Three-frequency resonances in coupled phase-locked loops

We construct an experimental model of a nonlinear dynamical system with three frequencies. With this analog electronic circuit made up of quasi-periodically forced coupled phase-locked loops, we investigate the structure of the scaling of three-frequency resonances or lockings in the dynamics. We hypothesize and confirm experimentally that for weak coupling the three-frequency resonance with the largest frequency-locked plateau in the space of parameters in the interval between two adjacent resonances p/q and r/s is given by the mediant (p+r)/(q+s). We expect this to be universal behavior in systems of three coupled oscillators.