Selectable Frequency And Multiple Frequency

[Steve Herschbach]

The latest issue of the ICMJ is out, and I have an article in it titled Selectable Frequency vs Multi Frequency Detectors. Those of you with a digital subscription can read it online.

The ICMJ has a policy against mentioning brand names in articles so I wanted to post this as a supplement to the article.

Most metal detectors process a single frequency. Low frequencies, that is single digit frequencies under 10 kHz, react well to high conductive targets, like coins, or large items, even if those items are of low conductivity. If you look at this typical metal detector target scale below you will note that non-ferrous items read higher not just based on conductivity but size also.

Low frequency detectors also do not "light up" the ground or hot rocks as much as detectors operating at higher frequencies. Many do not even offer ground balance controls because a factory preset level works well enough for some uses. Low frequency machines under 10 khz therefore tend to be aimed at the coin detecting market. There are too many models to list but most people have heard of the 6.5 khz Garrett Ace 250 as a perfect example.

High frequencies 30 khz and over have extreme sensitivity to low conductive and small items, but also struggle more with ground penetration and hot rocks. Their extreme sensitivity to tiny trash items like aluminum bits do not make them very practical for any detecting except gold prospecting. Machines 30 khz and higher tend to be dedicated prospecting machines. Examples would be the 48 khz White's GMT, 71 kHz Fisher Gold Bug 2, 56 kHz Makro Gold Racer, and 45 khz Minelab Gold Monster 1000.

In 2002 White's introduced the White's MXT at 14 kHz, and it is a perfect example of how detectors running in the "teens" make excellent "do-it-all" detectors. Since then everyone and their brother has jumped on that bandwagon, and there are too many machines running in the 10 kHz - 20 khz region to mention. Prospectors in particular would recognize the 19 khz Fisher Gold Bug Pro, but few know it is also sold in slightly different versions as the Teknetics G2, Fisher F19, and Teknetics G2+, all 19 kHz detectors sold to the general coin and relic market. Garrett has the 15 kHz AT Pro and 18 khz AT Gold to name a couple more popular metal detectors.

Here is some information for those of you who are more technically minded. George Payne was one of the engineers who patented many of the basic concepts used in VLF detectors to this day. Here is an excerpt from his article at http://jb-ms.com/Baron/payne.htm (2002):
 
"The r component acts differently. It is maximum at one particular frequency and decreases if you go up or down in frequency. We call the special frequency at which the r signal is maximum, the target’s “-3db” frequency. It also turns out that at the -3db frequency the x signal is one-half of its maximum value. This special frequency is unique to each target and is different for different target.

The higher the conductivity of the target the higher will be the targets -3db frequency. Conversely, the lower the conductivity the lower the -3db frequency. The -3db frequency of the high conductivity target will also make the r signal peak at a high frequency, normally well above the operating frequency of the VLF detector. This will make the high conductivity target have lower sensitivity on the VLF detector because the r signal amplitude drops if we are significantly below the -3db frequency. Simply put, maximum sensitivity on a VLF detector would be if we position the operating frequency directly at the target’s -3db frequency. For example, a dime and penny have a -3db frequency of about 2.7KHz. This is where their r signal peaks and would be the best frequency for picking them up using a VLF detector. However, a silver dollar has a -3db frequency of 800Hz. Nickels, on the other hand, have a -3db frequency, where its r peaks, at about 17KHz. Targets like thin rings and fine gold are higher still. Clearly there is no one frequency that is best for all these targets. The best you can do is have an operating frequency that is a compromise."

Well, if low frequencies are good for coins and high frequencies good for gold, why not make machines that can do both? Or both at once? Instead of picking a compromise frequency?

Selectable frequency refers to machines that can select from one of several possible frequencies, but process the signal from only one frequency at a time. The key is not what a detector transmits so much as what it processes. These may also be referred to as switchable frequency detectors. Multiple or multi frequency detectors process the signal from two or more frequencies at once. In theory this multifrequency analysis can be done simultaneously or sequentially at a very high speed. The end resultant is the same - the results from two or more frequencies are compared to derive information that cannot be had by analyzing a single frequency alone.

Multiple frequency detectors usually have a fundamental frequency, and then other "harmonic" or secondary frequencies they also use, but the power (amplitude) fades with distance from the primary frequency. From page 9 of Minelab's Metal Detecting Terminology:

You can find more information on harmonic frequencies at http://www.ni.com/white-paper/3359/en/ and here also.

