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Date: Thu, 5 Aug 2010 10:09:31 -0700 (PDT)
From: Daniele Tincani <danieletincani@yahoo.com>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <4C5ABD7D.2080301@iup.uni-heidelberg.de>
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Subject: Re: LF: Bandpass filter design
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Hello Stefan, LF,
may be this could be of some help?
http://www.aade.com/filter.htm
Regards
Daniele




________________________________
From: Stefan Sch=E4fer <Stefan.Schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
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 her=
e in=20
Heidelberg. It is an active E field antenna, using a BF981 and a 125 kHz=
 signal=20
that transforms the 137 kHz down to 12 kHz where some band filtering has=
 to be=20
applied. Then, i need another amp stage to drive the soundcards input (BF8=
62).=20
The high impedance of the wire input is first down transformed by a BF862=
 stage=20
as a source follower, then i allpy a double LF bandpassfilter that is coup=
led by=20
a C of some pF (about 4...8 pF). This signal is applied to the 2nd Gate of=
 the=20
BF981...

My question is: There may be better suited filter designs than taking a L=
=20
parallel C resonated at 12 kHz (after the mixing stage), between signal an=
d=20
ground since this gives a sharp filter, ie 137,7 kHz is already attenuated=
 by 25=20
dB compared to 137,0 kHz. What i want to have is a filter with a specific=
=20
bandwith and edge frequencies with about constant low attenuation in the=
=20
transmission range and relative sharp slopes so that 137,7 kHz is not real=
ly=20
attenuated but 138,83 kHz (DCF-39) as much as possible. DCF39 is 60 dB abo=
ve=20
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=20
there a web page where i just can type the filter oder, edge frequencies,=
 input-=20
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/lc=
filter=20
but i am not yet experienced too much about this stuff so i am not sure if=
 this=20
gives really useful answers.

Any other simple ideas to come quickly to the optimal filter type, oder an=
d=20
values? I do not want to spend too much time for that, so an "excellent fi=
lter=20
design book" is not the best hint ;-)

What about a cauer filter? I have read that it has the sharpest edges but=
 this=20
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 ro=
of=A0 of=20
the institute (the future QTH) from 0...48 kHz in SpecLab looks very promi=
sing=20
so far (see picture). DLF is 60 dB above noise although already attenuated=
 about=20
60 dB! So filtering before mixing and further amplification is necessary=
 in my=20
case, i assume...

Tnx for helping ideas.

73, Stefan/DK7FC



     =20

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<html><head><style type=3D"text/css"><!-- DIV {margin:0px;} --></style></h=
ead><body><div style=3D"font-family:arial, helvetica, sans-serif;font-size=
:12pt"><DIV>Hello Stefan, LF,</DIV>
<DIV>may be this could be of some help?</DIV>
<DIV><A href=3D"http://www.aade.com/filter.htm">http://www.aade.com/filter=
.htm</A></DIV>
<DIV>Regards</DIV>
<DIV>Daniele<BR></DIV>
<DIV style=3D"FONT-FAMILY: arial, helvetica, sans-serif; FONT-SIZE: 12pt">=
<BR>
<DIV style=3D"FONT-FAMILY: arial, helvetica, sans-serif; FONT-SIZE: 13px">=
<FONT size=3D2 face=3DTahoma>
<HR SIZE=3D1>
<B><SPAN style=3D"FONT-WEIGHT: bold">From:</SPAN></B> Stefan Sch=E4fer &lt=
;Stefan.Schaefer@iup.uni-heidelberg.de&gt;<BR><B><SPAN style=3D"FONT-WEIGH=
T: bold">To:</SPAN></B> rsgb_lf_group@blacksheep.org<BR><B><SPAN style=3D"=
FONT-WEIGHT: bold">Sent:</SPAN></B> Thu, August 5, 2010 3:32:45 PM<BR><B><=
SPAN style=3D"FONT-WEIGHT: bold">Subject:</SPAN></B> LF: Bandpass filter=
 design<BR></FONT><BR>Dear LF,<BR><BR>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 im=
pedance 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 Gat=
e of the
 BF981...<BR><BR>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 cons=
tant 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 muc=
h as possible. DCF39 is 60 dB above noise here although it gets already at=
tenuated by the input band filter!<BR><BR>Jim/M0BMU has designed a filter=
 for his VLF loop RX that looks quite good. Is there a web page where i ju=
st can type the filter oder, edge frequencies, input- output impedances an=
d so on and get the values?<BR>I have found such one at http://www-users.c=
s.york.ac.uk/~fisher/cgi-bin/lcfilter but i am not yet experienced too muc=
h about this
 stuff so i am not sure if this gives really useful answers.<BR><BR>Any ot=
her simple ideas to come quickly to the optimal filter type, oder and valu=
es? I do not want to spend too much time for that, so an "excellent filter=
 design book" is not the best hint ;-)<BR><BR>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?)<BR><BR>The picture sho=
ws what i have done so far. Watching the spectrum on the roof&nbsp; of the=
 institute (the future QTH) from 0...48 kHz in SpecLab looks very promisin=
g so far (see picture). DLF is 60 dB above noise although already attenuat=
ed about 60 dB! So filtering before mixing and further amplification is ne=
cessary in my case, i assume...<BR><BR>Tnx for helping ideas.<BR><BR>73,=
 Stefan/DK7FC<BR></DIV></DIV></div><br>

      </body></html>

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