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From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
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Date: Fri, 6 Aug 2010 00:38:09 +0100
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Subject: Re: LF: Re: Bandpass filter design
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Alan

Yes . your  right , any  'Q' up front is  a  bad thing , ringing etc
just go  for the  most  expensive   a/d  you  can  afford and
leave the  rest to the  dsp ... Simon Browns  new  sdr
leave the  filters in my  ra6790 well  behind ..

G ..

--------------------------------------------------
From: "Alan Melia" <alan.melia@btinternet.com>
Sent: Thursday, August 05, 2010 4:00 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: LF: Re: Bandpass filter design

>
> 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=
=20
> (113kHz)
> there should not be a problem. If you use a narrow passive filter yo=
u 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?=
?)=20
> just
> above 12Khz (to aid the anti-alias filtering) and another Butterwort=
h to
> remove any 50Hz and low harmonics of that say below 1kHz. this leave=
s 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 fe=
ed the
> sound-card. It is worth a look at some of the softrock workand circu=
its
> here. Also Paul did some work on this some time back. Jim may have=
 some=20
> more
> helpful ideas in this area. I have not found conventional filtering=
 in=20
> front
> of an FFT does a lot of good and it certainly has the potential to=
 "muddy"
> things up.
>
> Alan G3NYK
>
> ----- Original Message -----=20
> From: "Stefan Sch=C3=A4fer" <Stefan.Schaefer@iup.uni-heidelberg.de>
> To: <rsgb_lf_group@blacksheep.org>
> 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 grab=
ber
>> 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 som=
e
>> 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, th=
en 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 BF9=
81...
>>
>> My question is: There may be better suited filter designs than taki=
ng 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 abou=
t
>> constant low attenuation in the transmission range and relative sha=
rp
>> 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 altho=
ugh
>> it gets already attenuated by the input band filter!
>>
>> Jim/M0BMU has designed a filter for his VLF loop RX that looks quit=
e
>> good. Is there a web page where i just can type the filter oder, ed=
ge
>> 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 no=
t yet
>> experienced too much about this stuff so i am not sure if this give=
s
>> 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 edg=
es
>> but this may cause QRM in the pass band? (like clicks in a too shar=
p 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 altho=
ugh
>> already attenuated about 60 dB! So filtering before mixing and furt=
her
>> amplification is necessary in my case, i assume...
>>
>> Tnx for helping ideas.
>>
>> 73, Stefan/DK7FC
>>
>
>
>=20