Implementation Of Multi-modulus Fractional LO Divider For Multi-cellular-Band Applications

To RF integrated-circuit chips,one important issue is the purity of its local oscillator(LO)signal.It becomes harder when more and more RF system modules are integrated into one chip through SoC design.LO signal should have a large tuning range to cover all the cellular bands while it should be invulnerable to the interferers from the transceiver blocks on the same chip.One feasible way to solve this is to move and reuse the band,which can be realized by adding a multi-modulus LO divider.By changing the division ratios,the frequency band of the VCO is reused,hence a smaller VCO tuning range is enough to cover all the bands without utilizing several VCOs or dividers.Moreover,by using a fractional division ratio,the VCO pulling effect caused by strong signals like outputs from PA integrated on the chip and their harmonics can be avoided.Prior arts of the multi-modulus LO fractional LO divider are compared and summarized in this thesis.In this design,ring-counter-based downsamplers with reset synchronizer are utilized to enable the divider to work at a higher input frequency.Also,several modules were optimized for better matching.The circuit has been verified through post-layout simulation that it could work well with 9GHz input.This part has been taped out in TSMC 65 nm.Because of the nonideality of the real chips,calibration is needed.The relationship between the path mismatches and the output performance is modeled mathematically.A digital calibration scheme processing signals in the time domain is adopted to improve the output performance.Research on the measuring module of the digital calibration scheme—TDC has been made.The chosen structure of the TDC and the proposed time interval measuring procedure is designed according to specifications and verified through simulation;the digital algorithm is modified to get more correlation and a more-than-68%-faster convergence speed is achieved.At last,due to the structure of the proposed divider,its output duty cycle changes according to the division ratio.A brief introduction of the duty-cycle corrector(DCC)utilized in the divider is given.The output duty cycle is corrected to 50%±0.42%.