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.
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