Advances In Microwave Active

Abstract

Recent advances in microwave active filter design and realization are described.The techniques discussed include: transversal and recursive, cascading of transfer functions, predistortion, negative feedback and Q enhancement. A brief discussion of the merits and potential problem areas is presented along with experimental results obtained with both wide and narrow-band MMIC active filters. Introduction Recent trends in microwave components and circuits point in the direction of fully integrated subsystems that can be fabricated in large numbers at low recurring cost. The most popular solution for this requirement has been the design and realization of complete subsystems in monolithic integrated circuit (MMIC) form. A large number of devices have been succesfully implemented in MMIC with a high level of integration and performance, these include: low-noise amplifiers, switches, medium power amplifiers, oscillators, mixers and complete receiver front ends (involving the realization of a number of the above devices in a single ship). The microwave components that have so far lagged behind in this rush for miniaturization are those that require high unloaded Q factors such as band-pass filters and multiplexers. The difficulty involved in the size reduction of these devices is related to the rapid increase of ohmic losses with decreasing circuit sizes, which leads to low unloaded Q factors; about 30 to 150 for lumped or distributed resonators realized in MMIC, respectivelly. Over the past few years a number of techniques have been suggested and implemented to circumvent this problem. The purpose of this paper is to review some of these approaches, point out their advantages and disadvantages and present recent data obtained with a wide-band, cascaded function topology and a new realization of Q enhanced narrow-band MMIC active bandpass filter. Review of Design Techniques Four basic active filter design approaches have been succesfully implemented in the microwave range: transversal and recursive [ 1],[2] transfer function cascading [3],[4] negative feedback [5] Q enhanced resonators [6],[7I,l81. The transversal and recursive approaches are based on distributed filter concepts, borrowed from low frequency digital applications. The desired frequency domain filter shape results by combining a number of amplitude weighted, delayed signals as a truncated Fourier series(see Figures l a and lb). The sharpness of the filter slope at band edges (or selectivity) is proportional to the number of elements retained in the Fourier expansion which leads to practical limitations on the maximum out-of-band rejection obtained with this approach when circuit size is a design constraint. Another limitation is the spurious passbands, intrinsic to this design, that appear at harmonic frequencies [9].