# A Reflection-Type Dual-Band Phase Shifter with an Independently Tunable Phase
## 1 Introduction  
The phase shifter (PS) is one of the most important elements in wireless communication systems, as it is widely used in beam-forming, phased array, harmonic distortion, and multiple-input multiple-output (MIMO) systems [1, 2, 3, 4, 5, 6, 7, 8]. Tunable PSs are used with along/digital attenuators in phased array beam-forming networks where insertion loss (IL) and phase deviation (PD) must be as small as possible to steer the lobes and nulls of antennas. Tunable PSs are classified as transmission type [9, 10] and reflection type [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Reflection-type PSs can provide continuous phase shift, as well as excellent return loss (RL) due to their use of a 3-dB hybrid coupler and identical reflection loads at the coupler's through and coupled ports. Since the DC power consumption of a varactor diode is much smaller than that of a PIN diode or a microwave FET, varactor diodes are widely used in PSs.  
In [13], a PS with wide PSR was presented using L-type and \(\pi\)-type circuits, which provided a PSR of 201\({}^{\circ}\) for L-type and a PSR of 385\({}^{\circ}\) for \(\pi\)-type. Similarly, a reflection-type tunable PS using an impedance transforming quadrature coupler was presented in [14], in which a PSR of 234\({}^{\circ}\) was achieved at the center frequency. In [15], modifying the structure of [14] to cascading structure resulted in a PSR of up to 407\({}^{\circ}\). A compact tunable PS which achieved a PSR of 407\({}^{\circ}\) was presented in [16] using a vertical planar structure. Likewise, a tunable reflection-type PS at X-band was presented in [17] using a coupled line. This PS achieved a PSR of 392\({}^{\circ}\) with an IL of 2.1 \(\pm\) 1.3 dB over the operating band. However, the in-band maximum RL was always smaller than 10 dB. A wideband tunable PS based on acoupled line was demonstrated in [18; 19]. The coupled line compensated for the parasitic elements of the varactor diode, resulting in a wide PSR with small in-band PD and IL error across an extensive bandwidth. By using the series-shunt matching technique in a reflection-type PS, a wide PSR with minimum IL was achieved in [18].  
Multi-band RF circuits and systems have recently been in the spotlight. The multiple functions they perform are the key to next-generation wireless communication, sensing, and radar applications. Such multi-functional systems are designed to operate concurrently across multiple frequency bands. However, the traditional PSs can operate only on a single band. It is more effective to implement a multi-band PSs rather than rely on multiple single-band PSs. In [21], the PS operating in dual-band was present. However, in this paper, there is no tunable characteristic, and there is a disadvantage that only an arbitrary phase difference is implemented. A dual-band 90\({}^{\circ}\) PS using band-pass and band-stop designs was presented in [22]. Similarly, a dual-band PS that combined two single-band PSs was developed in [23]. In [24], a multi-band PS was included a distributed amplifier for loss compensation and an LC network for compact chip size. Likewise, a dual-band PS using a two-stage dual-branch phase tuning network topology was demonstrated in [25]. However, none of these previously developed conventional dual-band PSs possess an independent controllable phase shift at each operating frequency, except the PSs proposed in [25].  
In this paper, a dual-band reflection-type tunable PS is presented, in which the phase shifts at the lower frequency band (\(f_{\rm L}\)) and the higher frequency band (\(f_{\rm H}\)) can be independently tuned. As an experimental demonstration, the proposed PS was designed and fabricated for a \(f_{\rm L}\) of 1.88 GHz and \(f_{\rm H}\) of 2.44 GHz. The design was ultimately confirmed to provide a PSR of 114\({}^{\circ}\) in each band.