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
