Two-Tone Response in Superconducting Quantum Interference Filters

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 r f signals. The two-tone response of weak time harmonic electromagnetic fields has been detected on the spectral voltage output of the SQIF at frequency f₀ = f₁ - f₂, for various frequencies f₁ and f₂ 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.