Filter having an automatically controlled variable cut-off frequency

A measuring device for determining the period of an input signal independent of the frequencies of noise signals which may be superimposed on said input signal, comprising means for supplying the input signal whose period is to be measured to a filter apparatus which has an automatically controlled variable cut-off frequency, said filter apparatus comprising a filter circuit which includes a variable element the electrical parameter value of which may be controlled by application of an electrical control signal thereto, the cut-off frequency of said filter circuit being determined by the parameter value of said variable element and said filter circuit exhibiting a frequency cut-off characteristic having a declining portion;

• a level detector having its input connected to the output of said filter circuit for detecting a filtered output from said filter circuit the level of which varies with variations in the position of the frequency of the signal supplied to said filter circuit relative to the declining portion of the filter circuit cut-off characteristic due to the superimposition of differing frequency noise signals on the signal whose period is to be measured;
• means connected between the output of said detector and a control input of said variable element for supplying said detected level output as a control signal to said variable element thereby to control the cut-off frequency of said filter circuit in a manner operative to maintain said filtered output level at a given value;
• and a utilization circuit connected to the output of said filter circuit for receiving a filtered output of said input signal which has a level corresponding to said given value and from which noise signals having higher or lower frequencies than the frequency of the input signal are eliminated, said utilization circuit including means responsive to said filtered output for determining the period of said input signal.