Two 4-bit active phase shifters integrated with all digital control circuitry in 0.13-$\mu{\hbox {m}}$ RF CMOS technology are developed for X- and Ku-band (8–18 GHz) and K-band (18–26 GHz) phased arrays, respectively. The active digital phase shifters synthesize the required phase using a phase interpolation process by adding quadrature-phased input signals. The designs are based on a resonance-based quadrature all-pass filter for quadrature signaling with minimum loss and wide operation bandwidth. Both phase shifters can change phases with less than about 2 dB of RMS amplitude imbalance for all phase states through an associated DAC control. For the X- and Ku-band phase shifter, the RMS phase error is less than 10 $^{\circ}$ over the entire 5–18 GHz range. The average insertion loss ranges from $-{\hbox {3~dB}}$ to $-{\hbox {0.2~dB}}$ at 5–20 GHz. The input ${P}_{\rm 1dB}$ for all 4-bit phase states is typically $-{\hbox{5.4}}\pm {\hbox{1.3~dBm}}$ at 12 GHz in the X- and Ku-band phase shifter. The K-band phase shifter exhibits 6.5–13$^{\circ}$ of RMS phase error at 15–26 GHz. The average insertion loss is from $-$4.6 to $-{\hbox {3~dB}}$ at 15–26 GHz. The input ${P}_{\rm 1dB}$ of the K-band phase shifter is $-{\hbox{0.8}}\pm {\hbox{1.1~dBm}}$ at 24 GHz. For both phase shifters, the core size excluding all the pads and the output 50 $\Omega$ matching circuits, inserted for measurement purpose only, is very small, 0.33$\,\times\,$0.43 ${\hbox {mm}}^{2}$. The total current consumption is 5.8 mA in the X- and Ku-band phase shifter and 7.8 mA in the K-band phase shifter, from a 1.5 V supply voltage.

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