|LC Tuned Circuit.|
|Passes signals at fo. Attenuates signals at other frequencies.|
As Q = fo/Δf, for Δf to remain constant requires fo/Q to remain constant i.e. Q must vary in proportion with fo.
In the top diagram, inserting a resistance of value "R" in series with the L and the C sets the Q.
Q = ωL/R, where ω = 2*π*fo. For Q to vary in proportion with fo requires R to be constant.
This was achieved using wideband RF transformers to transform the source & load impedances from 50Ω down to a suitable "R" value. Here endeth today's trip around my brain.