Modern Radio Signals Filtering Devices Methods, Technologies, & Structures

The widespread use of multi-channel systems and signal processing methods is the development trend of modern radio engineering information systems. The case examples are multi-channel filtering and spectral analysis devices, which are discussed in detail in this monograph. Exceptional opportunities for improving methods and devices for information processing emerge from the use of information and, in particular, structural redundancy that appears as a result of the multi-channel processing of radio signals with a high-frequency resolution. This has resulted prominently in recent years. Solving the problems of study, design, and implementation of spectral analysis tools on acoustic waves are the most important tasks of applying the latest achievements in the development of technologies, methods, algorithms, structures, and devices for radio signals spectral processing in modern multifunctional multichannel radio information systems.

Filtering and spectral analysis of radio signals are used in various radio engineering information systems for a wide range of applications, including the parallel real-time analysis of large amounts of data. Along with devices for multi-channel radio signal filtering, both single and multiprocessor spectral analysis devices can be used in these applications. Since these spectrum analyzers subject the processed radio signals to the Fourier transform, we will call them Fourier processors. 

When studying the problems of spectral processing of radio signals, implementing their technologies, methods and algorithms for analysis, modeling, and development of structures and devices for filtering and spectral analysis, the final goal is, generally, frequency selection and frequency measurement of radio signals. A set of issues related to such measurements is considered. Algorithms, structures, and devices for measuring the frequency of radio signals after their spectral processing have been developed.

A review of methods for estimating signal parameters and the synthesis of devices for estimating and measuring their frequency showed that there are a large number of different options for constructing frequency discriminators. On this basis, open and tracking meters can be implemented on various elements. Each of them brings its own specifics to the processing of radio signals in the frequency measurement channels. Such a variety of measuring elements and structures prescribes different approaches to their research, but only relatively simple devices can be analyzed analytically, although most often under significant restrictive assumptions. Others, more complex and with less stringent constraints, can be studied through computer modeling or hybrid modeling approaches. The ultimate and most preferred method for analyzing the accuracy of frequency discriminators and meters based on them should be considered an experimental study in laboratory or natural conditions, which allows obtaining the most reliable and valuable results. 

An important stage in the creation of SAW devices is the modeling of its design providing required electrical parameters. This task can be solved within the context of a device design synthesis problem. Here, requirements are set for the electrical parameters, on the basis of which the design of the device is simulated. The simulation allows for selecting other design parameters, among which are the materials of the piezoelectric and IDT, the number of electrodes and their thickness in the IDT, etc. Since a number of SAW filter designs with specific design parameters are currently used, the task seems to be over-complicated and is not considered here.

The presented method assumes device simulation based on its electrical characteristics for the given design parameters of the selected filter type. This approach offers an analysis of the selected type of device design (irrelevant to the reasons for its choice) to predict its design parameters based on simulation of its electrical characteristics for different values of its design parameters. Obviously, based on the described method, it is also possible to build algorithms for device synthesis, e.g. within the context of electrical parameters optimization problem of the device with selected design type.

The problem of filter topology synthesis is investigated for only one type of ladder SAW filter. The described method can be used to construct algorithms for the synthesis of various SAW filter types. 

Low insertion loss and good temperature stability are the main requirements for filters used in communication system front ends. These requirements severely limit the design options for SAW filters that can be used in modern communication systems. It should be noted that until recently SAW filters with a relative bandwidth of 1..10% and low losses, and most demanded for communication systems, had low-temperature stability. This was due to the temperature coefficients of waveguide material medium frequency of the SAW filters. Such materials are strong piezoelectrics niobate and lithium tantalate.

Research aimed at improving the temperature stability of devices based on lithium niobate and tantalate was started a long time ago. Still, only emerging new technologies have provided the possibility of solving this problem for mass production conditions. Currently, the input circuits of almost all mobile communication devices incorporate filters (duplexers) on SAW.

High electrical strength and low losses compared to other transmission lines predetermined the widespread use of waveguide filters and multiplexers in on-board radioelectronic complexes of satellite communication systems. The electrical parameters of these devices, taking into account the specifics of their operation in near-Earth orbit, are subject to fairly stringent requirements: high power limit, temperature stability, weight, and size indicators. Their disadvantages can be considered as a limited range of operating frequencies, determined by the cutoff frequencies of the main and first higher modes, as well as relatively large sizes. Very high requirements are also placed on the accuracy of the manufacture of filtration devices. At present, in the literature, one can find a description of a large number of various designs of band-pass filters of the waveguide type. Some of them are considered in the present chapter.

