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Offline ebat
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« Reply #20 on: August 16, 2010, 06:39:46 am »
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Hello Eugene.  I could download all files from Luc on this side (reached 5 postings) - now I`m working on it.
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Erhard

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Offline xavier
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« Reply #21 on: August 16, 2010, 07:17:38 am »
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I wish you all the best on this project and truly hope that you succeed.

Regards Xavier

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So many questions so little time

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« Reply #22 on: August 16, 2010, 08:28:25 am »
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I've build a few PI machines but never a VLF.

I would love to come up with a 17Khz Gold machine in lines of the Treasure Barron type machine.

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« Reply #23 on: August 28, 2010, 11:34:07 am »
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http://vbox7.com/play:c3b5259d

This is my copy of Relic Hawk.Work good only for relics not for coins!

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« Reply #24 on: August 28, 2010, 11:51:01 am »
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Hi Homefire

Can you tell me why you want a 17Khz machine is there an optimum frequency for gold detection?

Regards Xavier 

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Offline GoldDigger1950
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« Reply #25 on: September 01, 2010, 03:03:37 pm »
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From the earliest days of VLF, 18khz has been arguably considered the ideal frequency for gold detection. Whites disagrees. Their Spectra V3i has a three frequency model that alternates with 22khz being the center frequency. Allegedly, it optimizes the detector for all precious metals.

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Garrett believes in the 15khz to 18khz spread. So does Minelab but oddly enough, they also sweep as high as 300khz.

Your audio scope setup is configured for spectrum analysis, Xavier. You could set up a sweep oscillator and measure the eddy currents by spectrum for a range of frequencies and develop your own reference. Of course, you need gold sources to do that. When I did my experiments, I used a solenoid coil (one of those that drives a large pin shaft like in a pinball machine or an old washing machine) as the receiver coil which was fixed near the gold object. I pulsed the transmit frequency and only looked at the eddy currents during the quiet time when the transmitter was off. It wasn't enough to look at just the amplitude, although that was important. I needed to see the entire spectrum of results to determine what frequency component I needed to watch for.

It's a complicated business, this detection of gold, mate. You should see my stack of notebooks from back in the 1980s when I started all this experimentation. There's a ton of useful information there if I ever get the time to open them again. I plan on repeating the experiments some day using a variety of waveforms - square wave, sine wave, sawtooth - and recording all the screens. Back when I did them, I had to hand draw the results from a spectrum analyzer display. I used a thermal printer for a while but those images have all gone black now from being stored in less than ideal temperatures.

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« Last Edit: September 01, 2010, 03:30:59 pm by GoldDigger1950 »
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« Reply #26 on: September 01, 2010, 03:52:33 pm »
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As GD says, Upper freqs find the goods.

Every Known gold finder in the VLF world is 12 KHz and up.

Ground penetrating suffers but you can find grain size nuggets with them.

I have a OLD OLD Garrett American S3.

It's basically a limited version of the Groundhog circuit and finds the small gold running at 15 KHz.



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« Reply #27 on: September 02, 2010, 01:35:49 am »
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Hi GD

I have printed this post as I find it very interesting. When I build my VLF I got it to run at about 14Khz. I found a frequency generator some time ago as it was my intention to use it for testing purposes now is possibly the right time to build it and start doing some research on the subject. This VLF that I build actually works quite well but only up to 200mm now that I have added a ground balance I have found that it will only go to about 160mm this will not do but I do have a suspicion that most of my problem comes from the coil.. will keep you posted on this one.

Regards Xavier 

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Offline GoldDigger1950
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« Reply #28 on: September 02, 2010, 02:24:25 am »
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Xavier, the timing is probably the issue. You have to allow the field in the TX coil to diminish before you read the RX coil or it will interfere with the received signal. The delay is normally less than 3ms with a TX off time that is set to match the optimum coil swing time. Your TX signal may be hiding the actual return signals from the metal.

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« Reply #29 on: September 02, 2010, 10:35:25 am »
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GD now that is great info the only thing is I'm not sure on how to solve that kind of problem see the way I have it configured is the receiver picks up the return signal from the RX coil that goes through a primary Op-Amp then to a secondary Op-Amp to turn on a transistor that in turn 'turns on the audio generator  my gain is controlled from the first Op-Amp going to the second. Now if I get what you are telling me to do is retard the TX signal to allow the return signal to be analysed more but how? If it were square wave I'd have no problem doing this but sine wave  Huh? should I change the receiver frequency? Or should I turn of the transmitter while the receiver is turned on?

Best Regards Xavier

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