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This circuit shows a quick, clean way to tune a notch filter’s center frequency by varying a single resistor, which could be switched. The LTC1062 switched capacitor filter and A1 form a clock tunable notch (see LTC1062 data sheet). O1, running from the 5V supply, furnishes the clock, which is level shifted by Q1 to drive the ±5V powered LTC1062. In this case, three common notch frequencies are listed; others are selectable by tuning O1 in accordance with the equivalency listed.