Message-ID: <50722034.7090304@iup.uni-heidelberg.de>
Date: Mon, 08 Oct 2012 02:37:08 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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References: <5070C574.4030005@iup.uni-heidelberg.de> <op.wltdilzsyzqh0k@pc-roelof> <5071A24A.2000105@iup.uni-heidelberg.de> <6616A6979A1D48D8AD7E1CC48E89C0B0@gnat>
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Subject: Re: LF: GRRRRR!! Now i know...
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Hi Alan,

Yes, that's it! :-) Crazy, what a day again!

Today i discovered and solved several problems, eeeh i mean challenges :-=
)

First the AFN signal, overloading the probe. Now i've added a series LC=20
of 100uH and 180+4.7 pF across the drain-source of the first JFET,=20
forming a notch filter with this frequency response=20
https://dl.dropbox.com/u/19882028/MF/1143kHz%20dip2.png

This reduced the AFN signal to a level similar to DLF 153 kHz, i.e.=20
arround 0.5 Vp.

There are still IM issues though but i discovered that i have protection=20
diodes on the RX input, single antiparallel 1N4148 diodes, which of=20
course begin to conduct at 0.5Vp. So i will change this into 3x=20
antiparallel 1N4148 diodes in series. This will certainly help, if it=20
was the main reason now.

And i think i have to spend some care to a correct termination of the=20
cables with 50 Ohm. I measured the output impedance of the MF mechanical=20
filter circuit i'm using on MF and found that i have to add 22 Ohm in=20
series to achieve 50 Ohm. During the measurements i saw that i forgot to=20
add a 100 nF C behind the 220 uF coil in the supply. This killed the=20
input JFET which explains why the MF signals dropped by 20 dB and have=20
been very close to the RX noise today... Lastly i found that the BF862=20
seems to make VHF oscillations, giving a strange AM modulation on the=20
output signal. After adding a ferrite perl, everything was/is fine now!!
The circuit is now updated=20
https://dl.dropbox.com/u/19882028/MF/MF%20Filter%20schematic.png

VHF oscillations on the probe (?), bad termination of the 30m long RG58=20
cable and the single 4148 protection diodes on the RX are now the most=20
probably reasons why the system still suffers from QRM. Tasks for=20
tomorrow and new reasons to be optimistic! :-)

Thanks to all for the discussion about the AFN signals and probes.

73, GN, Stefan/DK7FC




Am 07.10.2012 22:18, schrieb Alan Melia:
> Hi Stefan Problems ?? .....no problems on LF, just interesting=20
> challenges :-))
> Alan
> G3NYK
>
> ----- Original Message ----- From: "Stefan Sch=E4fer"=20
> <Stefan.Schaefer@iup.uni-heidelberg.de>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Sunday, October 07, 2012 4:39 PM
> Subject: Re: LF: GRRRRR!! Now i know...
>
>
>> Hi Roelof,
>>
>> Thanks for your comments and nice to see you'r arround here :-)
>>
>> Am 07.10.2012 17:04, schrieb Roelof Bakker:
>>> Regarding strong signal handling capabilities of the antenna, are=20
>>> you sure that the JFET is the problem?
>> Not sure. Maybe it happens in the 2nd stage, i.e. the 2N3053. 4Vpp=20
>> should still be managable.
>>
>>> You can try to increase the standing current in the output=20
>>> transistor or try a LPF between the first and the second stage.
>> This is the plan for the coming few hours :-)
>>
>>> Raising the supply voltage to 24 volt will also help; this depends=20
>>> on the transistor you use of course.
>> Yes, looks we have the same ideas :-)
>>
>> I'm really looking forward to the new results. Thanks for your=20
>> comments and suggestions. It makes me even more optimistic.
>>
>> BTW What would this hobby be without having problems from time to=20
>> time !!?? :-)
>>
>> OK, the show starts now. Must go out before it becomes dark... Slept=20
>> until 14 o'clock local today ;-)
>>
>> 73, Stefan/DK7FC
>>
>>>
>>> 73,
>>> Roelof, pa0rdt
>>
>