Three-frequency resonances in coupled phase-locked loops

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