Tunable microstrip bandpass filter with constant absolute bandwidth using BST varactors and digitally tunable capacitors

Abstract

In this paper, two second-order electronically tunable bandpass filters are presented. The filters are implemented in microstrip technology using barium–strontium–titanate (BST) varactors and digitally tunable capacitors (DTC) for tuning the frequency response of the bandpass filters. The filter realized using BST varactors has a 35% tuning range from 900 MHz to 1.275 GHz with an insertion loss variation from 3.1 to 2.6 dB. The absolute bandwidth is nearly constant over the entire tuning range, varying from 64 to 72 MHz (around ±5% variation). The filter realized using DTCs also has a 36% tuning range from 850 MHz to 1.225 GHz with an insertion loss variation from 3.1 to 1.5 dB. The absolute bandwidth is constant over the tuning range, varying from 88 to 98 MHz (around ±5% variation). The bandpass filters are tuned using a single control signal. The tunable bandpass filters are proposed for use in reconfigurable radios.

Introduction

Due to integration and miniaturization efforts in recent times, multiband applications have revitalized interest in reconfigurable radios where tunable bandpass filters play a vital role. For high Q applications, such tunable filters with a single control mechanism have been recently reported using coaxial and waveguide technologies [1, 2]. However, at ultrahigh frequency, L-, S-, and C-band applications, due to miniaturization requirements, electronically tunable microstrip and suspended stripline filters are highly preferred. One of the earliest works in this regard is in reference [3], which demonstrates a varactor-tuned microstrip bandpass filter at 4.5 GHz with around 33% tuning range using varactor loading on the quarter-wavelength resonators. Reference [4] presents a tunable filter at 1 GHz with an enhanced tuning range of 60% by employing suspended stripline resonators. A varactor-tuned bandpass filter using step-impedance microstrip transmission lines has been presented with around 12.5% tuning range at 2 GHz [5]. A tunable microstrip bandpass filter using a coupled loop resonator is reported with a tuning range of 50% at 1 GHz [6]. To miniaturize the size of filter, corrugated resonators have also been used to realize tunable bandpass filters [7]. Through the coupling of two hairpin- defected ground structure (DGS) resonators, a unique compact electrically tunable microstrip bandpass filter is proposed in reference [8]. The tunability is attained by integrating varactor diodes into the resonators. The tunable bandpass filter’s measured relative tuning range is 23.5%, and the measured insertion loss is between 3.6 and 2.1 dB with 8% fractional bandwidth within the tuning range. An electrical reconfigurable second-order microstrip bandpass filter utilizing two coupled octagonal-DGS resonators is presented in reference [9]. The tuning is achieved over frequencies from 4.3 to 5.4 GHz with quasi-constant bandwidth of 12%, while measured insertion loss variation is from 5 to 3 dB. Tunable band stop filters (BSFs) using barium–strontium–titanate (BST) varactor chips utilizing the stepped impedance resonators is presented in reference [10]. The designed tunable BSFs show more than 30 dB of band rejection at the center frequency of 1.1 GHz with 10% of frequency tuning ranges. One of the limitations of the tunable filters demonstrated in the above-referenced works is that they all employ diode-based varactors, which do not have direct current (DC) and radio frequency (RF) isolation. Hence, they all employ RF choke and DC blocking capacitors. In reference [11], the digitally tunable bandpass filter utilizing the lumped elements is presented. The filter operates over frequency range from 450 to 940 MHz.

In this paper, microstrip resonator–based tunable bandpass filters using BST varactors and digitally tunable capacitors (DTCs) are proposed and realized. BST varactors and DTCs are not only high Q components but also have inherent RF and DC isolation, and hence, no RF choke and no DC blocking capacitors are required. In addition, they have excellent linearity (IIP3 > 50 dBm) and have low leakage current (<100 nA). For the proof of the proposed concept, two second-order tunable bandpass filters are realized with a tuning range of around 35% at 1 GHz (900–1275 MHz), one using BST varactors and the other using DTCs. The absolute bandwidth is nearly constant over the entire tuning range (around ± 5% variation). In addition, the bandpass filters are tuned using a single control voltage. The paper is organized as follows: The “Concept” section presents the concept and design methodology, the “Design methodology” section describes the fabrication and measurement results, and finally, the “Conclusion” section presents the concluding remarks.

