  We present surface impedance measurement of the vortex linear response in a
large untwinned YBCO crystal. The depinning spectra obtained over a broad
frequency range (100 Hz- 30 MHz) are those of a surface pinned vortex lattice
with a free flux flow resistivity (two modes response). The critical current in
the "Campbell" like regime and the flux flow resistivity in the dissipative
regime are extracted. Those two parameters are affected by the first order
transition, showing that this transition may be related to the electronic state
of vortices.
