Simulation and discussion of typical radio frequency filters

The demand for filters is increasing as the frequency bands for 5G expand. RF filters become the segment with the largest market share of RF front-end chips. More crowded frequency bands, as well as thinner and lighter communication equipment, necessitate higher filter performance. In this paper, several technologies for the implementation of Radio Frequency filter are presented. Essentially three types are considered: (i) Lumped Element (LC) Filter and Integrated Passive Devices (IPD) technology; (ii) Acoustic Wave Filter; and (iii) a composite filter design that combines acoustic and IPD technologies. Benefits and drawbacks of these diverse configurations in terms of their technical performance and current applications are examined, and several models are presented and simulated using ADS software.


Introduction
Such as LTE, LTE advanced and Fifth generation (5G), new wireless applications utilize several bands of radio frequency to instantly assign the bandwidth required to enhance data rate.In particular, as a subsequent network to 4th generation, 5G networks are anticipated to have a hundred times more reduced delay and nearly a hundred times greater telecommunication speeds compared to existing 4th generation mobile communication technology, in order to achieve the data efficiency and speed objectives.The rigorous low loss requirements in millimetre-wave frequency bands, in addition to the precise impedance configurations featuring reduced thick and size, constitute significantly problematical issues of the design on radio frequency fore end modules [1].As among the most critical constituents in 5G radio frequency front-end modules (FEM), the demand for higher-frequency filters like BAW, IPD and LTCC grows.
RF filters are used to reject or allow transmissions in certain sections of the radio spectrum.Although filter construction differs according on the application, with size, cost, and performance being the most important factors, filters used for RF transmissions are typically Bandpass filters designed with coupled resonator techniques.Discrete inductor-capacitor (LC) filters or lumped element filters are perhaps the most common of all electromagnetic filter types with low-cost structures of moderate performance and size.The LC elements are sometimes implemented as printed structures on substrates, which is referred to as an integrated passive device (IPD) [2].Acoustic filters, namely bulk acoustic wave (BAW) and surface acoustic wave (SAW), are another popular filter configuration for mobile devices.Some of the parameters used to determine whether filters fit the different requirements include the rapidest transition to the final roll-off, in-band ripple, and the biggest out-of-band rejection.SAW and BAW filters possess a high Q-value and low insertion losses, as well as outstanding band selectivity.Therefore, by combining LC and acoustic technologies, it is possible to achieve high frequency, wide bandwidth, low insertion loss, as well as steep roll-off all in the meantime.

Lumped Element (LC) Filter
For 5G networks, there are a total of 29 frequency bands divided into two primary spectrum ranges, with a total of 26 bands below 6GHz (collectively known as Sub-6GHz) and 3 mm-Wave bands.Sub-6GHz is the primary band now in use, and it contains seven bands, which are n1, n3, n28, n41, n77, n78, and n79 [3].To meet the requirements of wireless communications systems, passive modules in the 2.4 GHz band are commonly used on the RF front-end of electrical devices.LC filters are applicable from less than 100 kHz to a little over 3 GHz.They have a low insertion essentially while the switch from its passband to rejection is abrupt.Such benefits make them popular for years.

Schematic Design of RF Bandpass LC Filter
The modern network synthesis method for constructing microwave filters is based on a low-pass prototype filter with integrated components.As shown on the Figure 1, this prototype is used to derive the transmission properties of various bandpass, lowpass, highpass, and band-reject filters [4].To demonstrate the design (Figure 2), Agilent Advanced Design System (ADS) 2020 software is used to create a three-order filter with a centre frequency of 2.45GHz and a bandwidth of 1GHz.After calculation and simulation, the results bellowed satisfy the criteria:   S-Parameter simulation can signify reflection and transmission properties (amplitude and phase) in the frequency domain.Reflection coefficients S11 or S22 represent return loss, admittance, and impedance, while transmission parameters S12 or S21 show insertion loss, phase, and group delay.According to Figure 3 and Table 1, the simulation results reveal that the bandpass filter exhibits return losses of less than 15 dB across the required band of 2.35-2.55GHz and insertion losses of about 0.1 dB across the pass-band, which illustrates that the proposed filter fulfil the expectation.

