Can someone please explain to me how a passive filter could possibly have a negative insertion loss? In the attached picture, this can be seen in the C or D curves where the insertion loss dips below 0 dB. I understand the filter is resonant at this frequency, but I don't understand how resonance could lead to a negative insertion loss on a passive circuit. This seems to conflict with the fact that passive circuits can't generate power.
Passive circuits cannot produce a power gain but they can produce a voltage gain. A resonant circuit can produce a voltage gain equal to its Q. If the impedance levels are considered, there is no power gain.
There is a nice white paper from SCHAFFNER explaining the 0.1/100 Ohm curves.
Note that attenuation is measured as a voltage ratio (not a power ratio), so when working at different input/output impedances, you can actually have a voltage gain.
