Date: Thu, 5 Aug 2010 19:18:56 +0200
From: Michel F5WK <f5wk@mterrier.net>
Message-ID: <1592352911.20100805191856@mterrier.net>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Bandpass filter design
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Hi Stefan,

Check out http://www.aade.com/filter.htm for a good & free filter design
tool. It does support coupled resonator band-pass filters also.

Coupled Resonator band-pass filters are likely to be the best match for yo=
ur
requirement at the expense of high insertion loss. I don't think you'll ge=
t
better with Cauer or elliptic filters where you have to take the inductor'=
s Q
into account at design time.

You could also add another resonator at the antenna which will provide you
with some extra gain and filtering. In that case have the greatest L/C rat=
io
as possible and extend the length of the whip to 1 meter . This arrangemen=
t
is sometimes referred as 'tuned whip'.

Good luck,

Michel - F5WK


Le jeudi 5 ao=C3=BBt 2010 =C3=A0 15:32:45, vous =C3=A9criviez :

> Dear LF,

> Currently i am setting up my active antenna for the planned LF grabber=
=20
> here in Heidelberg. It is an active E field antenna, using a BF981 and=
 a=20
> 125 kHz signal that transforms the 137 kHz down to 12 kHz where some=20
> band filtering has to be applied. Then, i need another amp stage to=20
> drive the soundcards input (BF862). The high impedance of the wire input=
=20
> is first down transformed by a BF862 stage as a source follower, then i=
=20
> allpy a double LF bandpassfilter that is coupled by a C of some pF=20
> (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=
=20
> L parallel C resonated at 12 kHz (after the mixing stage), between=20
> signal and ground since this gives a sharp filter, ie 137,7 kHz is=20
> already attenuated by 25 dB compared to 137,0 kHz. What i want to have=
=20
> is a filter with a specific bandwith and edge frequencies with about=20
> constant low attenuation in the transmission range and relative sharp=20
> slopes so that 137,7 kHz is not really attenuated but 138,83 kHz=20
> (DCF-39) as much as possible. DCF39 is 60 dB above noise here although=
=20
> 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=20
> frequencies, input- output impedances and so on and get the values?
> I have found such one at=20
> http://www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter but i am not yet=
=20
> experienced too much about this stuff so i am not sure if this gives=20
> really useful answers.

> Any other simple ideas to come quickly to the optimal filter type, oder=
=20
> and values? I do not want to spend too much time for that, so an=20
> "excellent filter design book" is not the best hint =20

> What about a cauer filter? I have read that it has the sharpest edges=20
> but this may cause QRM in the pass band? (like clicks in a too sharp CW=
=20
> filter?)

> The picture shows what i have done so far. Watching the spectrum on the=
=20
> roof  of the institute (the future QTH) from 0...48 kHz in SpecLab looks=
=20
> very promising so far (see picture). DLF is 60 dB above noise although=
=20
> already attenuated about 60 dB! So filtering before mixing and further=
=20
> amplification is necessary in my case, i assume...

> Tnx for helping ideas.

> 73, Stefan/DK7FC