Observe amplitude of blue waveform over time and how green wave adds and subtracts amplitude of blue output. If displayed at faster rate the amplitude of wave swelling and shrinking would look like a "beat".
Adjust lower frequency up to 10 MHz. Even if out of phase , the amplitude of blue wave is level and up and down amplitude stops changing. YouTube clip of WWV setting test equipment shows one or two Hertz difference at 10 MHz. can be adjusted using " zero beat" to calibrated frequency gear.
