utilized having an input phase that is in quadrature to the other as previously described thus such variation can be reduced to a minimum as described hereinafter the outputs of the filters 38 39 are connected to control voltage comparators 42 and 43 the filter 38 is connected through diodes 44 and 45 while the filter 39 is connected through diodes 46 and 47 to the comparators 42 and 43 the voltage comparators 42 and 43 are conventional digital devices that generate an output signal when their input signals exceed a preselected threshold voltage thus since the detectors 33 and 34 operate in quadrature one or the other of the comparators 42 or 43 responds first and resets flip flop 50 through or gate 48 this minimizes any time variation in the threshold output of the filters due to variations in the starting phase a conventional voltage comparator 51 has its noninverting input connected to the output 36 of the detector 33 and its inverting input to a reference voltage in response to change from negative dc to zero dc which occurs when the loop goes out of lock which occurs when a frequency change is initiated causes the device to generate a positive output on line 52 to set the flip flop circuit 50 thus the flip flop circuit is set to produce a characteristic.