Hi Stephan....why do you think you need a narrow filter at 12kHz?? Why not
let the sound card sort it out? Provided you have killed the image (113kHz)
there should not be a problem. If you use a narrow passive filter you risk
rapid phase changes near the wanted frequency. This is probably not a good
idea. I suspect that a fairly "benign" low pass filter (Butterworth??) just
above 12Khz (to aid the anti-alias filtering) and another Butterworth to
remove any 50Hz and low harmonics of that say below 1kHz. this leaves a
fairly flat pass-band with a slowly changing phase response.

What may be more important may be getting a good low noise amp to feed the
sound-card. It is worth a look at some of the softrock workand circuits
here. Also Paul did some work on this some time back. Jim may have some more
helpful ideas in this area. I have not found conventional filtering in front
of an FFT does a lot of good and it certainly has the potential to "muddy"
things up.

Alan G3NYK

----- Original Message ----- 
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Thursday, August 05, 2010 2:32 PM
Subject: LF: Bandpass filter design


> 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
>