How do multiple-frequency detectors work?

[Lash]

How do multiple-frequency detectors, like Minelab FBS and Fisher CZ dual-frequency work? Do they broadcast those muiltiple frequencies simultaneously, or one at a time in sequence, or does the machine determine the optimal frequency for the situation, or ?

Why are multiple-frequency machines better than single-frequency machines in highly-mineralized soil and wet salt sand?

Thanks,

Lash

 

[John-Edmonton]

Those are some fairly complex questions you asked.....




http://www.garrett.com/hobby/techsupport/techsheets/frequency.pdf

 

[Lash]

Thanks, John. Very interesting. Multiple-frequency and pulse-induction systems are the same.

 

[John-Edmonton]

Well, A PI is a multi-frequency, but the Full Band Spectrum (Minelabs patented FBS) is not a PI or in reality a multi frequency, as it is only utilizing only a single frequency at a time to get the deepest signal. So, single frequency machines, multi-frequency machines and machines using the FBS technology all have their pros and cons. Some will work better over another in specific environments or operator 's need and requirements. You are asking some really good questions

 

[Lash]

What determines which of the 18 or 28 frequencies of the FBS system is transmitted? Does the machine analyze the ground conditions or is the frequency user-selectable by changing programs?

How does the CZ handle its two frequencies?

Thanks again,

Lash

 

[John-Edmonton]

I have heard (but not verified) , that this technology is borrowed from Minelab (at a cost as they have the Patent to FBS technology) and similar. It just has a different name. What determines which of the 18 or 28 frequencies of the FBS system is transmitted? Exactly how that information is processed is beyond my scope. I do know that there are a lot of processers working hard performing a lot of tasks on the Minelabs. That is why they are a little slower with target response. You may want to ask Fisher specific questions on the Fisher Forum and Minelab specific questions on a Minelab forum. I have extensive experience using an Infinium and Sea Hunter Mark II, some Minelabs, but not utilizing the FBS technology.

 

[JB(MS)]

n/t

 

[JB(MS)]

According to what one the guys who worked with Fisher when the CZ was developed said, the CZ's transmit at 5 khz and process both the 5 khz frequency and the third derivative of that frequency, which is 15 khz and a much weaker signal than the 5 khz signal. I owned 5 CZ's, including an original CZ-6 I had Fisher convert to a 6a and used as my main detector for 6 years, over a 10 or 11 year period and while they had excellent depth on high conductors in my mild ground, their sensitivity to small, low conductive targets wasn't anywhere near as good as I've gotten from single frequency detectors in the 10 to 15 khz range. As for the multi-frequency Minelabs, I had two back in the 1990's. Both were Sovereigns, and while they worked darn well in the wet salt sand on the MS and Florida Gulf Coast beaches and in some of that highly mineralized red stuff they call dirt in northwest Alabama, they didn't work worth a darn in the virtually mineral free ground here. I kept the last one for a couple of years, mainly because of the DTI meter, but never used it very much. Here's something someone who is currently an engineer for one of the major detector mfg's posted on Carls forum a few years ago about the frequencies on the Sovereign and Explorer. He had no axe to grind at the time, and based on the accuracy of other things he said and posted I have little doubt the info in his post was accurate according to the tests he used to determine it.

<i><b>Both the Sov, and the Explorer, have identical transmit signals. If you look at the frequency spectrum, there are only a very few frequencies with enough power to make them useful, nowhere near 17, much less 28. Both machines process 2 or 3 frequencies, not sure which, but certainly no more. True pulse machines, like the Infinium, are very wide-spectrum, but it's unfair to make any frequency claims, as they do not process in the frequency domain at all, but rather in the time domain.</b></i>

 

[John-Edmonton]

I think most people who really engulf themselves in this hobby tend to own several metal detectors as you have, as some work a heck of a lot better in some areas then other places. No detector does it all, and a lot of detectors are better suited for different regions of the country due to the mineralization in the soil and what type of targets they are hunting for. Of course the hunters ability to use his equipment to its maximum plays an intricate part too.

 

[JB(MS)]

I usually have anywhere from three to eight and currently have seven. From the early 1990's until about the middle of 2004 I had a sideline used detector business and always used each one a few times to make sure it worked okay before I sold or traded it. I never made much money, if any, from them but I got to try a lot of different brand and model detectors at little to no cost. I noticed you use mostly Garrett detectors, I had more of those than any other brand. The GTA models were good sellers, had quite a few of those and also a good many of the Master Hunter series. Still have a Master Hunter 5 with three coils and the two box "Depth Multiplier". My favorite Garretts were the Freedom III Plus models. I used one for 5 -6 years as my main coin detector and found more coins with it than any other detector I've used.

 

[Carl-NC]

Unfortunately Garrett's explanation leaves a lot to be desired. Yes, PI detectors produce a ton of frequencies, but they are not frequency-domain designs, they are time-domain. It is unfair (IMO) to call them "multi-frequency" detectors in the same way we refer to true frequency-domain designs that are multi-frequency. It is not the same.

Regardless of what you read in advertising, the Explorer, Sovereign, CZ, and DFX are all 2-frequency detectors. They all transmit a digital waveform (as opposed to sinusoidal) that create the two frequencies. With the Explorer and DFX, you can shift the digital waveform to slightly offset the frequencies for better noise rejection. This is identical to the way frequency offsets work in other detectors. Nothing magic in any of this.

The magic occurs in the processing of 2 frequencies. The use of 2 frequencies gives a non-motion indication of salt that one frequency can't do, and (with the right processing) allows the detector to effectively remove the salt signal.

Again, PI detectors work in an entirely different way, and the only reason they reject salt is that salt doesn't produce a signal in normal (15us) PI designs.

- Carl

 

[Lash]

Thanks, Carl.

On what do the Minelabs base their claims of 18 and 28 frequencies?

 

[Eric from NC]

Engineering is not my cup of tea however my Excal works better than any other detector I have owned (on the beach), several Garrett and Tesoros. All I care about is if the detector works and finds stuff, mainly gold, silver and platinum. Turn it on, swing, dig and smile. Each have their strengths and weaknesses, but the technical specs on the frequencies and how they work and why, I don't know.
If you need to know what kind of fish you caught, I'm your guy. Sorry

 

[Carl-NC]

Thanks, Carl.

On what do the Minelabs base their claims of 18 and 28 frequencies?

It's an advertising gimmick. If you look at the frequency spectrum of an Explorer, you'll see gobs of frequency components. Way more than 28 in fact. Same with DFX and CZ. It's just that Fisher and White's don't try to mislead you. Interestingly, the Sovereign has the exact same transmit waveform as the Explorer.

But read the advertising... all the way back the the introduction of the Sovereign, Minelab has only claimed their MF detectors "transmit" 17 or 28 frequencies. They have never ever claimed that they "receive" or "process" or "use" that many. Because they don't.

- Carl