Frequency discrimination circuit

A frequency discrimination circuit for generating an output signal which has an upper saturation value or a lower saturation value, when an input pulse signal is out of a predetermined frequency detection range and has a value being proportional to the frequency of the input pulse signal and lying between said upper and lower saturation values when it is within said detection range, comprising:

• (a) retriggerable monostable multivibrator means for generating a first square wave signal of a predetermined pulse width in synchronism with said input pulse signal except for the case where said retriggerable monostable multivibrator means is retriggered, said input pulse signal being applied as a triggering input to said retriggerable monostable multivibrator means;
• (b) gate means including a gate circuit for generating a second square wave signal, said first square wave signal being applied as a first input to said gate circuit, said gate circuit receiving as a second input one of said input pulse signal, a signal obtained by subjecting said input pulse signal to waveform conversion, and a signal obtained by subjecting said first square wave signal to waveform conversion, a leading edge of said second square wave signal being delayed by a predetermined time from a corresponding leading edge of said first square wave signal, a trailing edge of said second square wave signal being synchronized with a corresponding trailing edge of said first square wave signal;
• (c) a trapezoidal wave generator supplied with said second square wave signal for generating a signal, said signal being maintained at a minimum value during said second square wave signal is kept in a high level and being varied monotonically with time at a period when said second square wave signal is kept in a low level;
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• (d) a sample-and-hold circuit receiving said signal from said trapezoidal wave generator as its input to be sampled and a sampling pulse as its sampling input, for generating a frequency discrimination output signal, said sampling pulse being generated in synchronism with a leading edge of said input pulse signal.

In a speed control device including a speed detector for detecting a speed of revolution of a rotating body driven by a motor to generate an input pulse signal having a frequency proportional to said speed of revolution, a frequency discrimination circuit supplied with said input pulse signal for discriminating said proportional frequency from a predetermined frequency corresponding to a predetermined speed of revolution to generate as a discrimination output an error voltage corresponding to a deviation of said proportional frequency from said predetermined frequency, and a motor driving amplifier supplied with said error voltage for changing a revolution speed of said motor so as to maintain said revolution speed of said rotating body at a predetermined value, the improvement wherein said frequency discrimination circuit comprises:

• (a) a trapezoidal wave generator for generating a trapezoidal wave signal, said trapezoidal wave signal including a part having a constant inclination, said part beginning at a time delayed from said input pulse signal by about one period of said input pulse signal;
• (b) a sample-and-hold circuit receiving said trapezoidal wave signal from said trapezoidal wave generator as its input to be sampled and a sampling pulse as its sampling input, for outputting a frequency discrimination output signal, said sampling pulse being generated in synchronism with the application of each input pulse signal, said sampling pulse having a relatively small width;
• (c) a single retriggerable monostable multivibrator receiving said input pulse signal as a triggering input, for generating a first square wave signal having a predetermined pulse width in synchronism with said input pulse signal except when said retriggerable monostable multivibrator is retriggered;
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• (d) AND gate means receiving said first square wave signal and a pulse train as first and second inputs, respectively, for generating a second square wave signal, said pulse train being generated in synchronism with the application of each input pulse signal, a pulse width of said pulse train being at least greater than the width of said sampling pulse and being smaller than said predetermined pulse width, said second square wave signal rising at a time delayed from a leading edge of said first square wave signal by a predetermined period and falling in synchronism with a trailing edge of said first square wave signal, said second square wave signal being applied to said trapezoidal wave generator to cause said trapezoidal wave generator to generate said trapezoidal wave signal at a period when said second square wave signal is kept in a low level, thereby generating said frequency discrimination output signal having a value proportional to said frequency of said input pulse signal in a predetermined range of frequencies to be detected.