Coupled inductor, constant resistance, broadband delay filter

This paper develops a three-terminal delay filter whose topology is an interconnection of only ideally lossless, coupled inductors and capacitors. In the absence of significant capacitive loading at the input and/or output ports, the proposed filter emulates an allpass network whose low frequency delay is twice that afforded by the poles of the proposed circuit. Moreover, the driving point input and output impedances of the structure are frequency invariant resistances whose values over the frequency spectra of interest are independent of the envelope delays achieved. These impedance characteristics allow for the convenient implementation of a cascade of match-terminated delay filter sections, thereby allowing for reasonably large envelope delays without incurring bandwidth penalties in either the magnitude or the delay responses. An example demonstrates the feasibility of designing a filter providing zero frequency delays in the mid-hundreds of picoseconds that remain nominally constant for signal frequencies extending through a few gigahertz. Correspondingly, the magnitude responses of these delay structures offer 3-dB bandwidths that can be significantly larger than the frequencies at which the observed envelope delays decay monotonically to a user-defined percentage of their respective zero frequency values.