[HomeBrew] Pixie II Output Filter

[Ian Wilson]

The simplest way to answer this question is to locate one of the free
pi-network
calculators on the web and play around with the impedance presented to the
PA transistor, and the Q of the pi network. For the Pixie, an impedance of
40
ohms and a Q of 1.7 give values that are close to what's on the schematic.

The reason why a simple LC calculation doesn't work is because the impedance
of the C's is comparable to the other impedances. So you need to transform
the
pair of parallel CR's at each end of the pi network to series equivalents.
Now you
have a series resonant circuit with two resistors, two capacitors, and the
original
inductor. However, the original 820pF capacitors have been transformed to
larger
values. These two values in series (using series capacitor formula) looks
like
about 1025pF. This is close to what's required to resonate with 2.2uH at
3.5MHz.

Scaling both the L's and C's by frequency leaves the Q unchanged and moves
the
operating frequency as desired.

Hope this helps.

73, ian K3IMW