An active inductor fractional-N frequency synthesiser for cellular systems

A phase locked loop (PLL) frequency synthesizer based on fractional-N frequency synthesis techniques, employing direct charge injection for fractional spurious tone reduction and an integrated voltage controlled oscillator (VCO) with an active inductor tank, is described. Frequency synthesizer design includes output frequency range, channel resolution, output phase noise content, spurious tone content, and setting time for its specifications. The loop can be used to directly modulate base-band information with an increase in the accumulator resolution, thereby decreasing the overheads required in a normal, indirect mixing modulation scheme. The reference current for fractional spurious tone compensation is externally controllable that can tune the system to optimal performance at different operating frequencies. Moreover, the feasibility of integrating the synthesizer in a transceiver environment can be made apparent by using active inductors.