is also known that phase distortion can be deleterious at frequencies 200 hz or so away from f h and f l finally it is also known that it is desirable to restrict energy at half the clock frequency prior to squaring the periodic filter of the preferred embodiment has the desirable advantage of providing transmission nulls at 200 hz on either side of both f h and f l as shown in fig3 and of providing a null at 800 hz half the clock frequency while a periodic filter is used in the preferred embodiment any filter providing a characteristic like the periodic filter about f h and f l could be utilized thus a shape such as that shown in fig4 would suffice a filter providing a characteristic like that of fig4 is referred to herein as a 34 double restrictive bandpass filter 34 curve 47 in fig3 shows the result of computing the point by point product of x 2 u03c0f and x 2 u03c0f wo the amplitude of the resulting product is given by in the region 2200 gt f gt 3000h z after normalizing the maximum amplitude to unity curve 43 in fig3 shows the product of the amplitude response of.