Multiplexers are microwave products designed to add signals received from several microwave amplifiers in one output channel. There are no domestic analogues of these devices, and foreign analogues are Multiplexer 4-1/C (C-band), and Multiplexer 3-1/X (X-band) from «COM DEV International» (Canada). The basis of the design of the output multiplexers of onboard radio-electronic complexes of space communication systems is waveguide narrow-band microwave filters of high power levels. 

Two-dimensional (2D) periodic structures of the microwave and terahertz ranges are becoming more and more widespread as the basic elements of instruments and devices for controlling the parameters of electromagnetic signals in radio astronomy, nuclear physics, medical technologies, electronic engineering, instrumentation, and radar technology. They are known in the literature as “Frequency Selective Surfaces” (FSS). Their electrodynamic performance depends not only on the geometric dimensions and physical properties of materials but also on the periodical spacing of their constituent elements. In the case when their dimensions are much less than the wavelength of the incident radiation, they are called “metasurfaces”. This chapter analyzes the effect of dimensions and electrophysical properties of structural materials on the transfer characteristics of frequency-selective surfaces with crossshaped apertures. 

In modern microwave and terahertz technology, one of the most hot topic areas of using frequency selective surfaces is frequency filtering of electromagnetic signals. Such structures make it possible to reduce the weight and dimension parameters of the radio equipment, which is especially important for onboard systems of aviation and space technology operating under harsh environment, which imposes high-reliability requirements on the units and components of these systems. One of the trends in the development of filtering devices of various types and different element bases has recently become the design of broadband and ultra-wideband, as well as narrowband and ultra-narrowband systems.

Freestanding frequency selective surfaces with cross-shaped aperture elements are investigated numerically and experimentally in the present chapter. Transmission characteristics of such two-dimensional periodic structures as a function of sizes are analyzed using numerical methods.Employing the conjugate-gradient method narrowband filters have been developed and optimized. The results of measurements carried out in the present study agree well with the obtained theoretical data.

Symmetrical reactive bipolar networks, which include microwave filters, constitute the most commonly used class of passive electrical circuits in practice. This chapter develops analytical models of these circuits in relation to their implementation on the basis of multi-wire systems of coupled microstrip lines. Such systems are characterized by the inequality of the residue mode phase velocities, which is due to the inhomogeneity of the substrate-air medium. The substitution networks for this type of symmetric network are defined as an equivalent mode representation, which forms the basis for the analysis and synthesis of high-selective filters. The operating principle of these filters is built upon the inequality effect of residue modes phase velocities. The relationship between the amplitude-frequency response and the group delay time of symmetric networks of both minimum phase and non-minimum phase types has been revealed.

Circuit engineering methods for increasing frequency selectivity of microstrip filters based on half-wave co-directional hairpin resonators are studied, the principle of operation which is based on the rational use of the effect of inequality of normal wave phase velocities. It is shown that in these structures with N resonators, it is possible to form N + 1 operating attenuation poles at finite frequencies without introducing additional electromagnetic couplings between non-adjacent resonators. The conditions for splitting the operating attenuation poles in compact multiply coupled structures are determined. The results of experimental studies of this type of highly selective filter are presented. 

In the structures considered in the previous chapter, a limited number of operating attenuation poles (transmission zeros) P ≤ N + 1 is formed, which is a limiting factor for a further increase of filter selectivity using resonators with a relatively low intrinsic Q factor. This is due to the structural construction of these circuits and, as it seems to us, an insufficient number of degrees of freedom in the independent control of the frequency properties of partial two-pole impedances Za and Zb of their equivalent circuits. This chapter explores new properties of single- and dual-resonator MSF structures, in which up to six or more operating attenuation poles can be directly formed at finite frequencies, i.e., these structures with an extremely minimal number of resonators sometimes demonstrate the properties of multi-resonator microwave circuits.

The principles of constructing high-order microstrip filters based on single- and dualresonator highly selective sections are being developed. It is shown that a significant increase of microstrip structure selectivity with a minimum number of resonators can be achieved by using the method of "reverting" of the transmission zeros, the effect of combining the main and parasitic passbands, as well as the galvanic connection of the sections instead of their cascade connection.