Noise immune circuit for use with frequency sensor

A noise immune circuit for use with sensors to discriminate against transient noise pulses which modulate detection signals produced by such sensors, such circuit comprising:

• input means to receive detection signals from at least one sensor;
• a power supply for providing d.c. operating voltage to said circuit;
• a first voltage comparator having a first input receiving said detection signals from said sensor and a second input receiving d.c. voltage from said power supply, said first and second inputs being balanced for a preselected amplitude level of said detection signals;
• said first voltage comparator having an output at which it produces a clipped square wave whose frequency corresponds to the frequency of said detection signals;
• a low pass filter connected to the output of said first voltage comparator to limit the maximum frequency of the square wave output produced thereby;
• a second voltage comparator having first and second inputs and an output, said first input thereof being connected to said filter to receive said frequency limited square wave output of said first voltage comparator, and said second input of said second voltage comparator being connected to said output thereof to receive positive feedback therefrom, so that said second voltage comparator operates as a Schmitt trigger circuit having a hysteresis trigger voltage characteristic at the first input thereof and producing a square wave at said output thereof of the same frequency as the square wave output of said first voltage comparator;
• the output of said second voltage comparator further being fed to said second input of said first voltage comparator as a bias voltage which latches the output of said first voltage comparator until a detection signal received thereby from said sensor exceeds a predetermined threshold amplitude level, thereby eliminating transient noise pulses below such predetermined threshold amplitude level;
• said bias voltage being maintained by said second voltage comparator so as to cause said first voltage comparator to return to its latched state after termination of a transient noise pulse which exceeds said predetermined threshold amplitude level;
• whereby at the output of said second voltage comparator there is produced a square wave of a frequency substantially the same as that of said sensor detection signals, notwithstanding modulation of such signals by noise pulses.