The phase shift in the dual-band mode can be written as

Figure 4 Dual-band phase shifter operating modes: low-band (a), high-band (b) and dual-band (c).

Figure 5 Phase shift of the simple multi-mode dual-band phase shifter.

Figure 6 Multi-mode phase shifter phase error.

If ΔφDB(ωL) = ΔφDB(ωH) = θd, where θd denotes the desired phase shift, an accurate phase shift can be obtained in dual-band mode; however, phase errors emerge when the dual-band phase shifter operates in the low-band or high-band modes. Phase errors are caused by the nonzero phase shifts of the low-band phase shifter at ω = ωH and the high-band phase shifter at ω = ωL (see Figure 5). The phase errors of the low-band and high-band modes are given by

respectively, where p = ωH/ωL. Figure 6 plots the phase errors versus p and values of k, where kL = kH = k. As shown, the phase error diminishes with the increasing p or decreasing k.

OPTIMIZATION

A dual-band phase compensation technique eliminates the phase errors. Phase compensation cells are cascaded with the main cells, as shown in Figure 7. To compensate for phase errors, the phase compensation cells use the same topology as the main cell but with ratio factors kCL and kCH < 1.

Figure 7 Prototype multi-mode dual-band phase shifter circuit.

Figure 8 Simulated phase shift of the low-band (a) and high-band (b) phase shifter with kL = kH = 1.64 and kCL = kCH = 0.94.

Considering the low-band mode, the center frequency of the low-band compensation cell is set as ωCL = ωH. The phase shift of the low-band phase shifter is

To cancel the phase errors at ω =ωL and ω = ωH, consider the following equations:

When θd, ωL and ωH are chosen, the design parameters kL and kCL can be obtained by solving equations (13) and (14). The design of the high-band phase shifter follows the same process as the low-band phase shifter design. Ideally, kH = kL and kCH = kCL if identical phase shifts are desired for low-band and high-band.

Figure 8 plots the simulated phase shifts of the low-band and high-band phase shifters, respectively, with θd = 180 degrees, fL = 5 GHz and fH = 25 GHz. As shown in Figure 8a, the low-band phase shifter achieves exactly 180 degrees phase shift at fL = 5 GHz and 0 degree phase shift at fH = 25 GHz. Similarly, phase errors of the high-band phase shifter are cancelled with the use of the compensation cell, as shown in Figure 8b. Accurate phase shifts are obtained whether the low-band and high-band phase shifters operate individually or concurrently.

Figure 9 Multi-mode dual-band phase shifter schematic.

CIRCUIT IMPLEMENTATION

Figure 9 shows the multi-mode dual-band phase shifter schematic. For each phase shifter cell, the reference path and phase shifting path are combined into one all-pass network with inserted switches. The low-band and high-band phase shifters are controlled with 2-bit logic voltages, VL and VH. When VL = 1, SL1, SL4, SCL2, SCL3 are on and SL2, SL3, SCL1, SCL4 are off, the low-band phase shifter operates in the reference state. When VL = 0, SL1, SL4, SCL2, SCL3 are off and SL2, SL3, SCL1, SCL4 are on, the low-band phase shifter is in the phase shifting state. Similarly, when VH = 1, the high-band phase shifter operates in the reference state, and when VH = 0, the high-band phase shifter operates in the phase shifting state.