Broadband phase invariant attenuator

A 6-18-GHz variable attenuator has been realized, with less than 5 degrees phase error over a 32-dB attenuation range. The approach utilizes the biphase nature of a reflection from a resistive load. With the use of quadrature couplers, the first-order errors are reduced to fourth-order terms, resulting in significant phase error improvement. Phase error gracefully degrades at higher attenuation (15 degrees at 45 dB), although attenuation becomes very sensitive to control voltage. The speed of the attenuator is limited essentially by driver circuitry, and better than 150 ns within 0.5 dB of insertion loss has been achieved.<>