Reflectionless Filters for Generalized Elliptic Transmission Functions

Single-ended circuit topologies, and a theorem for the development thereof, are presented with which one may realize constant-resistance (or reflectionless) filters, having ideally zero reflection coefficient at all frequencies and from all ports, suitable for elliptic and pseudo-elliptic filter responses. The proposed theorem produces topologies of a type known as the coupled-ladder, which has been previously studied for only polynomial responses (e.g. Butterworth, Chebyshev, etc.). A comparison between these topologies and another classical approach known as the economy bridge reveals that those proposed here have a number of theoretical and practical advantages. The theory is tested by the construction of a sixth-order, low-pass reflectionless filter exhibiting a pseudo-elliptic frequency response. Measured results are in excellent agreement with theory, and show return loss better than 20 dB throughout the pass-band, the transition-band, and up to two octaves into the stop-band.