With all the filter discussion regarding uBITX I decided to take a look at design of
the output filter on the BITX-40.
This LTSpice run shows that the stock LPF starts to roll off at about 8 MHz, which is
pretty good. Problem is that the magic -43 db point does not come until about 21 MHz
or about the 3rd harmonic area. Can this be imporoved...maybe.
By adding just one capacitor across the output inductor we now get much more
attenuation at the 3rd harmonic point of 21 MHz. By changing the value of C3
it is possible to move that point up or down in frequency. This might be a way
to attenuate a specific harmonic or spur if it does not move about too much.
Next step is probably to scan the BITX-40 output to see if it really needs this type
of modification (how clean is it?).
It should also be possible to add a similar capacitor across the L6 inductor to have
another attenuation point. That is a test for tomorrow because it is late and us old
guys need their beauty sleep. 8-)
NOTE: The upper trace in the pictures is frequency response. The lower trace is phase
shift, which could be important if oscillation is encountered. Note the sharp phase
shift at the parallel resonance point in the 2nd picture.