  We theoretically investigate the tunable slowing and advancing of microwave
signals based on bichromatic electromechanically induced transparency in a
three-mode circuit electromechanical system, where two mechanical oscillators
with closely spaced frequencies are independently coupled to a common microwave
cavity. In the presence of a strong microwave pump field, we obtain two
transparency windows accompanied by steep phase dispersion in the transmitted
microwave probe field. The width of the transparency window and the group delay
of the probe field can be controlled effectively by the power of the pump
field. It is shown that the maximum group delay of 0.12 ms and the advance of
0.27 ms can be obtained in the current experiments.
