  A circular ribbon flare SOL2014-12-17T04:51 is studied using the 17/34 GHz
maps from the Nobeyama Radioheliograph (NoRH) along with (E)UV and magnetic
data from the Solar Dynamics Observatory (SDO). We report the following three
findings as important features of the microwave CRF. (1) The first preflare
activation comes in the form of a gradual increase of the 17 GHz flux without a
counterpart at 34 GHz, which indicates thermal preheating. The first sign of
nonthermal activity occurs in the form of stepwise flux increases at both 17
and 34 GHz about 4 min before the impulsive phase. (2) Until the impulsive
phase, the microwave emission over the entire active region is in a single
polarization state matching the magnetic polarity of the surrounding fields.
During and after the impulsive phase, the sign of the 17 GHz polarization state
reverses in the core region, which implies a magnetic breakout--type eruption
in a fan-spine magnetic structure. (3) The 17 GHz flux around the time of the
eruption shows quasi-periodic variations with periods of 1--2 min. The
pre-eruption oscillation is more obvious in total intensity at one end of the
flare loop, and the post-eruption oscillation, more obvious in the polarized
intensity at a region near the inner spine. We interpret this transition as the
transfer of oscillatory power from kink mode oscillation to torsional Alfv\'en
waves propagating along the spine field after the eruption. We argue that these
three processes are inter-related and indicate a breakout process in a
fan-spine structure.
