Frequency-Transformation-Based Co-Designed Lowpass-Single/Multi-Passband-Highpass RF Filters

The theoretical design and practical development of RF analog filtering devices with co-integrated lowpass, single $\boldsymbol {/}$ multi-band bandpass, and highpass transfer functions is reported. For this purpose, two different classes of generalized frequency transformation that convert the equivalent normalized lowpass filter prototype into the desired RF filter with several co-designed filtering actions are proposed. They realize a frequency mapping of the reactance of a normalized unitary capacitor into that of a one-port single $/$ multi-resonance cell with added lowpass and highpass filtering capabilities. For these devised frequency transformations, the theoretical foundations and various illustrative filter examples designed at the ideal-circuit-model level are presented. In addition, higher-selectivity filter architectures based on the generation of additional out-of-band transmission zeros (TZs) by means of cross-coupling techniques are shown. Afterwards, the extension of this design methodology to RF multi-functional filtering components, such as input-reflectionless $\boldsymbol {/}$ absorptive filters based on complementary-diplexer circuit networks and two-way filtering power-distribution circuits, is also demonstrated. Furthermore, two design examples of distributed-element and inverterless lumped-element RF filters are provided. Besides, for the distributed-element circuit, a proof-of-concept microstrip prototype is manufactured and measured as experimental validation.

I. Introduction

During the last two decades, a considerable research progress on the development of RFmicrowave filters with multi-band operational characteristics has been conducted. This has been largely motivated by their potential need for emerging multi-purpose RF-wireless-communications and remote-sensing applications, in which the exploitation of several spectral bands at the same time is advantageous [1]. In this manner, a vast plurality of multi-band bandpass filters for the simultaneous acquisition of multiple desired RF signals corresponding to different services has been reported [2], [3], [4], [5], [6], [7], [8]. The same applies to RF multi-band bandstop or multi-notched-band filters, as a hardware mechanism that may allow the implementation of the required protection for RF systems to unwanted jamminginterference RF signals in highly-congested electromagnetic (EM) environments [9], [10], [11]. Note that although the existing variety of prior-art RF multi-band filters in terms of underlying RF operational principles and circuit structures is very rich, those approaches using single-to-multi-band frequency transformations deserve special mention [12], [13], [14], [15], [16]. This is owing to their intrinsic simplicity, since they mostly involve the replacement of the basic resonators of a pre-designed bandpass filter counterpart—or of the normalized unitary capacitors of an equivalent normalized lowpass filter prototype for normalized-lowpass-to-multi-band frequency transformations—by sets of resonators to shape the distinct bands. Lately, more-evolved frequency transformations have been devised, such as those in [11] and [17] allowing to embed multiple stopbands inside a wide passband range for the design of RF pre-selection broad-band filters with co-integrated multi-interference suppression.