A zero crossing threshold detector is an electroniccircuit that consists of an operational amplifier with an input voltage at its positive input (see circuit diagram). Often used in conjunction with other circuit elements, it usually functions as a simple voltage switch.
When the input voltage is positive, the output voltage is a positive value, when the input voltage is negative, the output voltage is a negative value. The magnitude of the output voltage is a property of the operational amplifier and its power supply. When used with a ±15 V power supply and a 741C operational amplifier, Vsat+ is approximately 13.6 V and Vsat- is approximately -14.3 V.
This detector exploits the property that the instantaneous frequency of an FM wave is approximately given by
f_i=\frac{\Delta t}{2}
where \Delta t is the time difference between adjacent zero crossings of FM wave
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