  The present paper describes the effect of magnetic pick-up coil transfer
functions on mode number analysis in magnetically confined fusion plasmas.
Magnetic probes mounted inside the vacuum chamber are widely used to
characterize the mode structure of magnetohydrodynamic modes, as, due to their
relative simplicity and compact nature, several coils can be distributed over
the vessel. Phase differences between the transfer functions of different
magnetic pick-up coils lead to systematic errors in time- and frequency
resolved mode number analysis. This paper presents the first in-situ,
end-to-end calibration of a magnetic pick-up coil system which was carried out
by using an in-vessel driving coil on ASDEX Upgrade. The effect of the phase
differences in the pick-up coil transfer functions is most significant in the
50-250 kHz frequency range, where the relative phase shift between the
different probes can be up to 1 radian (~60{\deg}). By applying a correction
based on the transfer functions we found smaller residuals of mode number
fitting in the considered discharges. In most cases an order of magnitude
improvement was observed in the residuals of the mode number fits, which could
open the way to investigate weaker electromagnetic oscillations with even high
mode numbers.