Integrated Passive Device (IPD) Technology in RF Filter
Statistically as the function of mobile phone gets more powerful, more passive components need to be used support the operation, which will result in increased product volume.Therefore, the development of multifunctional and compact microwave circuits with reduced power loss is becoming an unavoidable trend.To satisfy rising demand, reduce size and cost, and boost functionality, integrated passive device (IPD) technology has emerged as a feasible option for RF front-end design.
IPDs (integrated passive devices), sometimes known as EPC (embedded passive components), or IPCs (integrated passive components), are IPD are electronic components that incorporate resistors (R), capacitors (C), inductors (L), baluns, impedance matching elements, microstrip lines, or any combination of these components in the same package or on the same substrate [5].While IPD Technology is the high density integration of multiple passive components by etching diverse patterns on silicon, glass, or ceramic substrates utilizing a photolithographic Wafer fabrication technique, can replace bulky discrete passive components.It is classified as Thick-film or Thin-film depending on the passive device fabrication technique [6].LTCC (Low Temperature Co-fired Ceramics) is a branch of the Thick-film technologies which are widespread and relatively low cost fabrication method of passive components.In literature [7], A miniature dual mode bandpass filter with 7th order harmonics suppression for 5G N77-Band applications is proposed.As shown in Figure 4, it is manufactured in a two-layer LTCC substrate, with the centre frequency of 3.75 GHz plus the bandwidth of 0.9 GHz.The total size of the chip is only 7*7*0.3 mm^3.The use of the meandering SIVFL (stepped-impedance-  .As the technology is racing ahead, the millimetre-scale Thick-film technology is miniaturized into micron-scale Thin-film technology, which can reduce the size of passive systems by a factor of 1,000 while also drastically lowering the cost.Nickel chromium sputtering on an alumina substrate is one method of producing thin-film chips.Additional strategies are established on the usage of chromium silicide and tantalum nitride on silicon [8].For IPD-based filter, one of the factors influencing the electrical performance is the substrate material chosen.The substrate for IPDs can be ceramic (alumina), glass, or semiconductor like silicon and GaAs (Gallium Arsenide).

Silicon IPD Technology
In literature [9], a highly selective and compact bandpass filter (BPF) in the 5G communication band derived from high-resistivity silicon (HRS) passive integrated device technology is proposed.For existing silicon integrated circuit technology, the Q-factor of passive components is primarily restricted by metal line resistive attenuation and substrate loss [10,11].To attain great performance, HRS IPD technology combines a high-resistivity silicon substrate with nominally 3000-Ω⋅cm resistivity or even higher, and high-conductivity metallic copper.As a result, it has the following advantages: low loss, high quality factor, inexpensive, easy integration, small in volume, and suitable for integrated circuit fabrication process.The structure is as demonstrated in Figure 5.With the size of 2*1.25*0.1 mm^3 and upper limit insertion loss lower than 1.76 dB, both miniaturization and high selectivity have been realized in the filter.

GaAs IPD Technology
GaAs can be used as semi-insulating high resistance materials for integrated circuit substrates.Gallium arsenide (GaAs) semiconductor devices possess the benefits of high frequency, good performance in low temperature, less noise, and strong radiation resistance.Si-based chips are produced by a physical etching process, whereas GaAs chips usually go through a multi-layer chemical stacking procedure.A significant strength of the mentioned GaAs IPD technology over silicon-based technology is its greater unloaded Q-factor of lumped elements.Because of the semi-insulated properties, the inductor's parasitic capacitance to ground is greatly reduced, boosting the quality factor and resonance frequency at high frequencies, while the filter based on silicon-based IPD technology has a larger insertion loss due to the higher substrate loss.Besides, the thickness of GaAs substrate is another critical factor in optimising inductor performance and size [12].One design of GaAs IPD substrate is shown bellowed (Figure 7).

Glass IPD Technology
Glass substrate is an ideal choice for integrated passive device due to its material properties.As an insulator, glass is generally chosen in preference to other materials for advanced packaging owing to its low electrical loss and ultra-high resistivity especially at high frequency [14].To further utilize properties of glass for tiny substrates, precision or Through Glass Vias (TGVs) are required.TGVs are typically produced in two sequential process stages.To begin, a slew of micron-sized holes are drilled through the glass.The previously drilled micro holes are filled with a metal of choice in the second stage.According to [15], a metal-insulator-metal (MIM) structure was used to obtain high-Q capacitance, therefore making improved diplexers and broadband filters.Based on the result of [16], Through Glass Via is estimated to considerably increase three dimensional integrated circuits (3D-IC) performance since it has lower parasitic than Through silicon via (TSV).
In literature [17], silicon, glass and LTCC are used as the substrate of same circuits.Then three types of materials are compared in terms of electrical performance and cost.According to the findings, silicon and glass based IPDs are smaller in size and less expensive in price with equivalent electrical performance compared with LTCC technology.

