Microdegree frequency and phase difference control using fractional-N PLL synthesizers

Two independently programmable on-chip delta-sigma fractional-N phased-locked loop (PLL) synthesizers were developed in 65 nm CMOS with a total die size of 2 mm × 2 mm to demonstrate a millidegree phase shifter. Both PLLs use a 24 bit fractional modulator, thus a theoretical phase shift as small as 21 microdegrees is possible. Due to limitations in the noise floor at microwave frequencies, data was collected at postdivided frequencies 50 MHz and 400 MHz resulting in a best case measured phase step of 21 millidegrees at 50 MHz with a high degree of measurement certainty.