  We are developing a kilo-pixels Ti/Au TES array as a backup option for Athena
X-IFU. Here we report on single-pixel performance of a 32$\times$32 array
operated in a Frequency Division Multiplexing (FDM) readout system, with bias
frequencies in the range 1-5 MHz. We have tested the pixels response at several
photon energies, by means of a $^{55}$Fe radioactive source (emitting
Mn-K$\alpha$ at 5.9 keV) and a Modulated X-ray Source (MXS, providing
Cr-K$\alpha$ at 5.4 keV and Cu-K$\alpha$ at 8.0 keV). First, we report the
procedure used to perform the detector energy scale calibration, usually
achieving a calibration accuracy better than $\sim$ 0.5 eV in the 5.4 - 8.9 keV
energy range. Then, we present the measured energy resolution at the different
energies (best single pixel performance: $\Delta$E$_{FWHM}$ = 2.40 $\pm$ 0.09
eV @ 5.4 keV; 2.53 $\pm$ 0.10 eV @ 5.9 keV; 2.78 $\pm$ 0.16 eV @ 8.0 keV),
investigating also the performance dependency from the pixel bias frequency and
the count rate. Thanks to long background measurements ($\sim$ 1 day), we
finally detected also the Al-K$\alpha$ line at 1.5 keV, generated by
fluorescence inside the experimental setup. We analyzed this line to obtain a
first assessment of the single-pixel performance also at low energy
($\Delta$E$_{FWHM}$ = 1.91 eV $\pm$ 0.21 eV @ 1.5 keV), and to evaluate the
linearity of the detector response in a large energy band (1.5 - 8.9 keV).
