Direct wide-band phase shifters

Two functions which differ only in phase and that by 90 deg, are related to each other by the Hilbert transform and to derive one from the other it is necessary to know the values of the input function at all times, past as well as future, right up to infinity. However, the amplitude error will be less than ± 1 dB if the input values are considered up to a time duration equal to the period of the lowest frequency component in the signal. It has been explained that speech waves vary comparatively slowly and hence, their future form can be taken to be the same as that at the present instant. A direct wide-band phase shifting circuit which utilizes these ideas to give an arbitrarily small error in phase and an amplitude response constant to within ± 1 dB, over a 14 : 1 range of frequencies, is described.