Baseband filters for IS-95 CDMA receiver applications featuring digital automatic frequency tuning

This paper presents fully-integrated dual (I and Q) low-pass seventh-order Chebychev continuous-time filters for IS-95 CDMA channel selection applications. The capabilities of digital signal processors, available in almost all modern transceivers, are exploited to adapt the filter bandwidth to the desired frequency with minimum additional hardware. To fulfill the requirement of handling signals with a wide dynamic range, an active RC filter topology is adopted. Digital tunability is provided by constructing each integrating capacitor with an array of binary-weighted capacitors. The capacitors are switched in or out under the control of the DSP. The ratio of the fixed capacitor to the total variable capacitors is chosen based on the expected RC time-constants variations.