A Filtering Switch Made by an Improved Coupled Microstrip Line

Abstract

In this paper, we propose a new filtering switch with excellent working performance made using an optimized coupled microstrip line. Upon analyzing the RF (radio frequency) front-end’s system structure, the switching device was simplified to a diode, which was connected to the microstrip circuit we designed to become a filter switch with both filtering and shutdown functions. First, we obtained an equivalent schematic of this filtering switch based on the relevant microstrip line theory. This switch consists of two coupled microstrip circuits, parallel-coupled feed lines and coupled-line stub-load resonators (CLSs), and a PIN diode. Second, the operating principle is described by the switching of the operating states, with ideal shutdown performance in the off state and considerable selectivity and excellent out-of-band rejection performance in the filtered state. Finally, a prototype filtering switch with a center frequency of 0.8 GHz was designed and tested. After subsequent optimization and improvement, the simulation and test performance results were noticeably consistent, consequently verifying the performance requirements of this filtering switch in two operating states in the center frequency band.

1. Introduction

As enhancements in communication technology become increasingly mature, the working performance and integration necessities of RF (radio frequency) systems’ front-end devices are also increasing [1,2,3]. In modern communication systems, an RF (radio frequency) system’s front end is usually composed of filters and switches designed separately and then cascaded. This cascading method will cause matching distortion, which undoubtedly increases the production cost and design difficulty of the RF (radio frequency) system’s front-end circuit structure. The solution to this problem is to design an integrated circuit with the required performance, i.e., a filtering switch with both filtering and shutdown functions [4].

Figure 1 depicts our idea for the RF (radio frequency) front-end system, namely, integrating the filter and switch modules in the RF (radio frequency) front-end system to form a filtering switch. The filtering switch is a microwave-integrated circuit, which can change the circuit’s working state by changing the external diode’s on/off state. This design method aims to improve the circuit’s integration, avoid the matching distortion of the filter and switch cascade in the RF (radio frequency) front end, and reduce the integrated circuit’s design cost. There are different types of filtering switches, such as those designed with external PIN diodes for harmonic filter switching [5], those designed with cavity-coupled bandpass filters for switching between filter and switch functions [6], and those with an integrated divider and filter as one circuit [7,8]. There are filter switches that are integrated after designing a triplexer using the aggregate component theory [1] and integrated filter switches based on the fractal stub-loaded resonator (F-SLR) design [9]. In our paper, we combine a theoretical analysis of a coupled microstrip line and PCB fabrication process to design and produce a filtering switch with two functions of being switched between filter and off.

Previously, many researchers have labored on the design and commissioning of filtering switches. The filtering switch proposed in [10] has a compact size and clear passband selectivity; however, it has a relatively high insertion loss (IL) in pass-state operation and less than optimal filtering operating performance. Although the view that the transmission zeros (TZs) generated by the filtering switch enhance the filtering switch’s through-state selectivity to some extent was introduced in [11], the selectivity impact exhibited by using the filter switch in the filtered state is still relatively unsatisfactory. Although the filtering switch mentioned in [2] exhibits excellent shutdown rejection, it is not the best for use in the filter function, and while the filtering switch designed in [12] has excellent turn-off rejection performance and excessive integration, its relative bandwidth (FBW) is only 8.96%, with a connection insertion loss of 1.72 dB, which indicates an unsatisfactory band pass filtering effect.

To avoid the above filtering switch problems and to fill knowledge gaps in the related fields, in this paper, we combine an improved filter response theory of coupled microstrip lines [13] with the basic design techniques of filtering switches [14,15,16]. We also studied related research in similar fields, such as MEMS switch-based filters [17], ferrite resonant filters [18], and enhanced gain in waveguide antennas [19]. Based on these theoretical approaches, we propose a filtering switch designed according to the filtering effect of coupled microstrip lines.

Our design idea is to choose parallel-coupled feeder lines and coupled-line stub-load resonators (CLSs). At a wider operating bandwidth, the parallel-coupled feeder lines can reduce the insertion loss (IL) and return loss (RL), and the coupled-line stub-load resonators (CLSs) can produce symmetrically controllable transmission zeros (TZs).

Compared to the design idea of using aggregate elements in [1], the design using these two coupled microstrip lines is easier to implement. There is no need to consider the specific length setting of the short route, and the design is less difficult. Compared to the design using a fractal stub-loaded resonator (F-SLR) proposed in [9], coupled-line stub-loaded resonators (CLSs) are able to achieve good bandwidth more easily while providing transmission zeros. Moreover, the results achieved with the help of simulation and the actual performance of the work performed will match a bit better and be easier to optimize and adjust.

