Designing phase-locked loops for instrumentation applications

Many transducers provide a pulsed output whose frequency depends on the specific measurand's property. Conversion of the signal from a frequency-analogue to provide a magnitude-analogue output requires an accurate estimate of frequency, which is difficult when there is significant noise. Two common approaches to this problem-zero-crossing (ZC) detection and phase-locked loops (PLL)-are analysed and compared. The classical PLL however has difficulty handling baseband signals, where the expected input frequency ratio is large. A carefully chosen PLL structure, using heterodyning and a Hilbert transform phase-sensitive detector, is shown to cope with input data having high noise and a wide spread of possible frequency. The corresponding PLL design equations are derived: these indicate the improvement in performance over that of a classical PLL, an all-digital PLL and a ZC algorithm. (C) 2002 Elsevier Science Ltd. All rights reserved.