BFO "Discussion"

[Qiaozhi]

I consistently see here the old wives tale that BFOs cannot discriminate. Nothing could be further from the truth. Anyone who has actually bothered to test one would know this.

BFOs have the capability of rudimentary discrimination between ferrous and non-ferrous metals. There is no old wive's tale saying they cannot discriminate. The shift in oscillator frequency is caused by the permeability of the target and/or eddy currents generated in the target. All targets exhibit a varying combination of both these effects, meaning that the discrimination capabilities of the BFO are compromised. When coupled with the well-known drift and stability problems of the BFO design, you can readily appreciate why no-one wastes time nowadays on this type of design. For a total beginner in metal detector design it is a very good place to start, and is an essential part of the learning process. Once you understand the BFOs capabilities, it is time to move on to better things.

However, as I understand it, you have constructed and tested a "Matchless Metal Locator" designed by Thomas Scarborough, which happens to be an induction balance detector. So why are you so defensive when it comes to the humble BFO?

[Tim]

Again I will repeat- I have never persevered with a BFO/ oscillator that has this wives tale of drift. I do not understand why you are so insistent with your dis-information. Surely you cannot sanely argue that a stable and powerful oscillator is unimportant to a good design?. The simple BFO I originally based the IB on was already more sensitive than the Matchless. With the same coil.
God forbid that a mere amateur as myself should suggest that all avenues of oscillator design have not been explored. God forbid that a mere amateur as myself would even hint that some may have overlooked the basics in their search for the eureka moment. There is an old saying Q. "Couldn't see the forest for the trees."
It seems to me that the only area flogged to death is PI design.
What you say flies in the face of my experience. Both this BFO and this IB DO NOT DRIFT. And both of these machines will see a copper Aussie 1c piece through a pair of fencing pliers. The "rudimentary discrimination" you bag BFOs for, is the same or worse with PI and VLFs
Your protestations to my logic and experience are becoming more bizarre, and I can only surmise this is because of some ulterior and corporate motive.
I am defensive of BFOs because I protest the dis-information, not because I see them as better. A good design must be based on a good oscillator, or will you contest this too?
These are just to prove it is not me who is full of procrastination.

[Qiaozhi]

God forbid that a mere amateur as myself would even hint that some may have overlooked the basics in their search for the eureka moment.

I highly doubt there is any "eureka moment" to be found with a BFO design.

The simple BFO I originally based the IB on was already more sensitive than the Matchless. With the same coil.

Firstly, the Matchless is an IB detector, not a BFO.
Secondly, the TX oscillator is based on a 555 timer, so it is a driven coil. i.e. the coil is not part of tuned circuit.
Thirdly, if you're using the original Matchless coil, your detector cannot be a BFO.

A BFO requires two oscillators. One uses the search coil as part of a tuned circuit such that any nearby metal target will cause a change in inductance. The second oscillator (known as the LO) is tuned a frequency close to the main oscillator. The difference is the beat frequency. In your photo there is one detector that appears to be using an AM radio as the LO/mixer, and is about the simplest BFO you can build. The other (I presume) is the Matchless.

[Tim]

WM6. Yes, I agree with this statement. The BFO will distinguish, The IB of the same (twinned) oscillators will discriminate. For the clinical laboratory definition you are clinically correct. For the purpose of a field-use, hand-held metal locator these are both means to the same end. The dumbing-down required for PI and vlf desensitises the machine, and the presented arguments actually favour the BFO, IB and Beat Balance over modern vlf and PI as the ground interference increases.
I hope my lateral and simplistic thinking has not confused you labrats. You are quite welcome to come to Australia and test your theories on some "real" hard ground.
I notice gcause gave up on his surf in the goldfields as a bad joke, and I am sure many gurus here could have helped, but didn't. Corporate alliances???
When I have the surf PI DD working, here, it will not be through the advice of your gurus, rather first hand experience.

[Tim]

Oh dear oh dear Q. Hot heads don't read real well, do they. Yes the IB receives via an AM reciever.(not the detector with the radio) Whether one of the Matchless coils is driven by a 555 or whatever means it is still an IB. 2 same or matching frequencies. The IB I made has 2 identical oscillators, but they both require a double d coil.
The radio detector(pictured alongside IB) was the original and excellent BFO circuit, I doubled up for an IB. Beat-balance if you insist on splitting hairs.
One pictured is the BFO the other is NOT the matchless, it is the IB circuit with AM receiver inbuilt I made. The other board.Dumb old me. That's the one that comes close to my GMT. It's not a Minelab. It's not complicated. It's not expensive, and it is not a BFO.
Good to hear that you cannot see any future in BFO design, it just proves my disagreement is warranted.

[Tim]

I highly doubt there is any "eureka moment" to be found with a BFO design.

I am not saying their is. I am just saying there is an absolute probability that all basics have not been covered in the race towards the top.

[Tim]

The stoning of the infidels continues.......

[Tim]

Q . You should read my posts in this thread in order, and then confirm the truths, rather than try to fray the edges or nit-pick in vehement disagreeance. This only undermines the analytical and factual search for the truth. I now realise your opinion of novices like myself is quite low, and this beating-down is warranted in your opinion. But the asphyxiation of those outside the inner circle is hardly warranted. Unless you are a corporate.

