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A circuit which places accurate frequency calibrations on swept-frequency recordings is presented. This circuit is useful in the calibration of direct blood pressure transducers and will have applications in the testing of other biological transducers as well. The design range of 1 Hz to 128√2 Hz is suitable for testing a broad range of biological transducers and systems. The circuit basically measures the periods between successive zero crossings. Pulses are generated that may be recorded on a swept-frequency strip chart record of an individual transducer. A series of sixteen marker pulses indicates 1 Hz, √2 Hz, 2 Hz, 2√2 Hz, up to 128√2 Hz. A special wide pulse at 16 Hz distinguishes the marker pulses.