  Microwave frequency domain multiplexing is a suitable technique to read out a
large number of detector channels using only a few connecting lines. In the
HOLMES experiment this is based on inductively coupled rf-SQUIDs
(Superconducting QUantum Interference Devices) fed by TES (Transition Edge
Sensors). Biasing of the whole rf-SQUID chain is provided with a single
transmission line by means of the recently introduced flux-ramp modulation
technique, a sawtooth signal which allows signal reconstruction while operating
the rf-SQUIDs in open loop condition. Due to the crucial role of the sawtooth
signal, it is very important that it does not suffer from ground loop
disturbances and EMI. Introducing a transformer between the biasing source and
the SQUIDs is very effective in suppressing disturbances. The sawtooth signal
has slow and fast components, and the period can vary between a few kHz up to
MHz depending on the TES signal and SQUID characteristics. A transformer able
to face such a broad range of conditions must have very stringent
characteristics and needs to be custom designed. Our solution exploits standard
commercial, and inexpensive, transformers for LAN networks used in a suitable
combination. A model that allows to take care of the low as well as of the high
frequency operating range has been developed.
