Dual-Band Recursive Active Filters With Composite Right/Left-Handed Transmission Lines

Abstract

Unique properties exhibited by metamaterial transmission lines have been previously used to design a large number of dual-band microwave passive devices, but not active ones. In this paper, a novel dual-band active filter scheme based on composite right/left-handed transmission lines is proposed. The inclusion of these types of lines as feedback sections in a first-order recursive topology can be used to generate a filtering response with two arbitrary passbands. Additionally, dual-band couplers are also required. These may be implemented by means of stub-loaded branch-line structures. As an added advantage, these elements produce a strong rejection level at the central stopband that improves the overall response. Theoretical analysis and design procedures are verified by means of manufacturing and measurement of a prototype containing a distributed amplifier.Index Terms-Active filters, composite right/left-handed (CRLH) metamaterials, microwave recursive filters.

Multiple Order Dual-Band Active Ring Filters With Composite Right/Left-Handed Cells

“…In fact, there are two types of stability problems in this type of band-pass filters, both of them caused when working with non-ideal power combiners. The first case yields out-of-band instabilities that are caused by the limited bandwidth presented by some narrowband power combiners (e.g., branch line couplers [12]). In this case, there are some frequency ranges at which the combiner does not isolate the feedback line and the input, and some part of the amplified signal may be fed back to the input.…”

“…As exposed in [12], if the value of |A| is higher than A th , it leads to unconditionally unstable schemes. Thus, it is desirable to limit the value of |A|, at least, under this threshold value A th , in order to avoid instabilities:…”

“…The use of CRLH lines to achieve dual-band responses with a first-order recursive filter topology was firstly presented in [12]. That scheme used a feedback topology with branch-line couplers to combine the signals at the input and output of the filter and presented several potential drawbacks.…”

“…In addition to miniaturization, one of the most exploited properties of the CRLH lines is their characteristic non-linear phase response that allows the design of many dual-band microwave antennas [2][3][4] and circuits [5][6][7][8][9][10][11][12]. Despite most contributions on CRLH lines are based on passive circuits, some multiband active devices have recently been proposed, including power amplifiers [9,10], active diplexers [11] and active filters [12]. These circuits replace the conventional transmission lines with CRLH lines in well-known active topologies to obtain novel properties at different frequencies simultaneously.…”

“…On the other hand, the emergence of many multiband services embedded in one single communication equipment makes the development of microwave components with several operation frequencies be a very powerful solution to cover this demand. With respect to dual-band active filters, a solution based on a first-order recursive structure with a CRLH line was proposed in [12]. The cited topology requires dual-band power combiners (e.g., branch-line couplers) at the input and output of the filter, what increases the overall size of the circuit and makes very difficult the practical implementation of multiple order filtering structures.…”