Concept

The concept of the proposed tunable microstrip bandpass filter basically involves a BST varactor (or DTC) loaded quarter-wavelength (λ/4 length) resonator, as depicted in Fig. 1. One end of the resonator is shorted to the RF ground, while the other end is loaded with the varactor. The change in the DC bias voltage (or control signal for DTC) changes the capacitance value, tuning the resonant frequency. By suitably coupling such resonators together and by suitably tapping the input and output, one can realize tunable microstrip bandpass filters.

Figure 1. Tunable microstrip resonator using BST varactor (or DTC).

Figure 2 shows the schematic of the BST varactor-based second-order tunable bandpass filter designed using Keysight ADS. Gap coupling is used for inter-resonator coupling, whereas tap coupling is used for input–output coupling. BST varactor (1.7–9.7 pF: 24–1 V) which has a minimum Q of around 50, is employed. A second-order filter has three design parameters (variables), namely resonator length, input–output tapping location, and the gap between the resonators. The gap corresponds to the inter-resonator coupling, while the tapping location corresponds to the input–output coupling. Inter-resonator coupling co-efficient can be determined using equation (1), which incorporates operating bandwidth (BW), normalized coupling coefficient (M_i,j), and the resonant frequency (f_r). Equations (2) and (3) relate input coupling (R_S) and output coupling (R_L) to the normalized coupling coefficients [12, 13]. A coupled microstrip transmission line model in Keysight ADS is used for the simulation. Since there are only three design parameters, the filter is designed using manual tuning of the parameters. The gap between the resonators determines the bandwidth obtained, and the return loss can be improved using the tapping location. Figure 3 shows the DTC (pSemi: PE64102) based second-order tunable bandpass filter implemented using a similar methodology.

(1)\begin{equation}{k_{i,j}} = \,\left( {{BW}*{M_{i,j}}} \right)/{f_{\mathrm{r}}}\end{equation}

(2)\begin{equation}{R_S} = \,\left( {{M_{s,1}}*{M_{s,1}}*{BW}} \right)/{f_{\mathrm{r}}}\end{equation}

(3)\begin{equation}{R_L} = \,\left( {{M_{N,L}}*{M_{N,L}}*{BW}} \right)/{f_{\mathrm{r}}}\end{equation}

Figure 2. (a) Schematic and photograph of the tunable microstrip bandpass filter using BST varactors. (b) Simulated reflection coefficient and transmission coefficient of the tunable bandpass filter using BST varactors: capacitance variation from 2 to 8 pF. (c) Simulated transmission group delay (capacitance variation from 2 to 8 pF) and spurious response (at 8 pF) [BST varactor-based bandpass filter].

Figure 3. (a) Schematic and photograph of the tunable microstrip bandpass filter using DTCs. (b) Simulated reflection coefficient and transmission coefficient of the tunable bandpass filter using DTCs: capacitance variation from 2.8 to 8 pF. (c) Simulated transmission group delay and spurious response (at 8 pF) [DTC-based bandpass filter].

Design methodology

Since RT5880 (dielectric constant of 2.2 and loss tangent of 0.0009) is a low loss substrate, and provides good performance, and hence is employed to design and fabricate the tunable microstrip bandpass filter shown in Figs. 2 and 3. The resonators and feeding line for the tunable bandpass filter are designed using 50 Ω transmission lines. The line width for the resonators and feedline is calculated using the design equations for microstrip technology. For a given characteristics impedance “Z ₀”, dielectric constant and substrate height “d”; the line width “W” can be calculated using the following equations [14]:

(4)\begin{equation}\frac{W}{d} = \left\{ \begin{array}{*{20}{c}} \frac{8 e^A}{e^{2A} - 2}\,\mathrm{for}\frac{W}{d} \lt 2 \\[3pt] \frac{2}{\pi }\left[ B - 1 - \ln \left( {2B - 1} \right)\right. \\[3pt] \left.+ \frac{e_{\mathrm{r}} - 1}{2 e_{\mathrm{r}}}\left\{ \ln \left( {B - 1} \right) + 0.39 - \frac{0.61}{e_{\mathrm{r}}} \right\} \right]\,\mathrm{for}\frac{W}{d} \lt 2 \end{array} \right.\end{equation}

where

(5)\begin{equation}A = \,\frac{{{Z_{{o}}}}}{{60}}\sqrt {\frac{{{e_{{r}}} + 1}}{2}} + \frac{{{e_{{r}}} - 1}}{{{e_{{r}}} + 1}}\left( {0.23 + \frac{{0.11}}{{{e_{{r}}}}}} \right)\end{equation}

(6)\begin{equation}B = \frac{{377\pi }}{{2{Z_{{o}}}\sqrt {{e_{{r}}}} }}\end{equation}

Length of the resonators is taken equal to quarter wavelength of the center frequency of operating band. However, the actual length in design is optimized to achieve the optimum performance. Copper is used as the metal layer with gold plating. Plated through hole via is used for RF short. A common DC bias voltage is applied to both the BST varactors, as depicted in Fig. 2(a). Similarly, common control signals using Arduino Due microcontroller (Vdd, Clock, Select, and Data) are applied to both the DTCs as shown in Fig. 3(a). Both BST varactors and DTCs have inherent RF and DC isolation, and hence no RF choke and no DC blocking capacitors are required.

Simulation results are plotted in Figs. 2 and 3. The measured reflection coefficient (S₁₁) and transmission coefficient (S₂₁) for the BST varactor-based bandpass filter are plotted in Fig. 4. The measured reflection coefficient (S₁₁) is better than −10 dB from 900 to 1275 MHz, thus achieving a tuning range of 35%. The transmission coefficient (S₂₁) varies from 2.6 (at 1275 MHz) to 3.1 dB (at 900 MHz). The measured absolute bandwidth is nearly constant and varies around 68 MHz by ±5%. The transmission group delay variation and spurious response are shown in Fig. 5. It can be observed that there is no spurious response till 3 GHz. It is worth mentioning here that BST varactors have excellent linearity (IIP3 > 50 dBm) and have low leakage current (<100 nA).

Figure 4. Measured reflection coefficient and transmission coefficient of the tunable bandpass filter using BST varactors: bias voltage variation from 7 to 24 V.

Figure 5. Measured transmission group delay (bias voltage variation from 7 to 24 V) and spurious response (at 7 V) [BST varactor-based bandpass filter].

The measured reflection coefficient and transmission coefficient for the DTC-based bandpass filter are plotted in Fig. 6. The measured reflection coefficient (S₁₁) is better than −10 dB from 850 to 1225 MHz, thus achieving a tuning range of 36%. The transmission coefficient (S₂₁) varies from 1.5 dB (at 1225 MHz) to 3.1 dB (at 850 MHz). The measured absolute bandwidth is nearly constant and varies around 93 MHz by ±5%. The transmission group delay variation and spurious response are shown in Fig. 7. It is worth mentioning here that DTCs have excellent linearity (IIP3 > 60 dBm). Table 1 compares the proposed tunable microstrip bandpass filters with those reported in the literature.

Figure 6. Measured reflection coefficient and transmission coefficient of the tunable bandpass filter using DTCs (state 0 → S0: 00000000, state 7 → S7: 00000111).

Figure 7. Measured transmission group delay and spurious response (at S7) [DTC-based bandpass filter].

Table 1. Comparison of tunable microstrip bandpass filters

λ= free-space wavelength; IP3 = third-order intercept point; NM = not mentioned; SI = stepped impedance; TW = this work.

Conclusion

In this paper, two second-order electronically tunable bandpass filters are presented. The filters are implemented in microstrip technology using BST varactors and DTCs for tuning the frequency response of the bandpass filters. The filters have a nearly 35% tuning range of around 1 GHz. The absolute bandwidth is almost constant over the entire tuning range (around ±5% variation). The bandpass filters are tuned using a common control signal. The tunable bandpass filters are proposed to be used in reconfigurable radios.