Combining the benefits of the two coupled microstrip lines for the design ensures the reduction in insertion loss under the filtering effect, as well as the filtered state’s selectivity through the symmetrical transmission zeros (TZs). When the diode is in on-state bias, the filtering switch is in the filter state with excessive frequency selectivity, extensive rejection band, and extraordinarily low IL. When the diode is in off-state bias, the filtering switch is in the off-state mode with true off-state suppression.

2. Materials and Methods

We first perform a theoretical analysis of the coupled microstrip line given in Figure 2.

Figure 2 shows the equivalent schematic diagram of the coupled microstrip lines required to compose the filtering switch, where the parallel-coupled feed lines exhibit bandpass filtering effects, and where the coupled-line stub-loaded resonators mainly produce transmission zeros to enhance the passband selectivity in the filtered state.

First, the Z-parameter matrix of the parallel-coupled feed line is obtained primarily based on the coupled microstrip line’s parity mode analysis principle:

$$Z=\left[\begin{array}{cc}{Z}_{11}& Z_{12}\\ Z_{21}& Z_{22}\end{array}\right]=$$

(1)

$$\left[\begin{array}{cc}-\frac{j}{2}(Z_{oe1}+Z_{oo1})cot{\theta }_1& -\frac{j}{2}(Z_{oe1}-Z_{oo1})csc{\theta }_1\\ -\frac{j}{2}(Z_{oe1}-Z_{oo1})csc{\theta }_1& -\frac{j}{2}(Z_{oe1}+Z_{oo1})cot{\theta }_1\end{array}\right]$$

(2)

It is easy to see that the enter impedance $Z_1$ of this coupled microstrip line is:

$$Z_1=\sqrt{Z_{11}^2-\frac{Z_{11}{Z}_{21}^2}{Z_{22}}}=$$

(3)

$$\frac{\sqrt{{(Z_{oe1}-Z_{oo1})}^2-{(Z_{oe1}+Z_{oo1})}^{2}cos{\theta }_1^2}}{2sin{\theta }_1}$$

(4)

The $ABCD$ matrix $M_1$ of the parallel-coupled feed lines can be obtained from the Z-parameter matrix:

$$M_1=\left[\begin{array}{cc}{A}_1& B_1\\ C_1& D_1\end{array}\right]=$$

(5)

$$\left[\begin{array}{cc}\frac{Z_{11}}{Z_{21}}& \frac{Z_{11}{Z}_{22}-Z_{12}{Z}_{21}}{Z_{21}}\\ 1/Z_{21}& Z_{22}/Z_{21}\end{array}\right]$$

(6)

The $ABCD$ matrix $M_2$ of the coupled-line stub-load resonators (CLSs) is:

$$M_2=\left[\begin{array}{cc}{A}_2& B_2\\ C_2& D_2\end{array}\right]=$$

(7)

$$\left[\begin{array}{cc}\frac{Z_{oe2}+Z_{oo2}}{Z_{oe2}-Z_{oo2}}co{s}^2{\theta }_2& \frac{j}{2}\frac{{(Z_{oe2}+Z_{oo2})}^2}{Z_{oe2}-Z_{oo2}}si{n}^2{\theta }_2-\frac{j2Z_{oe2}{Z}_{oo2}}{(Z_{oe2}-Z_{oo2})sin{\theta }_2}\\ \frac{j2}{Z_{oe2}-Z_{oo2}}& \frac{Z_{oe2}+Z_{oo2}}{Z_{oe2}-Z_{oo2}}co{s}^2{\theta }_2\end{array}\right]$$

(8)

The equal impedance $Z_2$ of the coupled-line stub-load resonators can be acquired from the $M_2$ matrix:

$$Z_2=\frac{B_2}{D_2}=$$

(9)

$$\frac{j}{2}(Z_{oe2}+Z_{oo2})tan{\theta }_2-\frac{j2Z_{oe2}{Z}_{oo2}}{(Z_{oe2}+Z_{oo2})sin{\theta }_{2}cos{\theta }_2}$$

(10)

According to [13], the transmission zeros ($f_a$, $f_b$) of the bandpass filter in the skip state are acquired from the equal impedance at $Z_2$ = 0.

$$f_a=\frac{2}{\pi }arcsin\sqrt{1-K_2^2}<f_0$$

(11)

$$f_b=2f_0-f_a>f_0$$

(12)

where $f_0$ is the center frequency at which the device operates, and $K_2$ is the coupling coefficient of the coupled-line stub-load resonators. Compared with the traditional quarter-wavelength stub, the coupled-line stub-load resonators (CLSs) can produce two symmetric transmission zeros to improve the passband selectivity. Compared to stub-loaded resonators, coupled-line stub-load resonators (CLSs) can exhibit better passband effects at the same operating frequency.

