JPS576365A - Tachometer circuit - Google Patents

Abstract

PURPOSE:To accurately detect the micro signals of large level fluctuations and poor SN ratios by utilizing selective amplification and the comparison of a floating threshold system in a tachometer which detects the vibrations of a diesel engine as number of revolutions. CONSTITUTION:Engine vibrations are converted to an electric signal 1 which is removed of higher harmonics by a low-pass filter LF1 and only the 2MHz component thereof is selectively amplified by a tuning amplifier A2. An output 3 is amplified by as much as the quantity controlled by the difference between a reference voltage source 12 and the output 10 of an integrator 9 by an analog multiplier 4, is again selectivey amplified by the amplifier A2 and is made into a DC signal 8 by a detector 7. The integrator 9, a reference voltage source 11 and a multiplier 4 of long time constants constitute an automatic gain controlling circuit, which maintains the level of the output 8 roughly constant. The output 8 is removed of ripples by a low-pass filter LF2 and is made into a square wave 13. A comparator 16 makes level decision of 13 with the output 15 of an integrator 14 of a short time constant as a floating threshold, and after the wave is shaped by a monostable multivibrator 18, it is made into accurate revolving pulses.