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
of 100uH and 180+4.7 pF across the drain-source of the first JFET,
forming a notch filter with this frequency response
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.
arround 0.5 Vp.
There are still IM issues though but i discovered that i have protection
diodes on the RX input, single antiparallel 1N4148 diodes, which of
course begin to conduct at 0.5Vp. So i will change this into 3x
antiparallel 1N4148 diodes in series. This will certainly help, if it
was the main reason now.
And i think i have to spend some care to a correct termination of the
cables with 50 Ohm. I measured the output impedance of the MF mechanical
filter circuit i'm using on MF and found that i have to add 22 Ohm in
series to achieve 50 Ohm. During the measurements i saw that i forgot to
add a 100 nF C behind the 220 uF coil in the supply. This killed the
input JFET which explains why the MF signals dropped by 20 dB and have
been very close to the RX noise today... Lastly i found that the BF862
seems to make VHF oscillations, giving a strange AM modulation on the
output signal. After adding a ferrite perl, everything was/is fine now!!
The circuit is now updated
https://dl.dropbox.com/u/19882028/MF/MF%20Filter%20schematic.png
VHF oscillations on the probe (?), bad termination of the 30m long RG58
cable and the single 4148 protection diodes on the RX are now the most
probably reasons why the system still suffers from QRM. Tasks for
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
challenges :-))
Alan
G3NYK
----- Original Message ----- From: "Stefan Schäfer"
<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
you sure that the JFET is the problem?
Not sure. Maybe it happens in the 2nd stage, i.e. the 2N3053. 4Vpp
should still be managable.
You can try to increase the standing current in the output
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
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
comments and suggestions. It makes me even more optimistic.
BTW What would this hobby be without having problems from time to
time !!?? :-)
OK, the show starts now. Must go out before it becomes dark... Slept
until 14 o'clock local today ;-)
73, Stefan/DK7FC
73,
Roelof, pa0rdt
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