Novel automatic tuning method of RC filters using a digital-DLL technique

A novel automatic tuning method for RC filters is proposed for CMOS low-IF transceivers. The method is based on a digital-DLL-like technique and tunes the time constant of filters automatically. The tuning range and the tuning accuracy are analyzed theoretically. To verify the method, a sixth-order 2-MHz IF filter for Bluetooth and a tuning circuit were fabricated in 0.18-μm CMOS. The tuning circuit occupying 0.066 mm² includes only a reference first-order filter, a comparator, and a simple digital circuit. The measurement result of the filter and the tuning circuit shows that the maximum ±28% time-constant variation can be tuned within ±5% accuracy, which is consistent with theoretical results.