Wideband reflection type MMIC attenuator with constant phase

A method for reducing the unwanted insertion phase variation in an MMIC reflection type attenuator with wide bandwidth (>20 GHz) is described. The analogue reflection type amplitude control circuit uses cold-FETs as variable resistance reflection terminations. At high frequencies, very high phase variation is unavoidable due to the parasitic voltage-dependent reactance. Instead of tuning-out the parasitic reactance of the cold-FET, a novel symmetrical control method is presented for a two-stage attenuator. The phase shift variations from each stage cancel each other by the use of `complementary´ control signals