Coils normally must be wound specifically to make use of any given frequency or set of harmonic frequencies. A coil will usually work best at the given fundamental frequency making it difficult to get the best possible performance at all frequencies using one coil. The Minelab X-Terra series specifically requires a coil change to achieve a frequency change for this very reason. People who own them know 3 kHz coils weigh more than 18.75 kHz coils. Why? Because heavier windings are used at 3 khz for optimum performance at that frequency.

Here is what is probably an incomplete list of selectable frequency detectors and year of release:

1989 Minelab Eureka Ace Dual            8 kHz    19.5 kHz

1993 Minelab XT 17000                6.4 kHz    32 kHz

1994 Compass X-200                6 kHz    14 khz

1997 Minelab XT 18000            6.4 kHz    20 kHz    60 kHz

1999 Minelab Golden Hawk        6.4 kHz    20 kHz    60 kHz

2002 Minelab Eureka Gold        6.4 kHz    20 kHz    60 kHz

2005 Minelab X-TERRA 50                7.5 kHz    18.75 kHz

2006 Minelab X-TERRA 70            3 kHz    7.5 kHz    18.75 kHz

2009 Minelab X-TERRA 305            7.5 kHz    18.75 kHz

2009 Minelab X-TERRA 505        3 kHz    7.5 kHz    18.75 kHz

2009 Minelab X-TERRA 705        3 kHz    7.5 kHz    18.75 kHz

2009 XP DEUS                  4 kHz     8 kHz     12 kHz     18 kHz

2016 Rutus Alter 71        Variable 4 - 18 kHz

2017 XP DEUS V5           Additional 14 kHz    30 khz    55 khz    80 khz options

2017 Nokta Impact            5 kHz    14 kHz    20 kHz

2017 Makro Multi Kruzer            5 kHz    14 kHz    19 kHz

2018 Nokta Anfibio            5 kHz    14 kHz    20 kHz

 

Multiple frequency or multi frequency machines have become very confusing, as a lot of marketing material has focused on the number of frequencies transmitted. What really matters is what frequencies a detector receives, and how the information is compared and processed for results. Some commentary here. Many people look at the marketing material and assume that a machine processing multiple frequencies is somehow working across the board to deliver the best possible results at all frequencies. However, the two issues outlined above do apply. The machines are employing harmonic frequencies, and so cannot compete with a machine optimized at a single frequency as opposed to one of the distant harmonics running at less amplitude. Second, making one coil run perfectly at all frequencies is extremely difficult, again giving the dedicated machine an edge.

I highly recommend people not go down the technical rabbit hole but instead focus on what the machines do, on how they act. Two things are very apparent.

First, the big market for a long time was coin detectors, and the goal always was to identify coins as deep as possible while ignoring trash as well as possible. Processing two or more frequencies simultaneously gives the detector engineer more information to work with. All the focus was on developing great coin detectors and guess what, the multi frequency machines for all intents and purposes act just like very good lower frequency coin detecting machines. Good ground rejection, and great discrimination on coins for as deep as it can be achieved. The multi frequency machines don't really go deeper than single frequency coin detectors, they just do a better job delivering clean discrimination results to depth.

Here is a list of introductory models of multi frequency detectors and year of introduction. I am not listing all the derivative models to reduce clutter. I will post that later. 

1991 Fisher CZ-6            5 & 15 kHz

1991 Minelab Sovereign            BBS

1999 Minelab Explorer S/XS        FBS

2001 White's DFX            3 kHz & 15 kHz (Simulates single frequency by ignoring half the dual frequency signal)

2012 Minelab CTX 3030            FBS2

2020 Minelab Vanquish             Multi-IQ

Second, single frequency detectors have a ground balance problem. They can ground balance to mineralized soil, OR they can ground balance to salt water. Multi frequency machines can reduce signals from both mineralized beaches and salt water simultaneously, making them ideal for saltwater use.

1993 Minelab Excalibur   BBS (Sovereign in waterproof housing)

1995 Fisher CZ-20             5 & 15 kHz (CZ-6 in waterproof housing)

2001 White's Beach Hunter ID   3 & 15 kHz (DFX in waterproof housing)

 

There is a third class of machine that can run either as selectable frequency OR multi frequency detectors. Quite rare at this time. 

