second plurality of threshold values at successive time intervals, thereby to generate a sequence of Q digital output signals, said Q digital output signals corresponding to respective relationships between said Q analog modulation signals and said plurality of thresholds at said successive time intervals, transforming said I and Q digital output signals into a sequence of digital modulation signals in accordance with a predetermined relationship between values of said I and Q digital output signals in consecutive time intervals, and modifying said variable divisor in accordance with said sequence of digital modulation signals.