The Structure Design of a Microstrip Tunable Attenuator Based on Few-Layer Graphene

There are innovative applications for graphene materials in microwave Radio Frequency (RF) due to their unique optoelectronic properties. In this paper, a series of existing tunable microstrip attenuators based on few-layer graphene are compared and studied, and a new microstrip tunable attenuator based on few-layer graphene is proposed on this basis. Firstly, the relationship between the performance of different pair spacing attenuators with 4 pairs of graphene structures is analyzed based on the existing pair spacing studies with 1-3 pairs of structures; after that, the performance characteristics of the existing 1-4 pairs of graphene structures on different substrates are tested and analyzed; finally, a new tunable attenuator structure is proposed based on the above research results. The new attenuator consists of four parts, including a dielectric substrate, a microstrip line, a total of five pairs of graphene flakes on both sides and corresponding metal vias. The resistivity of the graphene flake can be changed by changing the access bias voltage, thus regulating the insertion loss of the attenuator. The experimental results show that the proposed attenuator can operate in the frequency range from 3 GHz to 10 GHz, and the insertion loss can be adjusted from -2 dB to -63 dB, with a high tunable performance of 61 dB.


Introduction
With the advancement of material research, nanomaterials are gradually applied to various industries.Among them, graphene has attracted the attention of many researchers due to its unique electrical, optical, and mechanical properties [1][2].As the technology of graphene large-scale preparation has become more mature over the years, its application in microwave devices also has a richer scene [3].The tunability of graphene under the influence of bias voltage makes it promising for innovative applications in microwave and radio frequency [4][5][6].
The attenuator is one of the key devices in the phased array system, which is mostly used for gain adjustment and damage protection [7][8].In order to adapt to the changing use scenarios, the dynamically adjustable attenuator is being gradually studied as an improved power control device [9].It provides the appropriate input signal or electric field strength according to the situation [10].Based on research on graphene materials, it has corresponding applications in attenuator design [11][12].A graphene-based tunable attenuator is proposed after that, its structure consists of two microstrip lines with a few layers of graphene flakes inserted in the middle [13].Applying bias voltage at the two ports causes the resistivity of the graphene flakes to change, thus affecting the overall insertion loss of the attenuator.However, this design solution provides a relatively limited attenuation tuning range and available bandwidth.
In contrast, the structure used in the subsequent paper consists of a microstrip line with a pair of graphene sheets and a pair of metal grounding holes [14].The insertion loss adjustment depends on the bias voltage input to the port and ground.This structure provides a high tunable range and a relatively wide available bandwidth.A new structure with 2-pairs of graphene is proposed in other studies [15].This structure increases the number of graphene flakes and the corresponding metallic grounded vias based on the previous one, further enhancing the adjustable range of attenuation of the attenuator.The structures with 3-pairs and 4-pairs of graphene proposed increase the number of graphene flakes and metal vias on the attenuator [16].And the tuning performance of the attenuator is improved again.
At present, there is still room for further expansion and improvement of this type of structure.If it can have a larger tunable range of attenuation, it will be more flexible in practical use.At the same time, whether there is a certain rule between the electrical properties displayed by this type of structure and the selected substrate materials still needs to be further studied.
In the paper, the existing 1-4 pairs are tested and compared on four different dielectric constant substrates.By improving a series of previous models, the new 5-pairs of graphene flakes model proposed is shown in Fig. 1.We tested and studied this structure.This attenuator is a symmetrical structure consisting of a central microstrip line with 5 pairs of graphene flakes and corresponding 5 pairs of metal vias on both sides.The two ends of the microstrip line are the signal input port and output port respectively.This structure can further extend the tuning characteristics of the attenuator and improve the input-matching effect.Figure 1.The geometry of a tunable attenuator based on 5 pairs of graphene flakes.

Parameters of Attenuator Design
In this work, a comparative test study on the performance of attenuators with different dielectric constant substrates is conducted.According to the research results, the choice of substrate is determined and a new attenuator structure is proposed.The newly proposed attenuator structure consists of a microstrip line with 5 pairs of graphene flakes and the corresponding metal vias.The signal is transmitted from one port to another, and the resistivity of the graphene flakes is adjusted by varying the voltage applied between the input port and the ground plane, thus regulating the insertion loss of the attenuator.Compared with the attenuator structures in [14][15][16], this design further improves the tunable range and input matching performance of the attenuator by increasing the number of pairs of graphene flakes and metal vias to 5 pairs.
The attenuator model used in the comparison experiment on the substrates with different dielectric constants are from the 1-4 pairs of graphene flakes attenuator structures, and the four models mentioned therein will be simulated for comparison tests on four substrates with different dielectric constants respectively.The parameters of the four substrates used for testing and the width of microstrip lines on the four substrates are shown in Table 1.The distance between graphene pairs is always 5mm, and the graphene flake size is 1.4mm in length and 0.66mm in width.The width of the square metal patch is 1.4mm and the radius of the metal via is 0.4mm.
Based on the research on the spacing and insertion loss of 2-pairs of graphene flakes [17], the distance between the graphene flakes pairs in the structure with 3-pairs and the 4-pairs are further analyzed in this paper.The purpose is to explore whether the distance between graphene flakes has the same rule on various structures.The design simulations were carried out using RF-60 organic ceramic laminate substrates and microstrip lines with a width of 0.94 mm on the structures of 3-pairs and 4-pairs.The properties of graphene pairs with the spacing of 5 mm, 10 mm, and 15 mm were studied.The insertion loss at a center frequency of 5GHz and a graphene resistance of 20Ω was analyzed for three cases, and the results are shown in Fig. 2. In contrast, the insertion loss performance of 5mm graphene pair spacing is relatively excellent on 3 and 4 pairs of models.Combined with the research results of the paper [17], in the comparative test of the existing structure and the newly proposed attenuator structure, the electrical performance curve is the smoothest when the spacing between graphene pairs is 5mm.The newly proposed structure will also be designed based on 5mm graphene pairs spacing.The attenuator model is proposed by studying the structure of the substrate and attenuator.To facilitate comparison, the substrate is made of RF-60 organic ceramic laminate material, thickness h = 0.64mm, dielectric constant ε r = 6.15, loss tangent angle tan θ = 0.0028.In order to match the 50 Ω input and output impedance, the microstrip line width is 0.94mm.The parameters of the graphene flakes, square metal patch, and the corresponding visa are the same as those of the previous models.