2009 White's Spectra Vision   2.5 Khz or 7.5 kHz or 22.5 kHz or all three at once

2018 Minelab Equinox   5 kHz or 10 kHz or 15 kHz or 20 kHz or 40 kHz plus multi frequency options

2020 Garrett Ace Apex   5 kHz or 10 kHz or 15 kHz or 20 kHz plus multi frequency options

In my opinion multi frequency has delivered well on its promise. The Minelab BBS and FBS machines are renowned for their ability to discriminate trash and detect coins due to their sophisticated processing. Again, focus on what they do. Not even Minelab in their marketing tells anyone these are prospecting detectors. Second, the Fisher CZ-20/21 and various Minelab Excalibur models are without a doubt the most popular and successful non-PI saltwater beach detectors made.

I have a White's DFX and I think it is a fantastic jewelry machine in particular. A good coin machine but lacks a bit of punch. The Vision/V3i upped the ante but while amazing on paper suffers from interface overload. The Minelab units are simple by comparison and a lesson on how people in general just want the detector to get the job done. Feature overload is not a plus. However, I think White's has the right idea. The ability to run either separate frequencies or multiple frequencies at once is very compelling. I just think nobody has really done it right yet in a properly configured package. The V3i has the ingredients, but needs to be stuffed in something like an MX Sport with a simplified interface and improved ground balance system. (2018 note - Minelab Equinox released). It really never did beat the White's MXT in some ways and many people when "upgrading" to the V3i end up going back to the MXT.

Selectable frequency has yet to really deliver on its promise in my opinion. So far it has been difficult to produce a selectable frequency machine that truly performs at all frequencies on par with a dedicated single frequency machine. The Minelab Eureka Gold at 60 kHz just never gets mentioned in the same breath as the White's Goldmasters/GMT or Fisher Gold Bug 2. Also, most selectable frequency machines in the past have been very feature limited prospecting machines, restricting their overall market appeal. 

I personally think we have seen enough variations of single frequency detectors. I do not believe much can be done to exceed the performance of the dedicated single frequency VLF type machines we currently have. What can obviously be done is a better job of packaging machines that deliver true punch at different frequencies, or multi frequency machines that bring across the board performance closer to what is expected of PI detectors. I do think we are seeing this happen now. The new Nokta Impact and the new DEUS V4 update are expanding the available options in selectable frequency in more usable packages. The Minelab GPZ and other hybrid platforms blur the line between what is traditionally considered PI and VLF and simply need the addition of discrimination to go to the next level. There is still a lot of potential to deliver machines that might reduce the number of machines many of us feel compelled to own by delivering more across the board performance in a single machine that would now take several detectors. Exciting days ahead.

1/20/22 - More on frequency, and why it's more about target size than type of metal detected

For those who want to try and get their head around selectable frequency and multi frequency technology, Minelab and White's have a gold mine of information in a few of their references. Dig into the following for some great explanations and diagrams.

Minelab - Metal Detector Basics and Theory

Minelab - Understanding Your X-Terra

White's - Spectra V3i Owners Guide

White's - V3i Advanced Users Guide

Better yet are the last three parts of the DFX instructional video by White's featuring engineer Mark Rowan explaining frequency and multi frequency methods:

 

 

 

 

[Shelton]

Good article. Thank you!

[Steve Herschbach]

OK, here is that full but probably incomplete list:

 

Selectable Frequency

1989 Minelab Eureka Ace Dual            8    19.5 kHz

1993 Minelab XT 17000                6.4    32 kHz

1994 Compass X-200                6    14 khz

1997 Minelab XT 18000            6.4    20    60 kHz

1999 Minelab Golden Hawk        6.4    20    60 kHz

2002 Minelab Eureka Gold        6.4    20    60 kHz

2005 Minelab X-TERRA 50                7.5    18.75 kHz

2006 Minelab X-TERRA 70            3    7.5    18.75 kHz

2009 Minelab X-TERRA 305            7.5    18.75 kHz

2009 Minelab X-TERRA 505        3    7.5    18.75 kHz

2009 Minelab X-TERRA 705        3    7.5    18.75 kHz

2009 XP DEUS            4    8    12    18 kHz

2016 Rutus Alter 71        Variable 4 - 18 kHz

2017 Nokta Impact            5    14    20 kHz

 

