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Message-ID: <00bb01cb34cd$e4675b70$4001a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4C58579F.30406@telus.net> <8CD01F14619A000-1C48-3D75@webmail-d073.sysops.aol.com> <8CD02ACE55ECBAC-1CF4-9FFF@webmail-m086.sysops.aol.com> <4C5ABD7D.2080301@iup.uni-heidelberg.de> <008501cb34c4$51e99cd0$4001a8c0@lark> <4C5AFD94.4080902@iup.uni-heidelberg.de>
Date: Thu, 5 Aug 2010 19:41:53 +0100
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Subject: Re: LF: Re: Bandpass filter design
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Hi Stefan I was surprised to see that message come though I thought I=
 had
had second thoughts and deleted it........ nevertheless the sentiment=
 is
still valid.

If the signal from DLF overloads the input maybe you need to look a bi=
t more
carefully at the first stage.. I suspect that fet is maybe not the bes=
t
choice. Look at the AMRAD design which uses a CP600 I believe. This is=
 a bit
difficult to find now but devices like the J310 run at fairly high cur=
rents
do quite a good job, There is an alternative to the AMRAD design using=
 the
J310 on someones  web site where the performance is compared with the
original. It might be Jay W1VD who did these tests. They are dogged wi=
th
local MF BC stations in the States so it needed to be quite good. DLF=
 may be
far enough away to drop a bridged-T notch filter on, as an alternative=
..
see http://www.alan.melia.btinternet.co.uk/BR-t-Notch.htm
It needs a low impedance feed and a high impedance load from memory so=
 would
fit well in the placce of your band pass filter. Hugh M0WYE used one=
 of
these with a simple receiver. It will only give 20dB+ possible 30dB
reduction, but that is well worth having..

...and yes we are luckier here DCF39 is not so much trouble :-)) it is
around 1mV/m in the UK in daytime.

Alan G3NYK




----- Original Message -----=20
From: "Stefan Sch=C3=A4fer" <schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, August 05, 2010 7:06 PM
Subject: Re: LF: Re: Bandpass filter design


Thanks for all answers so far.

Alan, i think that filtering is essential for me since DLF (153 kHz,
500kW) is just 40km distanced. I am afraid that the amplifier gets
overdriven else. For DCF39, which is now almost not filtered (maybe
10...20 dB) it is the same. This level is 60 dB above noise and i need
some 20 dB more gain after the BF981 since the level is still to low=
 for
the soundcard. SpecLab can do a lot, clear, but what if the amplifier
becomes nonlinear due to those high levels? This is my worry. In UK,=
 DLF
or DCF39 is not that issue i assume (?) ;-)

What is the problem with rapid phase changes? I am no communication
engineer, sri, but i like to learn on that! (remembering that before
some months i asked what is the "gain" when going from qrss3 to qrss30
;-) ).

Michels idea sounds good. In principal, this is the same what i have
done before the mixer. Applying 2 further band filters means parts
effort but this doesn't matter really. If i put one filter to 12,7 kHz
(QRSS3) and the other to say 11,6 kHz (CW and european transmit window=
)
this would work, regarding the attenuation of DCF39. And i can adjust
the filter characteristic directly by watching the SpecLab window and=
 a
broad band noise source (my soldering station does a good job there wh=
en
placing the antenna near it :-) ).

I will keep your suggestions in mind. Maybe a call to Markus can
convince my completely ;-)

73, Stefan



Am 05.08.2010 17:00, schrieb Alan Melia:
> 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 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?=
?)
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
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 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
>>
>>
>