Figure 3 shows the equivalent schematic of the filter switch based on the above analysis of coupled microstrip lines. The physical structure of the proposed filtering switch consists of two coupled microstrip lines, a parallel-coupled feed line, and coupled-line stub-load resonators (CLSs), where the coupled microstrip line $T{L}_1$ is connected to a PIN diode to achieve a good on/off control effect. The equivalent schematic diagram that makes up this filtering switch is given in Figure 3.

Figure 4 depicts the equal circuit used for the filtering switch’s PIN diode and the bias’s specific states. The filtering switch behaves as a bandpass filter characteristic when the switch is in on-state bias, and as a shutdown characteristic when it is in off-state bias.

First, based completely on the impedance evaluation and response of the coupled microstrip lines cited above, we conclude that the return loss (RL) of this filtering switch underneath on-state bias is the main decision, with the aid of the parallel-coupled feed lines $T{L}_1$ and $T{L}_2$; this is proven in Figure 2, while the transmission zero and out-of-band rejection are decided with the aid of the coupled-line brief-reduce loading resonators $T{L}_3$ and $T{L}_4$.

Second, according to our analysis of the individual coupled microstrip lines [13], we derive the final system matrix $M_d$.

$$M_d=\left[\begin{array}{cc}{A}_d& B_d\\ C_d& D_d\end{array}\right]=$$

(13)

$$\left[\begin{array}{cc}1& 0\\ 1/Z_2& 1\end{array}\right]\left[\begin{array}{cc}{A}_1& B_1\\ C_1& D_1\end{array}\right]\left[\begin{array}{cc}{D}_1& B_1\\ C_1& A_1\end{array}\right]\left[\begin{array}{cc}1& 0\\ 1/Z_2& 1\end{array}\right]$$

(14)

Finally, the filtering switch’s S-parameters are calculated from the system matrix $M_d$.

$$S_{11}=\frac{A_d+B_d/Z_0-C_d{Z}_0-D_d}{A_d+B_d/Z_0+C_d{Z}_0+D_d}$$

(15)

$$S_{21}=\frac{2}{A_d+B_d/Z_0+C_d{Z}_0+D_d}$$

(16)

3. Results

3.1. Design and Measurement

Figure 5 shows the physical layout of our proposed prototype filter switch based on the performance analysis of the selected microstrip line (Figure 2) and the construction of the schematic diagram (Figure 3). Based on the physical layout of Figure 5, the filter switch’s physical prototype is fabricated (Figure 6).

To effectively control the implementation of the on- and off-state function of this filtering switch in practical engineering, PIN diodes are connected to the parallel-coupled feed line. Figure 5 depicts the typical design of the filtering switch, with the complete circuit used being fabricated on a Rogers _RT_Duroid5880 (${\epsilon }_{re}$ = 2.2, h = 0.8 mm, tan $\delta$ = 0.0009) substrate. Murata capacitors C${}_b$ = 470 pF are used for DC blocking off and RF sign pass devices. In this paper, a PIN diode (SMP1345-079LF) from Skyworks is used, and the equal circuit [10] is given in Figure 4, i.e., $L_s$ = 0.7 nH, $R_s$ = 2 $\mathsf{\Omega }$, $C_s$ = 0.15 pF. An external DC voltage equal to 16 V is applied to the diode through a 1 k$\mathsf{\Omega }$ resistor, which adjusts the on-state or off-state of the diode, thus achieving the filtering switch’s on-and-off effect.

Under the simulation and debugging of the ADS 2019 software, the impedances of the coupled microstrip lines we chose to analyze in even and odd modes are $Z_{oe1}$ = 205 $\mathsf{\Omega }$, $Z_{oo1}$ = 83.7 $\mathsf{\Omega }$, $Z_{oe2}$ = 186.1 $\mathsf{\Omega }$, and $Z_{oo2}$ = 76 $\mathsf{\Omega }$. According to the actual situation, the external equivalent switching device will produce an excitation effect on the filter circuit using the ADS software to optimize the overall circuit, and the physical dimensions obtained after the system processing are as follows: W${}_1$ = 2.38, L${}_1$ = 17.8, W${}_2$ = 0.25, L${}_2$ = 73.1, g${}_1$ = 0.193, W${}_3$ = 0.30, L${}_3$ = 68.828, g${}_2$ = 0.17, W${}_4$ = 0.33, L${}_4$ = 71.45, g${}_3$ = 0.18, W${}_5$ = 0.4, L${}_5$ = 70.98, and g${}_4$ = 0.14 (unit: mm).

