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 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. 
P. Caputo, J. Tomes, J. Oppenlaender, Ch. Haeussler, A. Friesch, T.   Traeuble, and N. Schopohl