For the design of filters (band pass, notch, low pass and high pass), microstrip technology is widely used due to its small footprint, good compatibility with other external circuits, low cost of manufacture and good reproducibility. However, this technology does not make it possible to obtain resonators with high quality coefficients. In addition, it is an open structure where the problems of sensitivity to the environment are not negligible. To overcome these problems, we can move towards stripline solutions or waveguides for which electromagnetic radiation problems no longer exist, and which allow the latter to obtain good quality factors. In addition, waveguides tolerate high power signals. They are unfortunately very bulky and their integration with other planar circuits is not easy. There are then SIW structures which achieve a compromise between volume and planar structures. It consists in making rectangular waveguides in a dielectric substrate according to planar manufacturing processes. there is then a reduction in manufacturing cost and good compatibility with other circuits while maintaining a high quality factor.
Elliptika therefore has technologies and techniques for producing microwave filters. In Figure I you can compare the different filter technologies in terms of size, insertion loss and associated quality factor.
Figure n ° 1 Comparison of the performance of filter technologies according to their size and cost.
Planar technology is widely used in the field of microwave filters because it allows a small size and great topological flexibility. Indeed, the manufacture of the circuits is good and has a low manufacturing cost. This technology consists of a dielectric substrate metallized on one or two sides. Several technological solutions can be found: microstrip, coplanar, multilayer or triplate.
Cette technologie est composée d’un ruban métallique situé sur une face du substrat et d’un plan de masse, situé sur l’autre face. The fundamental propagation mode of such a structure is a quasi TEM mode (figure 2). From this technology we can realize band pass filter, notch filter, low pass filter and high pass filter in printed circuit board technology, the advantage of which is the reduced cost (figure n ° 3a). We also have ceramic solutions with high permittivity in order to greatly reduce the bulk size (Figure 3b).
Figure n°2. Microstrip technology
multilayer technology has a reference plane, a dielectric substrate as a support. It is then a question of working on the partial or total superposition of the conductive lines through dielectric layers.. It is a question of associating various solutions of planar propagations (cf figure n ° 4). On the other hand, the complexity is transferred to the level of 3D circuit modeling. The combination of different 3D configuration with dielectric makes it possible to move towards a multi-layer, multi-technology integration solution. In our case we use the solder mask as a dielectric. It allows more complex structures to be produced that meet tough electrical specifications without increasing the cost of production.
We present here an example of a varnish-based multilayer filter. The first dielectric support used is a Rogers 4350B substrate with a permittivity 3.66 and Tanδ: 0.003. The deposited solder mask, initially intended to protect the PCB, is therefore used here as a dielectric layer between the outer and inner metal layers, it has a thickness of 30 µm, a relative dielectric permittivity of 5.4 and its tanδ of 0.02. Thanks to the low thickness between layers, it allows strong couplings to be achieved and thus achieves a very wide bandpass filter.
Figure n ° 4: broadband band pass filter (3 – 12 GHz) made from a solder mask-multilayer technology
Volume technology consists of using a resonance mode located inside a cavity, which acts as a resonator. Electromagnetic waves resonate in several modes. Each resonance mode is characterized by a particular arrangement of the electric and magnetic fields, as well as by a resonant frequency. The TE and / or TM modes are the most used in waveguide or dielectric resonator filters; while the TEM mode is preferred for filters with coaxial air or dielectric cavities. The cavity technology has good electrical performance thanks to the very high no-load quality coefficients of the cavity resonators. This technology is therefore widely used in narrowband filters in order to obtain the minimum of insertion losses and very selective rejection. On the other hand, the size and the manufacturing cost can be the main drawbacks of cavity filters.
Waveguide filters are made with rectangular or circular metal air cavities. The resonant frequency of cavities is defined from their shape and size. The coupling between the different cavities is generally of the magnetic type, it is carried out with irises. Les modes de résonances dans les cavités sont les modes TE et TM.
Figure n ° 5: 4th order bandpass filter (center frequency 16 GHz) in waveguide technology
In 2001, D. Deslandes and L. WU developed a new waveguide technique using TEm0 volume propagation modes within a substrate, while preserving planar production methods. The principle of SIW (Substrate Integrated Waveguide) technology is therefore to produce a waveguide in a dielectric substrate, the two faces of which are metallized to form the upper and lower metal walls. Rows of metallized holes are used to form the side walls of the cavity (Figure 6). This technique therefore makes it possible to considerably improve the quality factor compared to planar resonators to result in filters with low insertion losses. However in the VHF and UHF band, despite the integration of the cavities in a dielectric substrate, the latter remain bulky.
Figure n ° 6 SIW band pass filter in Ku band (16 GHz) of order 4 (Alumina) tunable from PIN diodes
Coaxial resonator filters are based on coaxial cavities with a fundamental TEM type propagation mode. Coaxial resonators provide a compromise between lumped element filters and waveguide filters. They have good quality factors of between 200 and 5000, for dimensions much smaller than those of traditional waveguides. The design of this kind of filters is well mastered, however manufacturers and researchers have carried out several studies to improve the performances of coaxial resonators in terms of size, and reduction of manufacturing costs by using new manufacturing and manufacturing techniques.
Uniform coaxial air resonator filters are the most commonly used. They consist of short-circuited λ / 4 resonators. Les filtres à bande étroite sont réalisés avec des cavités coaxiales, les résonateurs sont couplés entre eux soit magnétiquement à travers des iris de couplage ou électriquement avec des tiges métalliques (figure n°7) . The combline and interdigital structures are the most used for wideband filters. The magnetic coupling between the resonators is in this case carried out by proximity. The input and output excitations are carried out with coaxial probes. In order to reduce the size of filters and improve their performance in terms of parasitic frequencies, resonators are generally capacitively charged “on the open circuit side”. Here is an example of a bandpass filter with coaxial resonators dedicated to 5G
Figure n ° 7 Bandpass filter with coaxial resonators of bandwidth 3.3-3.7 GHz
The principle of impedance jump is mainly used in resonators in TEM or quasi-TEM mode. Uniform impedance resonators have the disadvantages of higher harmonics which, in the design of microwave filters, are considered as parasitic returns. To overcome the problem of length in VHF band, it is common to load the resonator with capacitors, which results in shortening the physical length of the resonator. This method is limited in frequency, and above all the losses associated with the capacitors increase rapidly beyond 1 GHz. One solution is to use step-change resonators. The stepped impedance resonator (SIR) is composed of two or more transmission lines of different characteristic impedances Zi and electrical lengths θi. It is terminated with a short circuit or an open circuit. Figure 8 shows a SIR filter structure. A judicious choice of the characteristic impedances of the different sections makes it possible to reduce the resonant frequency of the fundamental mode while moving the harmonics away, or to bring the two together. There are therefore two interests, the miniaturization of the filter, or the distance from the first harmonic.
Figure n ° 8 Band pass filter with coaxial resonators dedicated to PMR networks (430 MHz)