Coils and Multiple Frequencies

[Steve Herschbach]

Hi Guys,

Thanks for all the great information. I'm a layman and user, and am trying to understand some basic concepts. I've read the previous posts here on multiple frequency units.

I've always assumed coils must be ''tuned'' for use at a particular frequency. So most White's coin detectors can use the same coils (since the detectors operate at the same frequency) but the Goldmasters need their own coils. Or the Gold Bug and Gold Bug 2 use coils that appear identical, but are not interchangeable. As an aside, why should a GB2 coil cost so much more?

Anyway, we now get multiple frequencies. Let's use the XT18000 as an example, since it is switchable between three frequencies. I've used the machine and it really works well at 20 kHz. At 60 kHz, however, it does not seem as hot as it should be. If blamed this on the coil not really being optimized for 60 kHz, but I do not know the truth of this. Can one coil really work well at 6.4 kHz, 20 kHz, and 60 kHz? Would the detector theoretically benefit from a coil designed expressly for 60 kHz?

Then to really get complicated we get detectors working with multiple frequencies simultaneously, or in very quick sequence, if that really matters. In any case I still wonder about the coils themselves.

I gather that the Fishers transmit a single frequency and receive back the same (5 kHz) and a ''harmonic'' (15 kHz). Is it the idea of harmonics that would allow the coil to receive efficiently at two frequencies? Or can circuitry compensate? Or is some fudging going on?

I also am given to understand that the DFX transmits and receives two separate frequencies, 3 kHz and 15 kHz, and so the same question arises. How does the coil do this? And does transmitting two separate frequencies have any real advantage over transmitting one and using a harmonic?

That of course leads to the Minelabs, which seemingly somehow pick frequencies to receive from a choice of 17 or 28 whether by transmitting or using harmonics. They seem to be a can of worms all their own, so I almost hate to bring them up. My small understanding of how this works is that the frequencies compared should have a rather large spread to get any real information from the difference, so looking at more than a few at the same time would be a waste of time.

So I gather that the Minelabs pick a few frequencies from a range and look at them. I wonder if the Explorer actually looks at any more frequencies at one time than the Sovereign? Perhaps the Explorer picks from a wider range, but when it comes down to analyze time they both compare the same number of frequencies?

And what about those darn coils. How can the Minelab coils really work perfectly across such a wide range of frequencies?

I must apologize, as I've loaded this post with more questions than I intended when I started typing it! Coils was the main focus.

And PLEASE... I do not care to debate the so-called merits of the different brands per se. Let's try and avoid the ''who's best'' nonsense. I'm just interested in the concept of multiple frequency use. I own a CZ-20 and DFX, and have an Explorer XS at my disposal. I think of detectors as tools in a toolbox. My toolbox does not have ''good wrenches'' or ''bad wrenches''. I just try to use the correct wrench for the job at hand!

The CZ-20 gives me little to work with. Just set the knobs. I'm working hard with my DFX right now trying one frequency or the other, or both. Very fun! Tons of options. And next I'm going to dig into the Explorer to learn it's secrets.

Thanks in advance for your patience with this ''to many questions'' post!

Steve Herschbach



Steve's Mining Journal

[Sean Goddard]

Re: Coils and Multiple Frequencies

Well first off,

Two frequencies, and, A frequency & A harmonic are the same thing!

A harmonic is a MULTIPLE of the FUNDAMENTAL. I.e. 15KHz is the THIRD harmonic of 5KHz.

As to coils, I occasionally do consultancy work for the telecomms industry, and when we use inductors

(coils) at VHF frequencies, we are often passing signal with a BANDWIDTH of 1MHz or more. And you're

worried about a few KHz?

No but seriously, it's mainly down to the "Q" or QUALITY factor of the coils. The higher the Q, the

narrower the bandwidth. OR, put in laymans terms If you have ONE turn of TX coil, then it will have a low

INDUCTANCE, and generally a lower Q. Therefor you will be able to use it over a wider BANDWITDH. Vice

versa for a coil with a lot of turns.

I believe the Explorer looks at THREE frequencies at ANY ONE TIME, selecting the best ones to use from a

98.5KHz SPECTRUM containing TWO "main" frequencies, and harmonics of these.

RIGHT, TWO frequencies? well the Explorer SEEMS to transmit square waves of two different PERIODS, which

means TWO distinct fundamentals. Now a square wave is made up of ODD harmonics, E.G 3, 5, 7, 9KHz etc.

If you transmit one square wave of 3KHz FUNDAMENTAL, and one at 15KHZ fundamental, then your ACTUAL

frequencies in your transmit waveform will be;

3, 9, 15, 21, 27KHz, Etc. in ONE part, and 10, 30, 50, 70, 90KHz, Etc. in the other do you follow?

