I. Introduction
In wireless systems, a balun is vital in converting unbalanced signals to balanced ones or vice versa. Traditionally, baluns are made in low-temperature co-fired ceramic (LTCC) and monolithic microwave integrated circuits (MMIC) technologies. Commercially, baluns are implemented in LTCC technology, but recently, there has been a movement of technology to integrated passive device (IPD) technology for miniaturization and integrated circuits. Much research has been conducted to incorporate individual baluns [1, 2]. Most of them are baluns using transmission lines developed for RF applications. The circuits using transmission line topology are larger in size because of multiple λ/4 wavelengths.
Thus, many papers have suggested that the lumped LC passive component provide a size reduction [3–9]. However, these papers’ suggestion has the effect of reducing the target bandwidth due to frequency-dependent factors. The conventional work in [10] is implemented with low-pass filter (LPF) and high-pass filter (HPF) structures using lumped elements, but it is difficult to obtain a uniform magnitude characteristic. Thus, magnitude balance and phase difference are uniform only at the center frequency. However, as magnitude balance deviates from the center frequency, the magnitude and phase deviation increase.
In this paper, we propose a wideband balun using through-silicon via (TSV) IPD technology. The TSV process provides a solid ground plane with a copper layer at the wafer’s backside. The design minimizes GND influence from external packaging variation sources, and the overall tolerance is primarily controlled by the wafer manufacturing variation [11]. Reduced thermal resistance through the backside via [12] and excellent thermal conductivity of evenly distributed thick copper plane improves thermal performance, which is critical for compatible use with high-power gallium nitride (GaN). The proposed balun is composed of lumped components, and the component values are derived by differentiating the magnitude of the S-parameter with the constant value. The proposed balun implemented using IPD technology to smaller and low loss.
II. Design Theory and Simulation Results
1. Design Theory
An ideal balun consists of distributed lines using λ/4 and 3λ/4 of differential electrical lengths with
2 Z 0
The inserted parallel resonator is an open circuit at the resonant frequency, and the inserted resonator serves to control the deviated magnitude values to constant values. To derive the element values of the proposed circuit of Fig. 1(b), Eq. (1) is obtained by terminating Port 3 as Z0, which is the ABCD parameter between Ports 1 and 2.
(1)
Similarly, the ABCD parameters obtained between Ports 1 and 3 are derived by A2, B2, C2, and D2, as shown in Eq. (2). The input admittance Yin terminated as Y0 at Port 3 in Fig. 1(b) is expressed as Eq. (3).
(2)
The total ABCD parameter is obtained between Ports 1 and 2 using the cascaded Yin and A1, B1, C1, and D1 parameters as Eq. (4).
Eq. (4) is arranged as AT = A1, BT = B1, CT = A1Yin + C1, and DT = B1Yin + D1. Using the relation between the S-parameter and the ABCD parameter, S21 is obtained as Eq. (5).
The magnitude of S21 is shown as Eq. (6).
By differentiating the magnitude of the derived |S21|, the derived |S21| must be frequency independent in terms of the frequency over a wide bandwidth. The differential value of the derived |S21| at the center frequency ω0 is zero to make the frequency-dependent term reduced in wideband; then the value of LP is expressed as Eq. (7).
The CP of the inserted parallel resonant values are derived from the resonant circuit.
2. Circuit Simulation Results
To verify the derived theory, the conventional circuit in Fig. 1(a) and the proposed circuit in Fig. 1(b) must be compared. The center frequency is 3.6 GHz, and the conducted simulation tool is Ansys, Electronics Desktop. The obtained values of Fig. 1(a) and 1(b) are shown in Table 1.
Fig. 2(a) and 2(b) show the circuit simulation results of the conventional and proposed circuits. The conventional circuit has variations ranging from −2.3 to −5.8 dB, while the proposed circuit has a constant value from −2.893 to −3.117 dB from 2.8 to 4.4 GHz. From the simulation results, the magnitude balance of the proposed circuit is compensated using design theory.
III. 3D Structure and Fabrication Results
Fig. 3(a) and 3(b) show the layer configuration of the IPD inductor and capacitor, respectively. It is a cross-section view in which several layers of silicon nitride (SiN) and metal are stacked on silicon carbide (SiC). The inductor consists of M1 (Metal1) and M3 (Metal3) layers and is implemented in the form of a spiral inductor using M1 of the SiN2 layer. Then, the M1 and M3 layers are connected using the SNO2 (SiN open) metal layer penetrating the SiN3 dielectric (DE) layer. The capacitor uses M2 and M3 layers and is implemented in the form of a metal-insulator-metal (MIM) capacitor by inserting a SIN3 DE layer between the metals. A DE layer is inserted between each metal layer of the inductor and capacitor to prevent short circuits. Table 2 shows the details of the layers used in the structure of Fig. 3.
Fig. 4(a) and 4(b) show a spiral inductor and capacitor implemented as an IPD, respectively. Inductors require 1.56 nH and 3.12 nH for the design of the proposed balun at 3.6 GHz. Capacitors require 1.2 pF and 0.56 pF for the proposed balun. Fig. 4(c) is the proposed balun. The ground passing through the SiC layer is designed using the TSV method, which is implemented in the circuit in the form of back via (BVIA). In addition, the designed ground-signal-ground (GSG) pad is inserted at the input and output ports for measurement. The total size of Fig. 4(c) is 2.4 mm × 1.02 mm.
Fig. 5 is the microphotograph of the proposed balun fabricated by an IPD. The implemented structure shows that the designed 3D structure is well implemented.
Fig. 6(a) and 6(b) show the magnitude and phase difference results measured with Agilent Network Analyzer (N5230) by uploading the proposed balun made with an IPD to Cascade Microtech probe station (Probe station summit 11741B). Fig. 6(a) is the comparison results according to the ±0.2 dB magnitude results within 2.96–4.2 GHz. Fig. 6(b) is the comparison results according to the phase difference from 2.96–4.2 GHz with ±3°. The measured results have a fabrication error since the capacitor is dominantly affected by the DE thickness between metals.
The comparison results of the performance according to the IPD technology are listed in Table 3, which shows that the balun has the center frequency, amplitude imbalance, phase imbalance, and fractional bandwidth (FBW) based on a 15-dB return loss. Therefore, using the proposed design method, the proposed circuit has excellent measurement results according to the amplitude, phase imbalance, and FBW.
IV. Conclusion
In this paper, we proposed a wideband balun using IPD technology for smaller sizes and low insertion loss. The proposed balun is designed to maintain the magnitude slope constant. By differentiating the magnitude, the component values of the proposed balun are derived. The magnitude balance and phase differences resulted in about 0.2 dB and 3° from 2.96 to 4.2 GHz. Thus, the frequency dependency of the proposed circuit is well compensated for not only magnitude balance but also phase difference.
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