A novel Q -tuning scheme for continuous time filters

VLSI implementation of an automatic Q tuning system

“…At high frequency and high Q designs the problem of parasitic phase shifts can be especially important since Q will adversely affect the filter s pole, passband regions and bandwidth. Thus to obtain a desired accuracy the filter must be automatically tuned in the quality factor and in many cases the center frequency [7,11]. For high frequency, moderate Q designs the afore mentioned problems still exist though less severe and the desired accuracy of the filter is process dependent unless proper tuning is employed [7].…”

“…This will present a problem if the scheme of [4,11] is used to tune high frequency lowpass or highpass filters because the mean square error cannot approach a global minimum when these two signals are severely out of phase. To be able to use the LMS algorithm to tune the Q for lowpass or highpass biquads, the error signal must comprise of the difference between a deliberately phase shifted input signal and the output signal of the master filter to be Q tuned [7]. The deliberated phase shifted signal is needed to correct for the +90 0 or -90 0 phase shift in the output signal at the pole frequency.…”

“…Thus to obtain a desired accuracy the filter must be automatically tuned in the quality factor and in many cases the center frequency [7,11]. For high frequency, moderate Q designs the afore mentioned problems still exist though less severe and the desired accuracy of the filter is process dependent unless proper tuning is employed [7]. Researchers have used different techniques such as Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page.…”

“…The diagram of Fig. 1 taken from [7] shows that a combination of the true LMS algorithm and a modified LMS algorithm is used to implement the automatic Q tuning. The true LMS algorithm is used to adjust the phase shift of the allpass filter, which must be ideally +90 0 or -90 0 at steady state depending on the sign of the lowpass or highpass master filter.…”

“…This brief addresses the automatic tuning of the quality factor Q for high frequency, moderate Q , analog lowpass/highpass filters based on the Magnitude Locked Loop (MLL) method. The proposed method is well suited to CMOS circuits and had previously been applied to a discrete IC component implementation of a LP filter [7]. This method uses the well-known Least Mean Square (LMS) error algorithm to adapt the quality factor to a desired value.…”