3.2. Simulated and Actual Measured Data

The simulated and measured parameters of the filtered changes are proven in Figure 7. Figure 7a indicates the simulated and measured transmission and reflection coefficients in the filtered state. Figure 7b indicates its corresponding off state. In this paper, we measured the frequency response of the fabricated filtering switch by using a Keysight E5071C. The outcomes are in suitable agreement with the ADS simulation results.

The measured results, inclusive of the impact of the I/O SMA connector, show that the in-band return loss is higher than 15 dB for a core frequency of 0.8 GHz, while the IL = 0.65 dB has a −3 dB FBW of 44.5%, and the measured turn-off rejection is higher than 27 dB in the passband of the filter state. Out-of-band rejection indicates that the filter change has state-of-the-art frequency selectivity and overall performance in the on state.

4. Discussion

On the one hand, the coupling of the microstrip lines is adjusted by the bandpass filter response of the parallel-coupled microstrip lines to gain an accurate bandpass filter effect. The g${}_1$ and g${}_2$ values in Figure 5 decide the bandwidth of the filtering effect. However, the coupling of the brief reduce-load resonator can be adjusted to generate the preferred passband transmission zeros (TZs) to enhance the filter function’s passband selectivity. The microstrip line coupling decided via the g${}_3$ and g${}_4$ values in Figure 5 can similarly manipulate the vicinity of the transmission zeros. So, when the filtering switch is operating in the on-state filter state, it has good passband selectivity and out-of-band rejection. When the filtering switch is operating in the off state, the input and output ports are approximately fully reflected, and good off-band rejection is easily obtained by changing the PIN diode’s bias.

Because we chose to design a lower center frequency, the physical size of the prototype shown in

Table 1. Comparison with other reported works. −: not given.

Ref.	CF (GHz)	IL (dB)	FBW (%)	RL (dB)	OSS (dB)	SIZE (${\mathit{\lambda }}_{\mathit{g}}^2$)	Feasibility of 5G Band Operation
[2]	0.903	1.58	10.6	20	37.6	0.0355	Yes
[2]	0.914	1.36	11.4	−	>40	0.0355	Not
[10]	1.246	1.9	15.3	<15	32.8	0.0201	Not
[10]	0.901	1.8	14.3	<15	27.8	0.0191	Not
[11]	0.991	1.08	14.73	20	48.2	0.029874	Not
[12]	0.994	1.72	8.96	−	>40.6	0.033115	Not
This work	0.8	0.65	44.5	16.3	27.3	0.1178	Yes

is inevitably too large; however, this is only at the prototype stage, and we will conduct more research to further reduce the size and production cost while ensuring the operating performance. Due to our limited experimental resources, unavoidable tolerances occurred during the fabrication of the prototype. The prototype in Figure 7b showed differences between the out-of-band simulation and the actual measurement in the off state, but it did not affect the operating condition in the operating bandwidth. In addition to this, the filtering switch prototype operates at a 0.8 Ghz center frequency and 44.5% FBW in the on state. In this case, part of the passband range is within the low frequency band of 700 MHz, and the relative bandwidth of 44.5% can cover most of the band; therefore, the prototype is expected to be applied to the 700 MHz band in 5G communication.

For the uncontrollable losses that appear above, we will refer to the work on LQG control of linear lossless positive-real systems [20] in our subsequent work to optimize and improve our prototype design, so that it can achieve better operating performance in the future.

5. Conclusions

The performance of the filtering switch made with the PCB fabrication technique was compared with that of the filtering switch discussed earlier, and the information is summarized in Table 1. In comparison with the performance of the filtering switches reported in [2,10,11,12], the suggested filtering switch in this study exhibits strong selectivity in the on state, as well as a high degree of in-band insertion loss. In the off state, it possesses good out-of-band rejection capabilities. Furthermore, this filtering switch has a low in-band insertion loss. In this paper, we proposed a filtering switch based mainly on coupled microstrip lines, and we experimentally verified its theoretical overall performance. The proposed filtering switch has good frequency selectivity, low insertion loss in the on state, and good overall rejection performance in the off state. Its operating frequency also satisfies the specific 5G frequency band, which means that it has good application prospects for future 5G communication systems.