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Re: LF: GRRRRR!! Now i know...

To: [email protected]
Subject: Re: LF: GRRRRR!! Now i know...
From: Stefan Schäfer <[email protected]>
Date: Mon, 08 Oct 2012 02:37:08 +0200
In-reply-to: <6616A6979A1D48D8AD7E1CC48E89C0B0@gnat>
References: <[email protected]> <op.wltdilzsyzqh0k@pc-roelof> <[email protected]> <6616A6979A1D48D8AD7E1CC48E89C0B0@gnat>
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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 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" <[email protected]>
To: <[email protected]>
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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