An Improved PLL for Tune Measurements

The key element determining the dynamic performance of such a PLL is the phase detector between the beam oscillation and the internal oscillation. Most circuits use a quadrature phase detector, for which the high frequency carrier at twice the excitation frequency is attenuated by a low-pass circuit. The remaining ripple of this component contributes to the bandwidth/noise performance of the PLL. In this paper we propose an alternative solution for the filter, notably an adaptive notch filter. We explain in detail design considerations and the resulting improvements in PLL bandwidth and/or noise figure.