Design of an adjustable active CMOS true time delay circuit for wideband beamforming applications

A Ka-band true time delay circuit has been realized in 0.18 µm CMOS which incorporates a new technique for providing variable delay for broadband operation. Precise delay variation capability is obtained by introducing a tunable active inductance structure. Changing the inductance value of the proposed active inductor is achieved through enabling and disabling auxiliary transistors in the structure. The relative delay resolution and maximum delay of the implemented circuit are 520 fsec and 1.56 psec, respectively. These values correspond to the performance of a 6-bit phase shifter with phase shift step of 5.625° at a center frequency of 30 GHz. Establishing linear phase shift and accurate fine delay tuning with the proposed active inductor has resulted in 80 fsec maximum delay variation and less than 30 fsec average error over the intended frequency span of 26.5–35 GHz. The finalized circuit occupies less than 0.03 mm2 and is associated with an insertion loss of about 8 dB with a variation of 1 dB per state.