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Offline HomefireTopic starter
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« on: August 30, 2014, 09:33:40 am »
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    I see this a real old time.   Multivibrater.   AKA .  Two Transistors making up a Flip Flop. Oscillator.  With the side line uses as a another ocilator.  Like the Falcon this is so simple it could work.




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« Reply #1 on: August 30, 2014, 10:00:05 am »
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It's BFO mate, will be very unstable.

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So many questions so little time

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« Reply #2 on: August 30, 2014, 11:59:06 am »
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   BFO is good for what they are.  Yes Unstable in ways but functional.   I had to retune my first BFO every 7 or 10 steps but it found me black sand deposits that produced a Oz or more of gold a few times.   This one is just a Multivibrater with a Side stepped Local Osilator tracking the same.  As long as they both drift the same way it will work.    

  The UCC 96 multiplex I worked with in the USAF had a 96KC sink signal.  The only way they could get every thing to Drift the same was to have them track a single source.  96Khz.  Using four deffernt 96Khz signals averaging the same by time they went through 12 mux it was all within time and usable at the end.  

  It makes no difference if it drifts!   As long as the two Oscillators are on the same page.   Detector Osc is working to do the Ref Osc.   Both are on the same page and will produce the same Signal.  5, 20 or 40 Hurts.

Posted on: August 30, 2014, 11:55:02 am
 The Flip Flop is driving he Other Osc.  Both keep the same Difference.   It makes no Problems.

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« Last Edit: August 30, 2014, 12:15:51 pm by homefire »
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« Reply #3 on: August 30, 2014, 06:56:31 pm »
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Boy that was a lot to go through just to get stability. I guess I am getting too used to modern technology. Not much was available back then.

In a BFO you can get a little more stability using a crystal oscillator for the reference, and a tunable Colpitts oscillator for the search oscillator. Just feed a sample of each oscillator to two 5.1 volt zener diodes connected anode to anode, and a resistor from there to ground for a signal you can feed to an amp.

Why 2 zeners for a detector? If you look at the chart for all zener voltages, you will see that some have positive temperature drift and some negative drift. At the crossover point of zero drift is the 5.1 zener diode. It will work up to about 20 KHz. The audio beat frequency from the output is much lower in frequency than that so you are ok. The only thing left is the caps, which are notoriously drifty with temperature. Maybe NPO polystyrene would help there.

Any other suggestions from anyone? Would love to hear what you think

Carl

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« Reply #4 on: August 30, 2014, 07:36:24 pm »
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   I was wrong.   It's two Colpitt Osc back to back.    Cheesy

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« Last Edit: August 30, 2014, 07:38:32 pm by homefire »
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« Reply #5 on: August 30, 2014, 08:26:39 pm »
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AU thanks for posting the circuit description.

The article states that the oscillators should be between 50Khz and 1 MHz, which I think is a little too high. It is easier to get the 100 Hz difference beat frequency or lower if the oscillators are around 10 KHz to 15 KHz, which also makes it easier to detect smaller objects. What say you Xavier?
Carl

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« Reply #6 on: September 02, 2014, 05:16:01 am »
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I say that it's going to be a big headache for nothing go PI get all the depth that you need, find small gold and it will be OK in the wet (damp) and dry patches that you may come across. But by the same token I'm convinced that there is a possibility to make a great BFO detector IF you have a lot of time to spare for R&D, there is one on here somewhere that has discrimination on it, it was posted by Cornelius some 4 or 5 years ago.

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So many questions so little time

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