In this novel 6-bit phase shifter, a conventional switched transmission line phase shifter cell is replaced with a structure where phase is controlled by a voltage applied to a barium strontium titanate (BST) ferroelectric material for tunability, improved performance and reduced size. Performance is simulated and measured from 2.7 to 3.5 GHz.
Phase shifters are key components in phased array radar and, increasingly, wireless communications. Well known approaches use PIN diode or FET switches to select fixed lengths of transmission line, providing adequate performance in general; however, conventional transmission line concepts lack tunability. The CRLH-TL structure has been used;1-2 however, this approach degrades insertion loss and return loss. A substrate integrated waveguide (SIW) structure uses varactors for tunable performance; however, this increases circuit complexity.3 The use of a ferroelectric material, which offers phase tunability and reduces circuit size, has been implemented successfully in phase shifter design.4-7
This article describes the design of a 6-bit digital phase shifter employing the ferroelectric material BST for tunability, improved performance and reduced size. Conventional transmission line phase shifter cells (5.6, 11.25 and 22.5 degrees) and tunable phase shifter cells (45, 90 and 180 degrees) are configured in a novel two-layer structure. It is electrically adjustable, well matched and has low insertion loss. Measured results closely agree with simulation.
PHASE SHIFTER CELL
The structures of conventional and tunable phase shifter cells are shown in Figure 1a and b, respectively. The conventional phase shifter cell, constructed on one microstrip layer, has phase differences given by8
where Qs and Zs are the electrical length and impedance, respectively, of the phase shifter cell at the design frequency f0, and Q0 and Z0 are the electrical length of the reference microstrip line.
The tunable phase shifter includes two layers. A conventional transmission line phase shifter cell is on the top layer and the ferroelectric phase shifter cell is on the bottom layer. Rogers 4350 substrate material is used for the upper layer cell, and BST is used for the bottom layer. The top and bottom layers are connected by via holes.
For tunability and small size, a BST thick film with a height of 2 µm is screen printed and sintered on an Al2O3 substrate with a height of 620 µm and εr = 10.1. Using a relative permittivity of εr,BST = 125, the circuit is matched with the 50 Ω transmission line of the upper layer. The phase of the structure is flexibly changed by modifying the BST voltage.
6-BIT DIGITAL PHASE SHIFTER
A layout of the tunable 6-bit digital phase shifter is shown in Figure 2. Conventional 3-bit phase shifter cells (5.6, 11.25 and 22.5 degrees) on the Rogers 4350 material forms the upper layer, while the tunable, ferroelectric, 3-bit phase shifter cells (45, 90 and 180 degrees) reside on the BST bottom layer.
The phase shifter line lengths of the tunable cell are l1, l2 and l3, while the width and gap are w1 and S, respectively (see Figure 1). The via radius between upper and bottom layers is R. Optimized dimensions are l1 = 10.2 mm, l2 = 18.9 mm, l3 = 27.2 mm, w1 = 0.36 mm, s = 0.18 mm, l4 = 6.3 mm, w2 = 0.16 mm and R = 0.15 mm. With these parameters, simulated phase as a function of BST voltage is shown in Figure 3. The relationship between phase and voltage is nearly linear, so phase control is simplified, especially when several phase shifters are controlled in an array. 40 V is chosen for bias.
FABRICATION AND MEASUREMENT
Photographs of the 6-bit digital phase shifter are shown in Figure 4. The upper layer (Figure 4a) is manufactured on the Rogers 4350 substrate. Skyworks’ SMP1321-079 PIN diodes are used as the switches in the conventional phase shifter cells. The bottom layer is fabricated using the BST-based ferroelectric material. Figure 5 is a photograph comparing the size of the new structure with that of a conventional 6-bit phase shifter.
In Figure 6, the simulated and measured values of |S21| for the tunable phase shifter at 360 degrees are compared with measured values for the conventional phase shifter. Similarly, Figure 7 compares the simulated and measured values of |S11| for the tunable phase shifter at 360 degrees with measured values for the conventional phase shifter. Comparisons of phase variation over frequency at 360 degrees are shown in Figure 8.
Measured insertion loss and return loss of the tunable phase shifter are approximately 3.6 and 18 dB, respectively, in the frequency range from
2.7 to 3.5 GHz. Insertion loss is about 1 dB lower and return loss is about 5 dB higher when compared to the conventional phase shifter. Phase variation for a 360 degree phase shift indicates an improvement of about 1 degree compared with the conventional phase shifter. Total phase variation over frequency is within ±2.5 degrees.
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