Fast settling frequency synthesizer with two-point channel control paths

A frequency synthesizer with low-power and very short settling time is introduced, which utilizes two-point channel control paths. While the main-path is the same as normal channel controls, a digital-to-analog converter (DAC) with tunable gain is used for the compensation-path to form a feed-forward direct voltage-controlled oscillator (VCO) control path. When the two paths are ideally matched, the two-point control can show zero settling time regardless of the amount of frequency change. However, the settling time performance can be significantly degraded if there exists any mismatch between the two paths. In order to remove the mismatch, a simple compensation method combining a linearized VCO with a resistor-loaded tunable DAC is presented. We show that the overall mismatch can be effectively tuned out by controlling the DAC load resistor, since the mismatch caused by process-voltage-temperature variations is dominated by the resistor variation. We have achieved near-zero settling time for 75thinspaceMHz frequency jumping from 2.4 GHz even with the use of narrow phase-locked loop (PLL) bandwidth of 20 kHz. When the phase noise at 1 MHz offset from 2.4 GHz is - 116.6dBc/ Hz, the total PLL power consumption using 0.18 μm CMOS technology is only 4.2 mW.