A time-to-digital converter based AFC for wideband frequency synthesizer

The automatic frequency calibration (AFC) technique is routinely used in wideband frequency synthesizers that contain multiple voltage-controlled oscillator (VCO) tuning curves. In this paper, a counter-based AFC that uses a time-to-digital converter (TDC) in the counting process is developed. The TDC is able to capture the fractional VCO cycle information within the counting window. This significantly improves the frequency detection accuracy over the existing counter-based AFC techniques. In addition, a quantitative model is developed to determine the minimally required error-free AFC calibration time for a given VCO tuning curve characteristic. An AFC circuit using the proposed TDC-based counter is designed in a 0.13-μm CMOS technology. Simulation results show that the proposed AFC significantly improves the frequency detection accuracy and consequently for a given frequency detection resolution reduces the AFC calibration time. The simulated error-free AFC time is <2.5 μs with a frequency resolution of 0.04 %.