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From: "Alan Melia" <alan.melia@btinternet.com>
To: "RSGB LF Gp" <rsgb_lf_group@blacksheep.org>
References: <4CCC8022.90003@iup.uni-heidelberg.de> <004e01cb7883$96949580$4001a8c0@lark> <4CCCA351.1070508@iup.uni-heidelberg.de>
Date: Sun, 31 Oct 2010 00:32:00 +0100
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Subject: Re: LF: Re: SBL-3+
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Hi Stefan No, you are right you do need low frequency paths on both si=
des of
the mixer. I have a "lateral" approach to technical questions which st=
arts
by saying "why do you want to do it that way"!

I guess the SBL-3 unit you wanted will solve the problem. If you need=
 more
reduction in the 125kHz, look at my web site for a bridged-T passive=
 notch
which was designed for 139kHz but would tune easily to 125. It might=
 give
you 25 to 30 dB rejection of the
LO. Watch the load requirements though....I seem to rememebr you may=
 need to
terminate with a high impedance buffer......emitter follower....to get=
 best
results from it.
http://www.alan.melia.btinternet.co.uk/BR-t-Notch.htm
It wont matter if this drifts a bit :-))

I am a little surprised the filter drifts that badly with
temperature.....wrong type of ferrite ?? You didnt have DC passing thr=
ough
any of the windings I suppose....that might give trouble.

Great experimentation.....well done

Alan G3NYK


----- Original Message -----=20
From: "Stefan Sch=E4fer" <schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, October 30, 2010 11:59 PM
Subject: Re: LF: Re: SBL-3+


Hi Alan,

I want to try an alternative to the BF981 which runs in my current LF
grabber RX. It downconverts 137 kHz to 12 kHz and feeds the soundcard
with this 12 kHz signal. It is running not too bad but i have some
problems with temperature drifting bandpass filters (since weeks) and
have thrown them out of the Drain-to-Plus path. So, now the setup is
running with a 2 stage RC LPF. I have had some problems with a to high
125 kHz level on the soundcard input and have applied a series LC
circuit for 125 kHz now which reasonably solved the problem. But the=
 LF
noise is now just about 10 dB above the soundcard noise, in the
afternoon. There is still a lack of sensitivity. This RX QTH should be
very well for a grabber antenna but currently the setup is the problem=
.
So i want to try something different, like a DBM. I am not an expert
here and can/have to learn a lot, it seems ;-)

73, Stefan

Am 31.10.2010 00:41, schrieb Alan Melia:
> Hi Stefan why a difficult to get mixer?? the IF port on most DBMs is=
 DC
> coupled so feed the siganl in there and (i presume) mix up ?? Or is=
 the
> RF/LO response still too high on the easily available ones?
>
> Alan G3NYK
> ----- Original Message -----
> From: "Stefan Sch=E4fer"<schaefer@iup.uni-heidelberg.de>
> To:<rsgb_lf_group@blacksheep.org>
> Sent: Saturday, October 30, 2010 9:29 PM
> Subject: LF: SBL-3+
>
>
>
>> Dear LF,
>>
>> I want to try a LF converter using this mixer. Does anybody know a
>> (cheep!) distributor in Germany or EU for 1...3 pieces of it? I've=
 just
>> found one where it costs about 40 Euro and this is to much!!
>>
>> 73, Stefan/DK7FC
>>
>>
>