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Does anyone know the pros and cons lowering the frequency of vlf machine to ulf.?
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Do you have any specific freq in mind, other than "below 3KHz" ( I assume) ?
Are you wanting to modify a commercial machine? Most are VERY optimised for the one freq, even those that could be modifed will require considerable skill. -
600-900hz, I'd like to start with no MCU VLF detector project.Originally posted by Skippy View Post
Do you have any suggestion/info/test result skippy??
Regards,
JohnComment
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If you're going that low, one problem you will have is the search-coil. Commercial coils are generally wound with a wire gauge that gives them a transmit Q-factor of about 15. If you try and force them to run lower freq, the Q drops too much. And coils are also induction-balanced at the single design frequency, and when operated away from this freq (higher or lower) the residual signal coming from the RX becomes too great.
Many commercial analogue machines have been reverse-engineered, and the circuit diagrams published. This would help you with the re-design.
You might also investigate industrial pipe locators, they often run at low frequencies. Possibly the C-Scope CS880, as an example.Comment
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Firstly I'd like to say thank you for the information above. Really helps me to understand the VLF machine better.
Another questions came to mind. How to figure out the Q-factor from a coil, double D coil also super D coil ? Does it have to be 15??
If commercial made it for single frequency is it possible to make homemade coil which can be used for multiple frequencies??
RegardsComment
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A Q value of 10 -> 25 seems common for the transmit coil of commercial machines. If it goes too high, problems such as large metal (particularly iron) affecting the frequency/phase of the oscillator, by changing L or loading the transmitter. Low Q-factors can result in poor sine-wave shape. Q is mathematically: (2 x pi x Freq x L) / R.
Multi-freq coil building is not discussed much on here. There are commercial multi-freq coils that may possibly be useful to you, such as those for the Fisher CZ machines (CZ-3, CZ-7, etc) which work at 5 -> 15 KHz, but how well they will perform at 1K or lower is unknown. They are normally force-driven with a H-bridge drive circuit, and are not tuned to any significant degree.
DetectorPro make a fairly low freq VLF machine, at 2.4KHz. I don't know if the search-coil is available seperately, as it's normally hard-wired to the headphones.
http://www.detectorpro.com/headhunter-underwater12.htmComment
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Whites CM V. Supreme, a very sensitive detector (I think our colleagues KT315 has related schematic on his CD), is working even lower at 1750Hz.Originally posted by Skippy View Post
And thanks Skippy, for some interesting parts of MD theory.Comment
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I mentioned the HeadHunters, but forgot to add the Fisher 1280X, which I understand the HeadHunters are developed from. They both run at 2.4KHz. As the 1280X is waterproof, the coil is hard-wired, so obtaining one on its own would be difficult.
http://www.fisherlab.com/hobby/fishe...l-detector.htmComment
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Low frequency comes with less sensitivity to salt water, but also the phases of all poor conducting metals get grouped around 90°. Such detector is superior for finding (bulk) gold and silver, e.g. relics.Comment
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The corner/-3dB frequency of a US copper 1 cent, or a U.K 1 pence is close to 3KHz; the larger U.K 2 pence, and US 25c are nearer to 2KHz, and I'm sure mens silver finger rings would also be about 2KHz approx. , so machines like the HeadHunter/ 1280x should be capable machines on a beach.
Having said that, my Fisher F75, at 13KHz, is no slouch at finding these 'ULF' targets, but not on a wet beach.Comment
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