Acoustic Wave Filter
Acoustic waves are classified into three frequency bands: audio, infrasonic, and ultrasonic.For RF frontend components, ultrasonic can be used to transmit signal information.Acoustic filter technologies are evolving in response to the global transition to 5G networks.Surface acoustic wave (SAW) and bulk acoustic wave (BAW) are two common types for mobile phone applications.

Surface acoustic waves (SAW)
Surface acoustic waves (SAW) are bouncy waves that are formed and propagate transversely on the surface of a piezoelectric substrate material, and their amplitude drops fast as penetration into the substrate material increases.As an illustration, Figure 8 below shows the structure of a basic SAW filter which is made up of a piezoelectric substrate and two Interdigital Transducers (IDTs).The IDT on the input side converts the electrical signal into an acoustic signal, while the output side IDT transforms the acoustic wave back into an electrical signal.The wavelength of the electrodes determines the SAW filter's operating frequency.The essence is to combine piezoelectric materials such as quartz crystal and piezoelectric ceramics to create a specific filtering mechanism that takes advantage of its piezoelectric effect and the physical properties of surface acoustic wave propagation.[18].SAWs and temperature-controlled SAWs (TC-SAW) have long been popular because the technology is mature and widespread enough to keep component costs low.The present success of SAW filters is also related to their compact size, and high stopband rejection.They are widely used in receiver front ends, duplexers, and receive filters in 2G, 3G and 4G networks.The most widely used substrate materials of modern RF SAW filter are Lithium niobate (LiNbO3) and Lithium tantalate (LiTaO3).But the usage of devices based on these materials is limited due to decline in performance above 2.7GHz.In addition to the substrate material, the thickness of the substrate also affects the performance of the filter.In literature [19], the results reveal that thinner substrate is beneficial to enhancing the driving sensitivity of the IDT.When electrode width to substrate thickness ratio is equal to 0.33, the piezoelectric driving performance is the best.Furthermore, above 1GHz, the selectivity of SAW filter declines.SAW also has poor thermal stability.The response and centre frequency of SAW filter will decrease sharply as the temperature rises.Therefore, TC-SAW is designed to improve the conventional SAW filter by covering its IDT with a temperature compensation layer.As temperature compensation, a thin film of Silicon Dioxide (SiO2) helps to reduce temperature-dependent filter frequency drift.

Bulk acoustic wave (BAW)
In a BAW filter, acoustic wave which is excited by metal patches on top and bottom sides of the quartz, propagates longitudinally and bounces up and down to generate a standing acoustic wave.To design a bulk acoustic wave (BAW) filter, multiple resonators are coupled together in a certain topology to form a passband, usually in a ladder configuration, which is set up using series and parallel resonators alternately.
Compared with SAW filter, BAW filter performs better at high frequencies.According to the frequency equation of SAW filter: f = v λ ⁄ , where λ refers to the spacing between the IDT electrodes.Since λ cannot be too small, SAW filter is not suitable for higher frequency band, while BAW filter can operate at high frequency up to 20 GHz.On account of the resonant frequency is inversely proportional to the thickness of the film, the size of BAW filter decreases with increasing frequency.Furthermore, high quality factor (Q<2000), very low loss and very steep filter skirts are typical advantages of BAW filter.However, as BAW filter is more elaborate and more expensive, both BAW and SAW will co-exist in the long term, playing to their respective strengths in the high frequency or low frequency.
BAW filters include FBAR (film bulk acoustic resonator) and BAW-SMR (solidly-mounted resonator BAW) devices.BAW-SMR filter forms a Bragg reflector by stacking thin layers of different stiffness and density to keep the wave to oscillate inside piezoelectric film, while FBAR use airgap or membrane to achieve same purpose.In literature [20], author analyses several sources of Q loss.They are energy leaking from the perimeter, thermal-elastic, Ohmic Losses and so on.

