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Re: LF: Bandpass filter design

To: [email protected]
Subject: Re: LF: Bandpass filter design
From: Daniele Tincani <[email protected]>
Date: Thu, 5 Aug 2010 10:09:31 -0700 (PDT)
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Hello Stefan, LF,
may be this could be of some help?
Regards
Daniele


From: Stefan Schäfer <[email protected]>
To: [email protected]
Sent: Thu, August 5, 2010 3:32:45 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

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