Evan,
That "Better" filter does look better.
The original was starting to reject rather seriously at 29mhz, probably too low.
This one is is more like 37mhz, too high?
You're right, when evaluating a filter we should drive it from a 50 ohm source impedance.
But measuring the filter impedance should be done at the node after that 50 ohms,
not before it.? This can be done by plotting V(n006)/I(Vin), with R1 set to 50 ohms.
The name of your wire between R1 and the filter is n006.? ?We could use V(n006)/I(R1),
but it then complains when we set R1 to 0 ohms.
Evaluating as above with a 50 ohm source impedance at R1 shows only a slight change
from evaluating with a 0 ohm source impedance.? But changes in the filter load
impedance at R2 bring on a far greater change in filter input impedance.
For this reason, connecting your new filter to Q90 instead of the 50 ohm load at R2
shows a large?variation in system input impedance.?
You can flip between any two settings easily with the undo/redo icons and then just hit the running man.
Whether those settings are in the plot window (perhaps V(n006)/I(Vin) vs 1v/(I(Vin) )? or in the
schematic window (perhaps R1=50 ohms vs R1=0 ohms) depends on which window is selected.
Jerry, KE7ER