Hybrid Filter with lumped-LC and acoustic technology
The q-factors of lumped LC filters and transmission line filter designs are usually insufficient to replace acoustic filters, so they are still employed in coupled with SAW/BAW in practical application.In order to make the best use of the strengths of both filters, a hybrid filter with high q-factor and wide bandwidth is proposed in literature [21].There are three major models for the simulation of BAW filters, which are the physical-based one-dimensional (1-D) Mason model, 2-D finite element method (FEM) model, and the equivalent circuit based Modified Butterworth-van-Dyke (MBVD) model [22].To verify the feasibility of the hybrid filter, ADS software is used to simulate a series circuit of acoustic and LC components.The schematic design is shown in Figure 9.The design inside red circles is equivalent circuit of acoustic part.As shown in Figure 10, the simulation results reveal that the out of band rejection is around 40 dB for the entire frequency range from 1.0 to 10.0 GHz.The bandpass filter exhibits insertion losses of about 0.1 dB across the 860MHz passband, which illustrates that the proposed filter fulfils the expectation.
However, owing to the coexistence of several assorted components, it is not easy to compound them together into a individual component unit and in the same process.In literature [23], the innovative SiCer compound substrate technology is introduced.As depicted in Figure 11, Through-Silicon Vias (TSV) are employed in the silicon layer and can be piled up with vias in LTCC, which enables the creation of a small RF-system-in-package (RF-SiP) with RF-MEMS and embedded passive or active components.

Conclusion
For 5G New Radio (NR) wireless communications, there has been a general and unavoidable tendency toward using higher frequency and greater bandwidth of the electromagnetic spectrum.The size of the filter will continue to decrease.However, the choice of filter is based on the final purpose, making a trade-off between cost and performance requirements.This study presents several techniques for implementing RF filters, compares and analyses their advantages and limitations.
Nonetheless, due to equipment and time constraints, the filter models presented in this paper are ideal circuit layouts and the materials and packaging methods used have not been simulated, nor have they been tested and optimised in kind.Future research can be directed in the following directions based on the overview of the preceding work.
In the aspect of the labour value of filters used in the RF front end, since each band requires a distinct filter to support it, the use of filters and the proportion of the labour value of filters in the RF front end increases as the number of frequency bands grows.Filters and other RF devices will see a trend toward miniaturisation, improved device shapes, and composite type in the future.
Advanced packaging solutions for RF devices will continue to innovate, such as tighter component layouts, Conformal shielding and Compartment shielding, double-sides PCBs, and high-precision and high-speed surface mount technology (SMT).

Figure 2 .
Figure 2. Schematic design of RF bandpass filter.

Figure 3 .
Figure 3. S-parameter graph of providing model.

Figure 4 .
Figure 4. 3D structure of the LTCC-based BPF[7].As the technology is racing ahead, the millimetre-scale Thick-film technology is miniaturized into micron-scale Thin-film technology, which can reduce the size of passive systems by a factor of 1,000 while also drastically lowering the cost.Nickel chromium sputtering on an alumina substrate is one method of producing thin-film chips.Additional strategies are established on the usage of chromium silicide and tantalum nitride on silicon[8].For IPD-based filter, one of the factors influencing the electrical performance is the substrate material chosen.The substrate for IPDs can be ceramic (alumina), glass, or semiconductor like silicon and GaAs (Gallium Arsenide).

Figure 8 .
Figure 8. Basic structure of SAW[18].SAWs and temperature-controlled SAWs (TC-SAW) have long been popular because the technology is mature and widespread enough to keep component costs low.The present success of SAW filters is also related to their compact size, and high stopband rejection.They are widely used in receiver front ends, duplexers, and receive filters in 2G, 3G and 4G networks.The most widely used substrate materials of modern RF SAW filter are Lithium niobate (LiNbO3) and Lithium tantalate (LiTaO3).But the usage of devices based on these materials is limited due to decline in performance above 2.7GHz.In addition to the substrate material, the thickness of the substrate also affects the performance of the filter.In literature[19], the results reveal that thinner substrate is beneficial to enhancing the driving sensitivity of the IDT.When electrode width to substrate thickness ratio is equal to 0.33, the piezoelectric driving performance is the best.Furthermore, above 1GHz, the selectivity of SAW filter declines.SAW also has poor thermal stability.The response and centre frequency of SAW filter will decrease sharply as the temperature rises.Therefore, TC-SAW is designed to improve the conventional SAW filter by covering its IDT with a temperature compensation layer.As temperature compensation, a thin film of Silicon Dioxide (SiO2) helps to reduce temperature-dependent filter frequency drift.

Figure 9 .
Figure 9. schematic design of hybrid filter.

Figure 10 .
Figure 10.simulation result of designed model.

Table 1 .
Electrical specifications of LC BPF.
variable feeding lines) and oblong resonator results in over 25 dB suppression up to 7th order harmonics as well as decline in size.