We designed two low noise amplifiers, one at 144 MHz and another at 440 MHz using this LNA; each design is tuned for the particular band. We noticed amplification at 440 MHz is clean and crisp, but amplification at 144 MHz is scratchy with FM audio signals. (The 144MHz LNA measures S21=+28dB and S11=-19dB with a VNA.)
Someone at a conference we went to recently told us that an RC filter is often used on an LNA output to reduce unwanted noise in the intended band. Later I wondered what type of filter to use and how it should be tuned, but the conversation was in passing I do not have access to the person to ask directly. He said that RC filters on LNA outputs have been used for years, so it sounds like this is a common thing to do.
Primary question:
What is the best practice for RC filters on an LNA output? HP, LP, or BP?
What dB drop across the filter is acceptable (or necessary) in the passband to operate as an appropriate filter? (We get 28dB gain at 144MHz so, if necessary, losing a few dB in the filter isn't that critical.)
More detail mixed with my speculation
Here they are, outputs are on the female SMA side (with the flat-side of the PCB down, output is on the right):
Below is the matching schematic, roughly oriented to the 2m picture above. Notice that S7 is disabled in the schematic (and bare pads/missing in the bottom-right of picture above), so we were thinking of adding a cap to pull down higher frequencies (in lieu of an RC filter?):
This is the output match by itself, that shows a highpass behavior on the 270pF output cap with S7 empty (bottom right of the 2m PCB picture):
This is a simulation with a 22pF shunt cap in S7 (hoping to avoid a board-redesign, but we can if necessary):
There is a big issue with this 22pF shunt: In S2P simulation (no EM), it kills the S11 input match, going from -27dB to -10dB. Smaller shunt caps affect the match less, but of course with less high-frequency attenuation. (Not sure how the real life match is affected without trying it, thought I'd ask the experts here, first.)
Suggestions?