LF: Bandpass filter design

[Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de>]

Dear LF,

Currently i am setting up my active antenna for the planned LF grabber 
here in Heidelberg. It is an active E field antenna, using a BF981 and a 
125 kHz signal that transforms the 137 kHz down to 12 kHz where some 
band filtering has to be applied. Then, i need another amp stage to 
drive the soundcards input (BF862). The high impedance of the wire input 
is first down transformed by a BF862 stage as a source follower, then i 
allpy a double LF bandpassfilter that is coupled by a C of some pF 
(about 4...8 pF). This signal is applied to the 2nd Gate of the BF981...
My question is: There may be better suited filter designs than taking a 
L parallel C resonated at 12 kHz (after the mixing stage), between 
signal and ground since this gives a sharp filter, ie 137,7 kHz is 
already attenuated by 25 dB compared to 137,0 kHz. What i want to have 
is a filter with a specific bandwith and edge frequencies with about 
constant low attenuation in the transmission range and relative sharp 
slopes so that 137,7 kHz is not really attenuated but 138,83 kHz 
(DCF-39) as much as possible. DCF39 is 60 dB above noise here although 
it gets already attenuated by the input band filter!
Jim/M0BMU has designed a filter for his VLF loop RX that looks quite 
good. Is there a web page where i just can type the filter oder, edge 
frequencies, input- output impedances and so on and get the values?
I have found such one at 
http://www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter but i am not yet 
experienced too much about this stuff so i am not sure if this gives 
really useful answers.
Any other simple ideas to come quickly to the optimal filter type, oder 
and values? I do not want to spend too much time for that, so an 
"excellent filter design book" is not the best hint ;-)
What about a cauer filter? I have read that it has the sharpest edges 
but this may cause QRM in the pass band? (like clicks in a too sharp CW 
filter?)
The picture shows what i have done so far. Watching the spectrum on the 
roof  of the institute (the future QTH) from 0...48 kHz in SpecLab looks 
very promising so far (see picture). DLF is 60 dB above noise although 
already attenuated about 60 dB! So filtering before mixing and further 
amplification is necessary in my case, i assume...
Tnx for helping ideas.

73, Stefan/DK7FC

LF active antenna under construction.jpg
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Spectrum at the gate of the BF981.jpg
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Circuit_under construction.jpg
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