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New for July 2009! A vector modulator can be used to perform a phase shift function, with the added benefit of amplitude control.
A signal is split into two signals that are 90 degrees apart (in phase and quadrature). There are many ways to generate quadrature signals.
The two halves of the signal are then passed through independent variable attenuators, which can also provide 180 degree phase shift. One way to do this is with a reflection attenuator, where the matched terminations can be changed from open circuit to short circuit or any impedance in between with constant phase angle. The lowest loss states are the open and short circuit, while the highest loss state is at Z0 (typically 50 ohms, and theoretically infinite loss)
Then the two signals are recombined using an in-phase combiner such as a Wilkinson.
By setting the magnitude of the I and Q vectors, any amplitude or phase is possible.
Often a vector modulator is provided on a silicon integrated circuit. Depending on the implementation, the vector modulator may or may not be a reciprocal network.
One complaint about vector modulators is that they often have very low compression point.