  In the presence of weak time harmonic electromagnetic fields, Superconducting
Quantum Interference Filters (SQIFs) show the typical behavior of non linear
mixers. The SQIFs are manufactured from high-T_c grain boundary Josephson
junctions and operated in active microcooler. The dependence of dc voltage
output V_dc vs. static external magnetic field B is non-periodic and consists
of a well pronounced unique dip at zero field, with marginal side modulations
at higher fields. We have successfully exploited the parabolic shape of the
voltage dip around B=0 to mix quadratically two external time harmonic
rf-signals, at frequencies f_1 and f_2 below the Josephson frequency f_J, and
detect the corresponding mixing signal at f_1-f_2. When the mixing takes place
on the SQIF current-voltage characteristics the component at 2f_2 - f_1 is
present. The experiments suggest potential applications of a SQIF as a
non-linear mixing device, capable to operate at frequencies from dc to few GHz
with a large dynamic range.
