Hi Kevin, I think Jim has beaten me to the draw......why are so many people
afraid of inductors these days?? This strange idea preceded the digital
revolution that everything active ( using "non-ideal" op-amps) was "good".
Passive filters take a lot of beating (they can be beaten in some areas with
a lot of technology) for simpicity. This is particularly when the resistor
component like inductors are made readily available from Toko. Whilst all
filter design programs will yield "silly" inductor values it is possible to
do what we always used to in
amateur design....pick the nearest "prefered" value from the Toko list.
These are not as good as home wound pot cores that Jim suggests but a lot
less trouble. After selecting a "nearest" value use an AC analysis computer
program to check that the response is still reasonable ( expensive software
?? ....there is a free one on my web site!) and tweak the caps a little if
necessary.
As to the effects of DCF39 ....(and it may be being useful as a BFO in that
circuit !!) it is possible at the LF frequencies to build a bridged-T notch
filter that will remove DCF39 with little loss in the 136 band. You will
need a high Q inductor as its is only 1030Hz above the band , so a Q of
about 130 will still leave you with 3dB loss at the top of the band. It
should be possible to make a circuit with a Q or 500 to 600, which should
make the in-band loss almost negligible. There is even an example in the AC
analysis ZIP download (I think) using a 50uH inductor. The only difference
might be it is easier to centre-tap the coil that use a capacitor "tap"
(which was easier in the program)
It good to see "Radio Destructor" articles surfacing again, even if the
reason for them was that in 1964 a full spec transistor cost about 10% of
the average weekly wage, valves were a lot cheaper.
Happy constructing
Cheers de Alan G3NYK
[email protected]
http://www.alan.melia.btinternet.co.uk
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