Abstract: A signal-frequency tracking band-pass filter useful for passing an input signal which changes rapidly in frequency. The tracking filter includes a voltage-controlled band pass filter, and a frequency-to-voltage converter for translating the output frequency of the filter to a voltage for controlling the pass band center frequency of the filter. The Q of the filter is controlled by a feedback circuit which makes the Q vary inversely with output signal amplitude. The Q is high for weak signals that are hard to lock on to, and the Q is low for strong signals to facilitate tracking of signals which change rapidly in frequency. The filter is especially useful for inclusion in internal combustion engine diagnostic equipment.

Abstract: A tachometer includes a transducer for sensing pressure variations at the end of the exhaust pipe, or other easily-accessible point, due to explosions in an internal combustion engine. The transducer produces a corresponding electrical signal having a prominent selected frequency component which varies in frequency in direct proportion with engine speed. As the engine speed is increased and decreased, the electrical signal is passed through an amplifier with automatic gain control, and through a tracking filter to separate the prominent frequency component from other components. The separated component may be applied to a frequency-measuring device calibrated to display the revolutions per minute of the engine, or to a computer which computes and displays the result of an acceleration burst test.

Abstract: An engine diagnostic apparatus includes a transducer for translating an exhaust pressure waveform emitted by an engine to a corresponding complex electrical signal having a prominent fundamental frequency component which represents the cylinder firing frequency and varies in direct proportion with engine speed, and having a subharmonic frequency component at a frequency which is a submultiple of the fundamental frequency. The subharmonic frequency component and the fundamental frequency component are separated from the complex electrical signal and from each other. A comparator compares the amplitude of the subharmonic frequency component with the amplitude of the fundamental frequency component to provide a measure of the unevenness of power strokes from individual cylinders of the engine.