Hi Stefan!
Tnx for nice qso last night!
My first qrq on mf!
73,
Df5qg

Christian Groeger

Stefan Schäfer <[email protected]> schrieb:

>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
>>>
>>
>