Synchronously tunable combined four-channel ferrite filter

The original design of the synchronously tunable combined four-channel ferrite filter is given. It is a functional unit consisting of four microwave systems of ferrite filters with coupling loops (one such microwave system in each channel) and a common magnetic system for these filters, which ensures their synchronous tuning in the frequency range of 2-4 GHz. The structural solution of the microwave system in the form of a module allows one to obtain both a band-pass ferrite filter and a band-reject ferrite filter. The results of experimental research are provided. They showed that such a four-channel controllable ferrite filter has not only improved design and technological indicators but also a high decoupling between channels. It can be successfully used not only in measuring equipment but also in modern multi-channel microwave communication equipment and radar.


Introduction
Frequency-selective electrically controlled microwave devices are important elements of modern radio technical systems.They are widely used in various measuring and special radio equipment [1 -5] .
Among the wide variety of frequency-selective controlled devices used in modern radio technical complexes and systems, ferrite filters are among the most popular.For their manufacture, ferrite resonators (most often spherical) are used, which work on the effect of ferromagnetic resonance.Ferrite filters, which have found wide application, use iron-yttrium garnet (YIG), made in the form of highly polished spheres, which are magnetized to saturation with a magnetic field.Spheres made of single crystals, placed appro-priately in the transmission line, can be considered high-quality resonators connected to these lines and returnable by changing the external magnetic field.

The main part
Structurally, the four-channel ferrite filter is a functional unit consisting of four microwave ferrite filter systems with coupling loops (one such microwave system in each channel), which have a common magnetic system to ensure their synchronous tuning.The image of the design of a four-channel ferrite filter and its microwave systems single-and two-resonator ferrite filters is presented in Fig. 1.The structural solution of the microwave system in the form of a module allows to receive both BPF and BRF.This is done by replacing the communication elements, which is technologically simple.At the same time, the modular design of the microwave system of the ferrite filters provides free access to all its elements and adjustments, which greatly facilitates the testing and setting of such filters.Note that hyperboloid connectors are used in microwave systems [11] .The modular implementation of the microwave system increases the decoupling between channels in a four-channel ferrite filter and opens up wide possibilities for varying the electromagnetic parameters of all the filters that it consists of.Thus, to ensure the conjugate tuning of ferrite filters in combined devices, it is necessary to mechanically adjust the frequency of ferrite resonators used in different microwave systems.
preselector, amplifier, postselector, mixer and local oscillator, as well as a high speed of tuning these circuits in a wide frequency range when feeding the filter, and the local oscillator from a single source of control current.
The American companies Micro Lambda Wireless, Inc. and Omniyig, Inc. [6 -8]  The company Micro Lambda Wireless, Inc. positions its products for the completion of satellite communication systems for civil and military applications, broadband receiving systems, telecommunication infrastructures and conjugate receiving, and transmitting devices [7] .Filters can be application-oriented or equipped with an analog or digital driver for the coil of their magnetic systems.
Note that the main positions in the advertised lines of selective devices of the American firm Omniyig, Inc., in most cases, are not occupied by individual filters but by combinations of synchronously tunable devices, for example, structures with two-channel multi-cascade filters synchronized by tunable frequencies or filter/generator systems in the bandpass filter (BPF) or band-reject filter (BRF) [7] variants.