Results of Substrate Comparison Experiments
The attenuator models of 1-4 pairs proposed in [13], [14], and [15] are compared and tested on different substrates, and the S21 curve is shown in Fig. 3.It can be seen that, for the attenuator structure with 1pair of graphene, the flatness of the S21 curve is relatively high for each substrate from DC to 10 GHz and slightly decreases between 5 GHz and 10 GHz for CER-10 and RF-35 substrates.However, given the relatively small tunability of this structure, a slight decrease in flatness can have a significant impact on the performance of the attenuator.Therefore, the available frequency range with the 1-pair graphene flakes structure is DC to 5 GHz on CER-10 and RF-35 substrates.On the attenuator structure with 2-4 pairs of graphene flakes, with the dielectric constant of the selected substrate, the attenuator will have a larger working bandwidth, and the smoothness of S21 will be better.However, on the RF-35 substrate, the center frequency shifts toward higher frequencies, and the S21 curve on the 4-pairs structure fluctuates to some extent in the operating frequency range.It should be because on the substrate with a lower dielectric constant, the signal transmission speed is faster and the frequency is higher.Based on the simulation results of insertion loss on various substrates on various structures, the comparison results shown in Table 2 can be obtained.It can be seen that the working frequency range of the attenuator is greatly affected by the substrate.On the whole, the working frequency range of the attenuator increases gradually with the decrease of the dielectric constant of the substrate.However, when the dielectric constant of the substrate decreases to a certain extent, the S21 curve will produce distortion in the operating frequency range.It is because the impedance mismatch of the system increases with the increase in signal frequency.High-frequency resonance will occur after reaching a certain critical value.Obviously, the smoothness of the attenuator S21 curve and the attenuation tunability of each structure on the RF-60 substrate are better, the working frequency range of the attenuator is wider, and the center frequency is closer to 5GHz.

Results of Five-Pairs Attenuator
In this section, the novel proposed a 5-pair structure is studied and tested for six graphene resistance values of 20Ω, 40Ω, 200Ω, 400Ω, 800Ω, and 1200Ω.The results are shown in Fig. 4. The operating frequency range is from 3GHz to 10GHz, the minimum insertion loss is -2dB at 1200Ω graphene resistance, and the maximum insertion loss is -63dB at 20Ω graphene resistance.Using RF-60 organic ceramic laminate as the substrate material, the new attenuator structure proposed in this paper is compared with various existing structures in parallel.The maximum tunable range of each attenuator is shown in Fig. 5, while the characteristics of each attenuator on the RF-60 substrate are shown in Table 3.The minimum insertion loss is the value when the graphene resistance is 1200 Ω, and the maximum insertion loss is the value when the graphene resistance is 20 Ω.It can be seen that the newly proposed 5-pair model has higher tunability in the frequency range of 3-10 GHz when the substrate materials are all RF-60 organic ceramic laminates.Combined with the comparison test results, it can be found that as the number of graphene flakes and corresponding metal pairs increases, the tunable amounts of input matching and insertion loss are improved.The newly proposed attenuator structure further enhances the tunable range compared to the attenuator with 1-4 pairs of graphene flakes.At the same time, the increase in the number of pairs of graphene also makes the input and output matching effect further improved, and the maximum return loss can reach -49dB.
Figure 5.The maximum tunable range for different attenuators on RF-60 organic ceramic layer substrates.

Conclusion
In this work, the effect of substrate on the performance of a graphene tunable attenuator is analyzed.From the experimental results, it can be seen that the RF-60 substrate has the best performance for each attenuator with a central frequency of 5 GHz.A new tunable microstrip attenuator structure based on few-layer graphene flakes is proposed.It is an improvement on the existing 1-4 pairs of models.The model is composed of a matched 50 Ω microstrip line and 5 pairs of graphene flakes with the corresponding grounding through vias.The maximum attenuation and tunability of the attenuator can be further improved by increasing the number of graphene flakes and the corresponding grounding through vias.According to the results of the test, the new attenuator can operate in the frequency band from 3GHz to 10GHz with insertion loss tuning range from -2 dB to -63 dB.Compared with the existing 1-4 pairs of graphene attenuators, the proposed attenuator has a higher tunable performance and more graphene pairs to further improve the input matching.Therefore, the proposed attenuator has greater potential in various applications and can be applied to scenarios with richer requirements for variable attenuation amounts.

Figure 2 .
The maximum insertion loss of different graphene pairs spacing when the resistance of graphene on RF-60 substrate is 20Ω:(a) 3-pairs; (b) 4-pairs.

Table 1 .
Substrate parameters and microstrip line width for different substrates.

Table 2 .
Comparison of available frequency ranges on different substrates with different structures.

Table 3 .
Comparison of simulation data with different attenuator models on rf-60 organic ceramic.