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Offline darkkitten2000Topic starter
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« Reply #10 on: May 06, 2010, 07:32:21 am »
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Hi,
my runs at about 110 KHz (searching oscilator).

d.

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Offline xavier
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« Reply #11 on: May 06, 2010, 11:36:48 am »
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Well then the reference oscillator must be at the same frequency and that is quite low for a BFO that's perhaps the reason why it's so stable I have had schematics where it was running at over 450Khz I never did figure out why they run at such high frequencies why not try to make one that runs under or at 30Khz? 

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So many questions so little time

Offline darkkitten2000Topic starter
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« Reply #12 on: May 06, 2010, 12:18:55 pm »
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The refference oscilator is built around the 3.57 MHz crystal (basic frequency) and is further divided using NAND gates down to some 900 KHz (divided by 4), this is used in mixer circuit, so 1 Hz of src. oscilator change causes 8 Hz of audio frequency change (ref is used as a IC clock), and there lies the key to sensitivity. Of course, to afford that both oscilators must be as stabil as possible. In reality, the ref osc. is remarkably stabil since basic frequency deviations are divided by 4, and so is the relative small drifting (hence the high frequency ref. osc.). The src. osc. is not so stabil, however the designer did it all to be; special capacitance adjustment using a transistor, week cupling via small capacitor, separated power source to avoid voltage fluctuations caused by speaker.... non the less, the drift is constant, however the frequency change caused by a metal target is so abrupt (multiplied by 8  ) it is easily recognized. This makes the machine unbelivebly sensitive and it can easilly be compared with simple VLFs, i have never seen anything like it. For a BFO, by testing it against a large metal target (i.e. refrigerator) the audiable change can be noticed at 70 cm from the target, amazing for the 15cm coil (you can't expect anything like that in soil; maybe 20-30 cm is expected max.), but very small coins (15mm diameter) could be found down to 10 cm - tested in soil, and that is unbelivable.

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d.


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« Last Edit: May 06, 2010, 12:25:14 pm by darkkitten2000 »
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Offline xavier
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« Reply #13 on: May 06, 2010, 12:46:34 pm »
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Yes and when you say 10cm you must add the distance above the ground normally about 5cm not to shabby I'd say if you had a bigger coil you would find that you will pickup a fridge at 100cm but then you must decide if you are looking for large or small objects.
The Beat-frequency oscillator (BFO) is the simplest (and oldest) type of metal detector technology and is a good starting point for learning how metal detectors work. The basic beat-frequency metal detector employs two radio frequency oscillators which are tuned near the same frequency. The first is called the search oscillator and the other is called the reference oscillator.

The outputs of the two oscillators are fed into a mixer which produces a signal that contains the sum and difference frequency components. This signal is feed to a low-pass filter removing the harmonics. As long as the two oscillators are tuned to the same frequency, the output will have no signal.

When a metallic object disturbs the magnetic field of the search coil, the frequency of the search oscillator shifts slightly and the detector will produce a signal in the audio frequency range.

Although once popular, BFO's are no longer being made by professional metal detector manufacturers. They are simple and inexpensive, but do not offer the accuracy and control of modern PI or VLF detectors. Attempts have been made to add new features such as discrimination and more advanced models were produced in the 1970s, but they were soon replaced by recent, more sophisticated technology.

BFO designs are still used in cheap hand-held devices and in low quality, toy type detectors. The vintage BFO detector is more of a curiosity and collector's item than a usable piece of equipment.


PS I got this from the net Cut and paste.

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« Reply #14 on: May 06, 2010, 12:57:49 pm »
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Exactly, but this is ment to be a shallow peeker, for small shallow (not deeper than 10cm) objects, a chip simple usefoul low end detector, fairly sensitive on small items. For anythig more, another detector (i.e. commercial - good VLF or even better, a PI) is required; 100KHz at minute power fades away quite rapidly.

d.
 
P.S. The technology is as good as it serves the purpose, regardless the age, sofisticattion etc.

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« Reply #15 on: May 06, 2010, 01:24:58 pm »
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I totally agree with you on that but remember that the high frequency has difficulty of ground penetration that is why VLF was so good compared to a BFO running at 110Khz a VLF will run at 4.5Khz giving it the advantage that lower frequency's penetrate better look at your PI it runs at what 20Hz? 

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« Reply #16 on: May 06, 2010, 04:30:15 pm »
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My first one was running around 300 Khz and could see a dime at about 4".

Wet Ground befuddled it.

It would be cool to build one running down at say 15 Khz and using say a VCO to amplify the changes.

 Wise

Some day when I'm Rich I will try it.



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« Reply #17 on: May 06, 2010, 06:17:19 pm »
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If using the colorburst crystal oscillator makes for instability, maybe you could use a TCXO. Temperature compensated crystal oscillator. The xtal oscillator with compensation is built into one module. All you need is VCC and 50ohm output. Another idea might be a phase lock loop and some divide by n. Bet there might be a PLL with built in division in one package. I've been away from the latest goodies available, so can't suggest anything though.

Jim

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« Reply #18 on: May 07, 2010, 04:51:45 am »
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Homefire I cant see why it can't be done why must it run at 300Khz is there a particular reason for this? like you just said it could run at 15Khz or even less mind you 15Khz is better if you are looking for gold

Mudflap I think that one of the reason for the instability is the reference coil is for one smaller than the search coil and there is a difference in the temperature change (time and temp) two the coil's are fare from one another so one will have short leads while the other long leads (box to search coil) now if you have a crystal for the reference oscillator it will remain stable(reference oscillator) but your search coil will change very rapidly I did some testing I was in the sun for 10 min and it was quite stable then placed the search coil under a bush within seconds 30 or 40 it had drifted so I don't think that there is a way to get it to remain stable unless you can incorporate an auto adjust.

Regards Xavier   

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« Reply #19 on: May 15, 2010, 03:14:34 pm »
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Hi,

assembled modules available.

d.


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