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Dear all
Following all the discussion of the "G3YXM design"
filter, I feel I ought to let you in on the time consuming process I used
to arrive at those values:
Stage 1. Look up a topband PA design ARRL
handbook.
Stage 2. Copy
the 2.02MHz lowpass filter design and multiply all the values by
10.
Stage 3. Try
it and see if it works!
I did
figure that second harmonic output attenuation wasn't goint to be an issue with
a square-wave PA so the 200kHz + cutoff shouldn't be a problem and measurements
back this up.
Incidently the
Ropex uses exactly the same values (weird eh? Did they work it out themselves,
copy me or use the same topband design??) but theirs has a nice little
refinement, they placed a 3.3nF C (I think) across the second inductor to
produce a third-harmonic trap.
I have tried
other filter values but those are the ones that work best for efficiency and
that's what was important for the absence of smoke in the
shack.
KISS
73
Dave
G3YXM
Dear Tom, LF Group,
There are 2 main requirements for a TX output
filter
a) Attenuate harmonics to acceptable
level
b) Provide the right load impedance for
the PA - usually a pure resistance at the output frequency.
Usually, the impedance at the input of the filter
is made the same as the load, I assume 50R. There are a number of ways of
doing this:
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