Actuality and purpose of publication
The improvement of the characteristics of the already created and the development of new ferrite devices continues even now, since the constant development of microwave technology puts forward new requirements and sets new challenges for developers.So, with a wide use of multi-channel receivers in modern radio-electronic systems, the issues of ensuring electromagnetic compatibility of both individual channels and systems as a whole become especially acute [9,10] .To solve some of these issues, it in the temperature range of 0 -+ 60 °C [3,8] .Currently, this task is solved using internal automatically regulated heating elements [12] .For local heating of ferrite resonators in microwave systems, one can use a selfregulating micro thermostat, shown in Fig. 2.This micro thermostat is characterized by simple manufacturing.The peculiarity of such a thermostat is that a posistor -thermoresistor with a positive temperature coefficient of resistance is simultaneously used as a heater and a regulating element.A combined magnetic system is used to form the magnetic field in the given four-channel ferrite filter.Such a magnetic system is shown in Fig. 3 and is formed by using a permanent magnet to create a reference magnetic field and an electromagnet tuning of the filter frequency [13] .It consists of E-shaped magnetic cores 1 and 2, which have pole tips 3 and 4, a permanent magnet 5, and coil 6. Holes 7 at the ends of the side rods are made for fixing microwave sys-tems 8 on the magnetic cores with screws 9. Magnetic core 2 is melted in the windows of magnetic core 1 so that two working gaps 10 are formed between the side rods of both magnetic cores, and between the ends of the rods of magnetic core 2 and the base of magnetic core 1 formed gap 11, which is greater than the working gap 10.Shortened middle rods of the magnetic cores 1 and 2, on which coil 6 is located, are butt-joined by means of a permanent magnet 5.This placement of the permanent magnet away from the working air gaps ensures the uniformity of the permanent magnetic field at the location of the ferrite resonators.The proposed design of the magnetic system allows the building of both multi-cascade ferrite filters and multi-channel combined filters.For example, when two modular microwave systems are installed in one of the working air gaps of a magnetic core, it is possible to obtain a two-channel filter (at the same time, the influence of temperature and changes in magnetic parameters are equally affected on all filter resonators), or, by connecting them in series, to obtain a four-resonator filter with increased selectivity.

Experimental results
An experimental research of the developed fourchannel ferrite filter was carried out.For this purpose, samples of the microwave system BPF and of the microwave system BRF were manufactured, as well as the above common magnetic system for them, which the metrological documentation for the device [14] ).It should be noted that the decoupling between channels in the given four-channel ferrite filter is at least 80 dB.

Conclusions
1.The design of a synchronously tunable combined four-channel ferrite filter is presented, consisting of four microwave ferrite filter systems with coupling loops (one microwave system in each channel) and a common magnetic system for these filters, which ensures their synchronous tuning in the frequency range 2-4 GHz.The structural solution of the microwave system in the form of a module allows us to obtain both band-pass filters and band-reject filters in a technologically simple way.
2. Due to the use of the original modular design of the microwave system and the design of the magnetic system, the four-channel ferrite filter has improved design and technological indicators and high decoupling between channels.
The main requirements for such devices are the availability of high electrical characteristics, compactness and simplicity of design, manufacturability in production, and low cost.Ferrite filters with electrical control of the frequency are used in the technique of measuring and controlling various parameters of microwave signals.One of the first applications of resonators on ferrite single crystals in measurement technology was the creation of microwave frequency meters.It is advisable to use both filters and heterodynes with electrical frequency control using ferrite single crystals in superheterodyne receivers.It is possible to ensure the necessary coupling of the resonant circuits of the have the largest range of ferrite filters among the main global manufacturers.Micro Lambda Wireless, Inc. currently offers the broadest product line of commercial and microwave filters in the 500 MHz -50 GHz frequency range.Different models of ferrite filters are mainly with the number of stages from two to eight.At the request of the customer, it is possible to supply filters of both commercial (0...+65 °C) and extended working temperature range (-54...+85 °C for military use).
It is known that during work, it is necessary to take into account possible changes in the parameters of controlled ferrite filters and especially temperature instabilities.The effect of temperature occurs, first of all, in a change in the resonant frequency of the filters, in this case, changes in insertion loss, bandwidth, and level of parasitic resonances are also possible.The temperature characteristics of controlled ferrite filters are determined mainly by the parameters of their ferrite resonators.The corresponding orientation of the ferrite spheres can reduce the change in resonance frequency to 8-12 MHz

Fig. 1 .Fig. 2 .Fig. 3 .
Fig. 1.Sample of a four-channel synchronously tunable ferrite filter: a -external view of the filter; b -external view of microwave systems of single-and two-resonator BPF and BRF The expanded uncertainty of filter attenuation measurement at a confidence level of 0,95 and a coverage factor of k = 2 (taking into account the assumption of the normality of the distribution law of the measurement result) does not exceed ± 2,2 dB.Of increased practical interest are exactly two-stage ferrite filters.The experimental data obtained during the research of three samples of twostage BPF and three samples of two-stage BRF are shown in Tables1 and 2, respectively.The samples of modular microwave systems of two-stage filters have overall dimensions of 56,6x26, 8x20,0 mm, and a weight of 70 g.

Table 1 .
Parameters of two-stage tunable ferrite BRF

Table 2 .
Parameters of two-stage tunable ferrite BRF