A new tuning technique for implementing very high Q, continuous-time, bandpass filters in radio receiver applications

This paper investigates the problem of developing very high Q (>100), integrated, continuous-time bandpass filters for use in radio receivers. A major problem in implementing such filters is the need for real-time tuning of the filter´s transfer function. This paper introduces a new tuning approach that uses a tuning reference signal which is orthogonal to the signal being processed so that both signals may be present in the filter simultaneously. A specialization of this approach that use a spread-spectrum modulated reference for orthogonality is investigated in detail. Unlike existing approaches for tuning high Q filters, the new technique uses no filter switching, makes no assumptions about the spectral content of the filter´s input, and does not require circuitry for measuring the input signal´s characteristics. The new approach is found to be suitable for use with two of the most popular signaling formats-wideband frequency modulation, and low dimension digital phase modulations such as QPSK