  We successfully exploit the parabolic shape of the dc voltage output dip
around B=0 of a Superconducting Quantum Interference Filter (SQIF) to mix weak
external rf signals. The two tone response of weak time harmonic
electromagnetic fields has been detected on the spectral voltage output of the
SQIF at frequency f_0 = f_1 - f_2, for various frequencies f_1 and f_2 ranging
from few MHz up to 20 GHz. The two tone response is a characteristic function
of static magnetic field B and of bias current I_b, related to the second
derivative of the dc voltage output.