So by CAREFULLY selecting your BASE frequencies, and LOWPASS filtering your transmit signal, you can

claim to have 28 transmit frequencies. HOWEVER, the processing power to analyse EACH of these is quite

frightening, and way beyond what Minelab claim for the Explorer, so they MUST be using some other trick.

Selecting a few, most effective frequencies is probably the way they do it.

[Dave J.]

Re: Coils and Multiple Frequencies

That's an excellent "Mechanic's Critereon" question, Steve!

Being unemployed, and having torqued that nut, I hope for the opportunity to answer your question with a better metal detector.

--Dave J.

[Robert Hoolko]

Re: Coils and Multiple Frequencies

Some manufacturers use a tuned coil. There is a capacitor in parallel with the coil inductance which forms a resonant circuit. Most of White's detectors use this design and most of their coils are tuned to 6592.5 Hz. This kind of coil can only operate over a very narrow frequency range around the resonant frequency. For example, the XLT uses frequencies between 6027.5 and 7274.5 and the response is down quite a bit at the ends of that range.

Other manufacturers use an untuned coil. It is just an inductor (with winding resistance). This type of coil is not confined to a narrow frequency range. There is still some limit on the frequency range over which it can be used because the impedance of an inductor is proportional to frequency. So if a certain voltage waveform applied to the coil causes a certain current to flow, then the same waveform at twice the frequency will only cause half the current to flow. (This assumes that the winding resistance is relatively low compared to the coil reactance at the frequencies we are talking about.)

Robert

[Steve Herschbach]

Re: Coils and Multiple Frequencies

Thanks for the help, guys. It's too bad Dave's reply seems to be broke, however!

Thanks again,

Steve Herschbach



Detecting Gold at Ganes Creek

[Dave J.]

second try; ignore my other nonfunctioning post in this thread

Some searchcoils have resonating capacitors in the searchcoil, so those searchcoils only work at a particular frequency. If there are no resonating capacitors in the searchcoil, then it can, in principle, be in balance over a wide range of frequencies. Minelab obviously knows how to do it well, and I do too, but some other competitors may not know, so that's a subject I'd rather not say more about for now.

The fact that the reactance of the coils is proportional to frequency presents some challenges in getting good operation over the whole range, but there are ways to achieve a pretty good compromise. Another area where I'd just as soon not say very much.

--Dave J.

[C.Pruysers]

Re: second try; ignore my other nonfunctioning post in this thread

Dave . I think that a coil is in resonance by using one specific capacitor for one specific frequency . By using different capacitors you can tune the coil to different requencies . Something I did very well in a BFO detector . Ofcourse you have to change the reference frequency in the same time . This can be done by using a IC 4060 and a Xtal of 8MHz . This will give you three ( more ) set reference frequencies , namely 500KHz , 250 KHz and 125 KHz . By tuning your coil three times to these frequencies you can use one coil and three search frequencies . My search frequencies are 125 , 62.5 and 31.25 KHz . Ofcourse I can go as low as I want to by using another Xtal and other tuning capacitors . By the way I thought this forum was to help eachother . You mention things and do not want to share anything . Why are you on this forum in the first place ? Kees

[Dave J.]

multiple frequencies, sharing on forums

I think the Minelab 17000/18000 series use a technique such as you describe. There have been attempts to switch the frequencies rapidly in sequence to get quasi-simultaneous "multiple frequency" operation, but there have never been any successful products based on this principle.

I hang out on three metal detector forums-- Carl's Geotech forum (this one), Eric Foster's PI Technology forum, and the Finders forum (which is not so technical). Most of my technical posts are on the PI Technology forum, where I have been quite generous with information. Many of the people who hang out here, also hang out on the PI Tech forum and are aware of what goes on there.

Since I work in this business, there are some things I can't talk about because they're confidential information, and other things I don't talk about because they represent what may be critical competitive information for future work. Upon occasion I mention the existence of withheld information as a means of advertising my services to engineering managers who monitor these forums, and also to let other people know that such information does exist even though I am not presently divulging it-- which in itself is sharing some information.

--Dave J.

[C.Pruysers]

Re: multiple frequencies, sharing on forums

Dave . That sounds fair enough . I am totally reworking Koehlers circuit and would not talk about it too much either at this moment . Kees

[Sean Goddard]

Re: Coils and Multiple Frequencies

Steve,

I can vouch that Dave has been VERY helpful on the PI forum, and his extensive knowledge is freely shared (except when that would infringe upon the advanced research I know he's working on).

Obviously experienced professionals like Dave, have to keep some secrets, and that's what makes him skills in demand by potential employers.

Good luck and HH

[Steve Herschbach]

Re: Dave Johnson

Hi Sean,

By Dave's reply being broke, I meant it was broke! It was giving an error. But Carl fixed it.

I owe Dave a lot, as I've found lots of gold with detectors he helped design. Deserves a spot in the Detector Hall of Fame as far as I'm concerned!

And thanks for your detailed reply to a novice.

Steve Herschbach