Multi Frequency

1991 Fisher CZ-6            5 & 15 kHz

1991 Minelab Sovereign            BBS

1992 Fisher CZ-5            5 & 15 kHz

1993 Minelab Excalibur            BBS

1994 Minelab Sovereign XS        BBS

1995 Fisher CZ-20            5 & 15 kHz

1996 Fisher CZ-7            5 & 15 kHz

1998 Minelab Sovereign XS2/XS2 Pro    BBS

1998 Minelab Excalibur 800/1000        BBS

1998 Fisher CZ-7a/7a Pro        5 & 15 kHz

1999 Minelab Explorer S/XS        FBS

2000 Minelab Sovereign XS2a Pro        BBS

2001 White's Beach Hunter ID        3 & 15 kHz

2001 Fisher CZ-70 Pro            5 & 15 kHz

2001 White's DFX            3 kHz & 15 kHz (Simulates single frequency by ignoring half the dual frequency signal)

2002 Minelab Sovereign Elite        BBS

2003 Minelab Explorer II        FBS

2004 Minelab Quattro MP            FBS

2004 Fisher CZ-3D            5 & 15 kHz

2005 Minelab Sovereign GT        BBS

2006 Minelab Explorer SE        FBS

2008 Minelab Explorer SE Pro        FBS

2009 Fisher CZ-21            5 & 15 kHz

2012 Minelab CTX 3030            FBS2

Selectable Frequency or Multi Frequency

2009 White's Spectra Vision        2.5 Khz or 7.5 kHz or 22.5 kHz or all three at once

2017 Minelab Equinox                  5 kHz or 10 kHz or 15 kHz or 20 kHz or 40 kHz plus multi frequency options

[Ringmoney]

Why is it AKA machines never get a mention at all? With their single frequency coils, frequency can be changed the same way as the X-Terra range. With their selectable frequency coils, they don't utilise harmonics to select frequency, in contrast they utilise a simple mechanical switch to increase or decrease the amount of windings used inside the coil.

[Steve Herschbach]

Because for all intents and purposes AKA machines do not exist in the United States. I wrote the article, and I only mention what pops into my head. I know next to nothing about AKA, and they are far enough off my personal radar that I simply did not think of them. Nothing nefarious, just the way it is. I will attempt to get more familiar with them, and perhaps even add them to the list when I know more, but the bottom line is if they want attention in the U.S. they need to do more to get it. A few dealers and a service center is the first step. I may be just a local yokel, but if I have to mail it out of the U.S. to get service than I am not interested.

[Geotech]

Good article Steve, but wrong on the harmonic info. Switchable frequency machines switch to different fundamental frequencies, not harmonics of the base frequency. Most of them run sinusoidal transmitters so there are no harmonics, and they switch in different capacitors to do so.

Also, the DFX doesn't really run in single frequency mode. It always transmits/receives 2 frequencies, just ignores one of them in "single" mode.

 

[Steve Herschbach]

And by the way Ringmoney, feel free to start a thread on AKA and bring us up to speed. Like you asked, why does nobody mention AKA? That does include you also. I post stuff all the time just to be informative. There is no forum rule against others doing the same. I would welcome it!

[Steve Herschbach]

5 minutes ago, Geotech said:

Good article Steve, but wrong on the harmonic info. Switchable frequency machines switch to different fundamental frequencies, not harmonics of the base frequency. Most of them run sinusoidal transmitters so there are no harmonics, and they switch in different capacitors to do so.

Also, the DFX doesn't really run in single frequency mode. It always transmits/receives 2 frequencies, just ignores one of them in "single" mode.

 

OK, I need to edit that. Now that you point it out and I think about it, kind of obvious. If you are running at a single frequency there can be no harmonic in use.

Thanks Carl - I love learning stuff!

[Geotech]

Just now, Steve Herschbach said:

OK, I need to edit that. Now that you point it out and I think about it, kind of obvious. If you are running at a single frequency there can be no harmonic in use!

Well, it's more complicated than that. Instead of a sinusoid, you could run a square wave drive which generates a triangle wave current, which does have a fundamental and harmonics. This is exactly what a Fisher CZ does. A SF detector could demodulate only the fundamental (ignore the harmonics), and vary the square wave frequency to whatever it wants, the advantage of which no coil capacitor needs to be selected. The V3 does it this way (in SF mode), and I'd guess (but don't know) that the Alter 71 does it this way, too. But I expect most of the switchable frequency detectors on your list run sinusoidal transmitters.

[Steve Herschbach]

I try but there is a reason Carl why you design detectors and I just use them. I try to learn just enough to understand why the things I observe in the field happen the way they do, but my brain taps out after that.  Luckily I stayed out of the whole harmonics thing in the magazine article.

I think my latest edit got it cleaned up - Thanks again.