# Highly Linear Loaded-Line Phase Shifter Utilizing Impedance Transformer and PIN Diode
## 3 Loaded-Line Phase Shifter  
The fundamentals and various implementations of loaded-line phase shifters are discussed in detail in [26]. In this type of phase shifter, the desired phase shift is achieved by selectively controlling stubs or lumped elements connected to the main transmission line. Regardless of the specific implementation, the core concept for achieving a target phase shift, \(\Delta\theta\), is illustrated in Figure 2. In each switching state, the phase shifter can be modeled as an equivalent transmission line with a characteristic impedance of \(Z_{eq}\) and length of \(\theta_{eq}\). To achieve a well-matched and effective phase shift, the design must ensure that \(Z_{eq1}=Z_{eq2}=Z_{0}\), and that the differential phase shift satisfies \(\Delta\theta=\theta_{eq2}-\theta_{eq1}\).  
In addition to the dimensions of the stubs--such as their width, length, and position relative to the main line--the parasitic parameters of the switching elements also play a critical role in determining the resulting phase shift and insertion loss (IL). These effects must be carefully accounted for during the design process. A comprehensive mathematical framework for calculating the required stub dimensions to achieve a specified phase shift is provided in [26; 29].  
Figure 2: Key concept of the loaded-line phase shifter: two different states of a loaded-line phase shifter can lead to the required phase shift with minimum IL.  
Although the loaded-line phase shifter offers linearity advantages by utilizing PIN diodes in a parallel configuration, its maximum achievable IIP3 can be even better by using impedance transformer. As reported in [8], an RF switch employing the BAR64-02V PIN diode in a parallel configuration demonstrated an IIP3 of approximately 90 dBm at 2.05 GHz.  
To further enhance the IIP3, the impedance optimization technique presented in [3] can be employed. This approach suggests that connecting the PIN diode to a lower impedance improves the overall IIP3 of the switch including the IIP3 (ON) and IIP3 (OFF). Our measurement results, shown in Figure 3, support this observation. In these experiments, a BAR64-02V PIN diode configured in a series topology was evaluated under varying impedance conditions. The results confirm that both ON- and OFF-state linearity are strongly influenced by the impedance seen by the diode. This finding reinforces the effectiveness of impedance transformation in enhancing the overall linearity performance of PIN diode-based switches.  
To adjust the impedance seen by the PIN diodes, impedance transformers are incorporated on both sides of the loaded-line phase shifter. The coupler-based impedance transformers at the input and output serve to present a reduced impedance to the PIN diodes--thereby improving linearity--while simultaneously matching the overall phase shifter to standard 50 \(\Omega\) ports. This dual transformation ensures both optimal PIN diode operation and seamless integration into typical RF systems. The structure of the proposed highly linear loaded-line phase shifter is illustrated in Figure 4. In this configuration, each switching element consists of a single PIN diode connected in a parallel-to-ground arrangement. Despite the addition of impedance transformers, the input and output impedance of the overall phase shifter is maintained at 50 \(\Omega\) to ensure compatibility with standard RF systems. The reduced impedance presented by the coupler minimizes voltage swings across the reverse-biased diodes, thereby directly enhancing the OFF-state IIP3, as validated by our measurements and analyses in [3; 19].  
Figure 3: Measured and simulated IIP3 of the BAR64-02V PIN diode in both ON and OFF state while connected to different impedances.