US Patent 11327098 Frequency detection circuit and method

During frequency detection, a constant current source outputs an output current to charge a variable capacitor for multi-period. In a calibration mode, according to a comparison result between a cross voltage of the variable capacitor and a reference voltage, a capacitance value of the variable capacitor is adjusted. In a monitor mode, according to a reference frequency and the cross voltage of the variable capacitor, a frequency under test of a circuit under test is detected.