2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/108439The authors demonstrate quadratic mixing of weak time harmonic electromagnetic fields applied to Superconducting Quantum Interference Filters, manufactured from high-$T_{\mathrm{c}}$ grain boundary Josephson junctions and operated in active microcooler. The authors use the parabolic shape of the dip in the dc-voltage output around B=0 to mix \emph{quadratically} two external rf-signals, at frequencies $f_{\mathrm{1}}$ and $f_{\mathrm{2}}$ well below the Josephson frequency $f_{\mathrm{J}}$, and detect the corresponding mixing signal at $| {f_{1}-f_{2}}| $. Quadratic mixing takes also place when the SQIF is operated without magnetic shield. The experimental results are well described by a simple analytical model based on the adiabatic approximation.3 pages, 3 figuresSuperconductivitytext