[WM6]

When I have the surf PI DD working, here, it will not be through the advice of your gurus, rather first hand experience.

I am sure (with you), that this this old and almost forgotten, today probably undervalued MD design, has his potential, especially with new approach ideas and new technical grip applied.

With some BFO solutions (par example using X-tal oscillator) I got very good results. In some cases better than using classical IB technology.

But in nowadays world of colorist LCD displays and all bells and whistles of modern, mostly IB designs, no one believe in poor look BFO appearance.

[Tim]

Thanks WM6. I have Xtals, but have never taken this path. It has been the oscillator circuits opposing the xtals that I have explored. If I was to take this path I would start with 2 xtal oscillators, say 32khz and x5 = 160khz, or 110khz and x5= 550khz..etc. With appropriate overlapped coils making the end number khz. I'll probably get shot down in flames for that stupid idea too!

[WM6]

Thanks WM6. I have Xtals, but have never taken this path. It has been the oscillator circuits opposing the xtals that I have explored. If I was to take this path I would start with 2 xtal oscillators, say 32khz and x5 = 160khz, or 110khz and x5= 550khz..etc. With appropriate overlapped coils making the end number khz. I'll probably get shot down in flames for that stupid idea too!

Don't worry about stupid ideas Tim.
Stupid ideas are immanent to high pro developer too.
The only difference between Pro and Amateur designer are in public approach: amateur are prone to show all his creations working and non-working, while the Pros are prone to show refined and working designs only.

[Tim]

Yeah, a round stone with a hole in it was probably someones stupid idea. Then and expert called it a wheel and took all the credit.

[Qiaozhi]

Unless you are a corporate.

I notice that you state in your biography: "I despise politics, disparity and corporates, as they beget each other."

Geotech is a place where people interested in metal detector technology meet and discuss various designs and projects. If you "show and tell", then don't be surprised if you get questions in return. It doesn't automatically mean that there's a conspiracy to steal your "Eureka moment" detector, or that everyone wants to tear your design apart and pick fault. Only a paranoid would think that way.

[Davor]

Well, not everyone's motives are corporate, mine the least of all.
I also wouldn't mind if this topic is split at about post #56 or so, because it turned an unintended path.

And this is going to be a long one.

Matchless is a cross-breed between a BFO and IB, in a sense that it mixes Tx with another oscillator. It improves frequency pull by decoupling means of IB coils configuration, and hence the sensitivity, but it is a BFO nonetheless. Just about every other technology goes deeper.
However, the physics of all metal detecting technologies are the same. BFO is only a type of a detector, and just because its sensitivity is proportional with frequency - it operates at LF.

LF as a radio band is quieter than VLF, and there's my motive to play with it. There are also limitations, and those stem from the target's response time, or tc, or tau, and in all cases it is a material-specific feature. It may also be spoiled with target geometry, but less so at LF than VLF, due to the skin thickness going deeper at lower frequencies. When target response is received with an IB Rx coil, it will show phase shift related to the material, and always within 90° span for non-ferrous targets. It is quite difficult to map all metallic targets with a single frequency, as common metals' time constants span between about 4us and 150us, roughly 1:40 ratio.
For that reason relic detectors use lower frequencies, and small gold detectors go to LF.

What happens when a low frequency is used for hunting low conducting targets (and the other way around) is that targets' response group tightly at extreme angles, making VDI distinction or discrimination impossible. In case of low frequency, all small gold, foil, etc. go to the upper extreme, and in case of high frequency all good conductors as bulk copper, silver, gold, lead etc. go to the lower extreme. With very high frequency you'd be able to discriminate foil thickness, but all non-ferrous metals would be grouped together, with no possibility to distinguish between them. And there is your BFO.

BFO discrimination works for distinction between ferrous (increasing inductance of a coil) and non-ferrous (effectively shorting a coil turn). That's it.

LF is very good for discriminating small gold and alloy relics, but it leaves good conductors grouped together. It is also more prone to salts and water, and hence the best candidate for applying multi frequency trickery.

PI detectors may assume any of these features as the first sample delay acts as a LPF, and first sample duration acts as a HPF. Both having sinc(f) frequency response shape. With short first sample delay, PI goes well into LF. Because large sample durations stretch to quite low frequencies, bulk targets benefiting from thicker skin depths at low frequencies tend to produce responses that seem to go way beyond the above mentioned 150us, but that's just a quirk of technology, not the physics.

The main thing is that the physics works the same for all technologies.

Various technologies have their shortcomings:
- PI has no meaningful discrimination and it is deaf to short tc targets, making attempts at shortening 1st sample delay silly and expensive
- VLF lacks some 15 years of development, mainly in fields of ground balance and air signal mitigation. Also the state of the art of gain blocks and 2-quadrant discrimination suck.
- BFO lacks everything above and some. But it is incredibly simple. It might be improved by pulse-counting FM detector, making it a nice micro-controlled pinpointer.

[Qiaozhi]

I also wouldn't mind if this topic is split at about post #56 or so, because it turned an unintended path.

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