Band-Pass Quadrature Filters

Two identical low-pass electrical filters, having any realizable low-pass frequency-response characteristic and synthesized from linear analog computer components by any convenient method, may be interconnected in a simple manner to form a band-pass filter whose frequency-response characteristic represents a translation along the frequency axis of the frequency response characteristic of the original low-pass filters. The resulting filter circuit has two inputs and two outputs. When either input is driven by an electrical signal representing a real function of time, the resulting filtered signals at the two outputs are in phase quadrature with respect to each other. When the two inputs are driven simultaneously by the real and imaginary parts of a complex signal, the two outputs represent the real and imaginary components of the filtered complex signal. The filter, when used in this manner, is able to differentiate between positive and negative frequencies so that its pass band exists for positive but not for negative frequencies, or vice versa.