  We study the transport properties of a quantum dot driven by either a
rotating magnetic field or an ac gate voltage using the Floquet master-equation
approach. Both types of ac driving lead to photon-assisted tunneling where
quantized amounts of energy are exchanged with the driving field. It is found
that the differential-conductance peak due to photon-assisted tunneling does
not survive in the Coulomb-blockade regime when the dot is driven by a rotating
magnetic field. Furthermore, we generalize the MacDonald formula to calculate
the time-averaged noise spectra of ac-driven quantum dots. Besides the peak at
zero frequency, the noise spectra show additional peaks or dips in the presence
of an ac field. For the case of an applied ac gate voltage, the peak or dip
position is fixed at the driving frequency, whereas the position changes with
increasing amplitude for the case of a rotating magnetic field. Additional
features appear in the noise spectra if a dc magnetic field is applied in
addition to a rotating field. In all cases, the peak or dip positions can be
understood from the energy differences of two available Floquet channels.
