variation in the strobe rate causes the notch frequency of the filter to change thus by varying the frequency of the strobe pulses in proportion to the variations in the carrier frequency the notch frequency is caused to track the carrier frequency this insures that the filter is always appropriately tuned to eliminate the carrier frequency leaving only the information carrying envelope fig4 illustrates the strobe pulse generator used in conjunction with the filter of fig1 it generates four strobes which together form a period t defined by the reciprocal of the carrier frequency therefore as previously pointed out the period of the strobe will follow the carrier 34 frequency period 34 the strobe pulse generator of fig4 comprises a phase locked loop 25 e g model ne564 obtainable from the signetics corporation the input to the phase locked loop 25 is the signal containing the variable carrier frequency to be rejected the output of the phase locked loop 25 is fed back to its input via tandem frequency divider 26 and 27 which divides the output frequency by 64 thereby causing the output frequency of 25 to be 64 times the input this output provides the clock which is connected to a four stage shift register 28 and as one input to each of and gates 29 30 31 and 32 in addition to the divide by sixteen output of divider 26 two additional output terminals are fed to and gate 33 the first output terminal provides an.