  SRON is developing the Frequency Domain Multiplexing (FDM) readout and the
ultra low NEP TES bolometers array for the infrared spectrometer SAFARI on
board of the Japanese space mission SPICA. The FDM prototype of the instrument
requires critical and complex optimizations. For single pixel characterization
under AC bias we are developing a simple FDM system working in the frequency
range from 1 to 5 MHz, based on the open loop read-out of a linearized
two-stage SQUID amplifier and high Q lithographic LC resonators. We describe
the details of the experimental set-up required to achieve low power loading (<
1 fW) and low noise (NEP $\sim 10^{-19} W/Hz^{1/2}$) in the TES bolometers. We
conclude the paper by comparing the performance of a $4 \cdot 10^{-19}
W/Hz^{1/2}$ TES bolometer measured under DC and AC bias.
