• Frequency selective power limiter
    You need some frequency-selective component to be able to discriminate between the 2 signals at different frequencies.

    LPF filter is the most general solution (works over the widest frequency range).

    If the frequencies are known in advance, a notch filter or harmonic filter might work better.

    The quarter-wave stub idea is a type of notch filter to create destructive interference at the frequency you want to reject. You could use a tuneable quarter-wave stub if you need the filter to be tuneable, though it'll only notch out one frequency at a time.

    A diplexer is a 3-port filter, with a common input and 2 outputs: LPF + HPF, typically with less insertion loss than splitting the signal and filtering each signal individually.

    You can make a poor-man's diplexer:
    input --> circulator <--> LPF --> low freq output
                       \---------> HPF --> high freq output
    Though you're relying on the LPF to reflect the out-of-band signals at the input port, which isn't always the case for LPF filters.


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