A diplexer presents a broadband termination to e.g. a mixer, which is connected on the left side. Due to the series resonance circuit of C1 and L1, only a small bandwidth can pass to the next stage e.g. the if - amplifier, which needs only to present a good match for this frequency range. All other frequencies, above and below, 'see' the two 50 Ω resistors.

For a Q=10, the Bandwidth is about 1 MHz at 10 MHz Centerfrequency. (as expected)

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