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D.I.Y. ON THE BW
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A Note from author:
I'm sure a real DIY, all receivers that have passed into my hands,
more or less have received some kinda of modifications.
But to modify
a BW is NOT a good affair. So suggest you or to leave modifications
more as possible OUTSIDE BW or better than ever, to buy a better
receiver instead!!!
If you can't
stay on hold and WANT to modify your BW, please almost leave circuit
board
and surrounding with a chance to back to original condition, there
aren't so many WORKING Barlows around the world, today...unfortunately
:( ...thanks :) FC.
1)
A Simple 30 Mhz Low Pass Filter
An annoying feature of BW sure
is the easy overmodulation of first RF stage to receive distorted
local strong FM signals from some mixing with local 45 Mhz upconversion
oscillator. Apart to that tuning trick described in FeedBack
page or in the Literature one, better
of all is to built a 30 Mhz Low-Pass filter.
With this simple circuit, all the
frequencies ABOVE 30 Mhz will be drastically cutted, and so
FM too.
Look the schematics:
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As you've seen, this is a very
simple circuit. It consist only in two impedances (L1= 1 microhenry
L2=2.2 microhenry) and three 10 pf ceramic capacitors. It
will be assembled in a metallic case with a metallic shield
between first Z and first capacitor and the rest of circuit
(see schematic and layout). Input impedance is 50 Ohm,and
this should be used with a low impedance antenna to achieve
max attenuation of unwanted frequencies. Not so effective
with built-in stilo antenna.
A possible layout:
Attenuation at 100 Mhz will be over 40
Db.
Enjoy it!!!
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