Discriminating BFO detector

		Quote:			Posted by Cornelius
	Golddigger . That is what Homefire said !!!!!!!!   Cornelius

No, he said that ONE of them was crystal controlled.

This is the problem with those who have only just entered into the design or analysis of metal detector technology. The earliest models of BFO machines were not crystal controlled. Then someone got the bright idea to use a crystal. Lo and behold, they were not as good as those models which drifted all over the place and tone discrimination didn't exist in that design. Today's modern, if you can call them that, BFO detectors have gone back to the unstable design.

I've been in the electronics design business for all of my adult life and late teens as a Ham Radio licensee. I know my stuff from the standpoint of education and practical use.

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Golddigger . I don't agree with you . In the discriminating BFO detector we find two oscillators  .  One for the search frequency and one for the reference . Now if the two oscillators are not stable  you have to readjust constantly  . The only thing you want is a stable reference oscillator and adjust the search frequency ( the coil ) to that . So instead of having two unknown ,,drifting ,, oscillators you only have one . I respect your education ( although I can not check it ) , but here you are wrong . With repect  Cornelius

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Golddigger you are not picking a fight are you ?  What does the name  ,, reference ,, oscillator  mean to you ?  Just a question !   Cornelius

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Homefire, the issue is that BFO requires, not just likes, the sympathetic oscillations that occur between drifting oscillators. Using de-tuned PLLs and Comparators takes care of the drift better than a crystal oscillator and still allows proper operation. This isn't theory any more. Patents have been issued on this and recently designed BFO machines use this method.

Bear in mind that I am not disputing ANYTHING you write here but you are not allowing for the fact that there are OTHER means to accomplish things of which you might be unaware. The wisest choice in this sort of discussion is to simply look it up.

What do you want me to do? Ignore current design and just roll over so you can incorrectly inform people that YOUR way is the ONLY way? At least I admit that the old ways work while simply informing you that there are new ways. New ways which I have not declared to be better. Just different. I leave the better or worse decision to those who use the circuitry.

		Quote:			Posted by Cornelius
	Golddigger you are not picking a fight are you ?  What does the name  ,, reference ,, oscillator  mean to you ?  Just a question !   Cornelius

I'm not picking a fight with you at all. Never have. However, you seem happy to take pokes at me from a position of not understanding what I am talking about.

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Hi,

hmm, i do not know where this is heading, beeing only a chemist my background in electronics is far inferior compared to you, guys, but even i understand a crystal conntrolled oscilator is better than a free running LC, what i do not understand is what would be a practical benefit of improving the reference oscilator in this circuit while the original one serves the purpose wery well. There is next to none frequency pooling (locking in) between the two oscilators and the reference is quite robust as is. The circuit looks well designed, oscilators are well "isolated" it will run without readjusting for a fairly long period (not counting the temperature infuence), and a very small change in frequency of searching osc is noticable. The unit is fairly sensitiv. Those are obviously not the problems of this unit, are they?

Would an old fashioned design really worked better as GD implied? I still have a beginer's few transistor  BFO i have made some 30 yares ago, as a kid. O.K. I cleaned the dust off it, put in a new battery and off we go! Now first adjusting the iron core or reference OSC coil to zero audiable and further more to some slow changing oscilations. Putting the searching coil to the ground and there is no audiable ground effect. Bringing the coil next to diamagnetic matter, it sure increases the frequency but the two oscilators are pretty much locked in and the frequency of REF OSC is pushed up as well, so no audiable is heard. This looks like a better behaviour, but wait, now i put a small target (i.e. 2 Euro cent coin) to the ground and sweep the coil a cm or two over it. Guess what happened. The oscilators are quite locked in and no audiable change is heard. The Cornelius' design is much finer and will clearly pick up this target, however, the change will be in the same magnitude as the frequency variations due to changing distance between ground (diamagnetic matter) and the coil, thus difficult to identify unless you know it is there. I'll let you decide which is performing better.

And i owe you one concerning "veneral" BFOs. I use to have an old military detector, i gave on to neighbour kids, that was quite veneral. Since these devices are now obsolete and no longer in military use they are sold as surpluse for some EUR 50 or so, i intend to buy another one, but i am looking for a  later model where electronics was not potted in plastic, to do some reverse. The unit was capable of reporting targets up to some 30 cm deep, not more, but was very sensitive to small items, capable of picking up 1 cm long piece of thin wire (from obvious reasones) and rock solid stabil, of course, ground effect cancelation. No discrimination. The circuit was remarkable simple few transistor design, but the search coil itself was tricky.  A large main coil, many windings of thin wire, on an transformer like iron core in a form of stick, with two sets of auxilary coils at both ends winded ortogonal to the main coil, every one carefuly tuned by a small ferrite core. The main coil and the two sets was ran from three ferrite pot transformers with some few hundred volts apperantly at a fixed frequency. By adjusting of driving transformers primary current the two auxilary coil sets tunes the main coil in inductively. Putting the probe to the ground, both auxilary sets and main coil is affected the same and it is cancelled out, but bringing one single auxilary coil out of balance and the unit will give the clear report.

regards,
d.


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