A LO generation system with optimized AFC scheme for multimode GNSS receivers

This paper presents a LO generation system used in multimode global navigation satellite system (GNSS) receivers in 55 nm CMOS. This system contains two PLL frequency synthesizers (FS) and supports simultaneous receiving of any two bands among GPS/GLONASS/Galileo/Compass systems with a flexible frequency plan. An optimized automatic frequency calibration (AFC) scheme using an error compensation logic enables fast and high-precision calibration process for optimum PLL operation. Measured results show that the phase noise of each FS is lower than -83 dBc/Hz and -110 dBc/Hz at 10 KHz and 1 MHz offset, respectively. And the locking time including AFC time and PLL time is only about 20 μs. The loop bandwidth varies by less than 6% for all GNSS signals and reference frequencies (10-80 MHz). Each FS consumes about 12 mA current from a 1.2 V supply, and the area of LO generation system is about 1.25 mm².