Vlf receiver (wilfried fritz design)

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dr_peppers

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Aug 13, 2017
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Hi Guys.
Not exactly lw, this is very long wave.
I built a version of wilfried fritz's Vlf receiver as found on vlf.it, I dont know if anyone here has any experience of one of these.
I'm finding it difficult to get the thing to work, I got the Lo to oscillate from around 400kc to about 650kc, so thats ok, then I tuned the Bfo so that the ceramic filter will pass it and generate a tone on receiving a Cw signal.
I also checked the audio bandpass filter with my sig gen, it performs as expected.
I used a J310 in place of the Bf966, and biased it slightly differently.
The problem is the circuit is as deaf as a post, I eve took the thing out in the fields to try it, same difference.
Anyone else any ideas.
Yes I'm new here.
 

majoco

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Dec 25, 2008
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Welcome to the forum, we'll help you all we can, but....

In no way is a J310 equivalent to a BF966 regardless of how you bias it. A BF966 is an insulated gate mosfet and has an extremely high input impedance which is why you should get good VLF reception on just a whip antenna. I suggest you try to get the right part and have another go - by the sounds of it, you have the rest of it working correctly so it shouldn't be too far away.... :)
 

dr_peppers

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Aug 13, 2017
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Hi,
You might be right, however a bf966 isnt an ignt, according to its data sheet its definately a fet, the major difference with it and a J310 is that the Bf device is dual gate, however one of the gates in grounded in the application anyways, the only other difference which may make a lot of difference is the gate source capacitance is double, I can compensate for that on the preselector, and were talking vlf not vhf.
I'll have another look at the datasheet for the J310, and maybe order some Bf966's.
 

dr_peppers

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Aug 13, 2017
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Ok so I'm a bit of a numpty.
I had pin 1 and pin 4 of the 455 kc filter module backwards, so the input went to the ground terminal and ground went to the input terminal, I should have wondered why there was a null at 455kc instead of a peak.
Now when I power the rx I get something, and it picks up my sig gen on 100kc across the bench.
However if I connect the Bfo the thing goes deaf as a post, I tried tuning the Bfo a little as the Bfo freq on this rx is critical, but its still as deaf as I am.
I think the o/p level of the BFo is so high its sending the agc into low gain.
Heres the page for the Rx I built:
http://www.vlf.it/fritz/pocketrx.html
Heres a vid of it:
https://www.youtube.com/watch?v=1aK_zNxkfuY
And heres another design I found that is similarish only Hf and dual conversion:
https://www.heinzstampfl.ch/wp-content/uploads/2015/03/junior1_assemblyguide.pdf
 
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