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Todd, an answer about 'Metal Detector Filters' regardless of operating frequency range.

January 10, 2020 11:20AM avatar
ALL makes and models of detectors will use 'filtering' for different applications. I have usually used the words "so-called" and similar terms when I am responding to a post and make any reference to some 'filtering', especially when grouping detectors as being a 4-Filter, 2-Filter or 3-Filter design. I have described the reasons for, and use of, those terms. It started forty-some years ago after George Payne designed our first VLF-Disc. or motion-based Discriminators in '78 when at Bounty Hunter. The Red Baron.

We had mainly been using conventional TR or TR-Disc. detectors, most of which operated at or close to a 100 kHz frequency. One of the bigger operating issues we had to deal with was the effects of ground mineralization on the Electro-Magnetic Field generated about the search coil. A coil should be operated at a uniform coil-to-ground relationship so as to eliminate or diminish the effects of 'false' signals when the coil height is changed. With a proper slight audio 'hum' Threshold level, the audio will increase abruptly if the coil is lifted away from the ground which moves it away from the offending mineralized ground signal. However, if the coil is lowered close to the ground, that increases the ground signal influence on the EMI as more ground is brought into the near-portion of the EMF close to the search coil.

In 1974/'75 George Payne developed our first metal detectors designed to cancel out the effects of the ground signal by designing detectors that operated at a much lower frequency in the Very Low Frequency range which is 1 kHz to 30 kHz. These detectors were able to be adjusted for a proper slight audio Threshold, and then with another control, adjusted to be able to ignore the effects fro ground mineralization, thus ignoring the effects of search coils changes (lifting or lowering) from the ground. Along with that new design, not only did they ignore the ground mineral signal, but it made that search mode reactive to ALL metal objects, to include both Ferrous and Non-Ferrous.. Thus, since the 'VLF' mode ignored the ground and responded to everything, to include ferrous objects, that was referred to as an All Metal mode

Consumers had the pleasure of ignoring the ground falsing and now getting much better depths because they cancelled out the ground signal. But the new problem was being able to reject or Discriminate all the unwanted trash, such as iron nails, foil, bottle caps, pull-tabs, etc. So they incorporated a 2nd search mode to go with the 'VLF' mode, and that brought us the VLF-TR-Disc. model. We could cancel the falsing effects of the ground mineral signal in the VLF mode, to find all metal targets, ferrous and non-ferrous, then switch to the conventional TR-Disc. mode to use Discrimination to isolate the better targets and reject, Discriminate, the unwanted trash. That was a really good thing.

But with many advances in any design there are going to be trade-off.moody smiley

Because these newer detectors were working at the VLF frequency rather than those much higher frequencies they used t, it made them more reactive to changes in coil-height from the ground. That meant when we used a TR-Disc. mode to check a target, there would be more reactivity to coil height from the ground. In short, it could false a lot easier while trying to Discriminate unwanted common trash.

One mode, VLF, to Discriminate the ground signal, and another mode, TR-Disc. to Discriminate the unwanted trash like nails and foil and tabs, etc., etc. Wouldn't it be great to ignore the ground signal AND reject the trash all at the same time? Sure it would! Leave it to George Payne, Once again, he combined VLF ground cancelling with Discrimination to reject unwanted trash in 1978 with a VLF-Disc. mode.

Now, by this point, we had been using, and progressively advancing, metal detector circuitry design, but hadn't really been using some terminology such as 'filters' or 'filtering.' To keep it simple, let me just explain that 'filtering' is any function that filters or removes something that is unwanted, such as trash. The TR Discrimination mode is Discrimination or cancelling or rejecting or ... filtering out ... lower-conductive and unwanted trash.

We were staring to see other name /description changes in the industry such as 'VLF.' To this point it only referred to a search mode that ignored the ground mineral signal and therefore responded to All metal targets. Here, and in the next five years we had new terms for 'VLF' such as:

GB ... Ground Balance
GEB . Ground Exclusion Balance
GC ... Ground Cancel
GNC . Ground Neutralizing Circuitry
GA .... Ground Adjust
GEN . General Search Mode
... but more popularly ...
All Metal because that's what the initial 'VLF' mode provided.

For a few years (with White's lingering linger than the others) quite a few manufacturers made 3-Mode detectors:

• VLF ......... a Ground Cancelling All Metal mode.
• VLF-Disc.. a motion-based Discriminate mode to reject trash, that also Cancelled the Ground Signal. at the same time.
• TR-Disc.... a conventional TR-Disc. mode that still dealt with the ground signal and the coil-height of operations.

But with many advances in any design there are going to be trade-off.moody smiley Guess what? it happened again.

With the new VLF-Disc. models, starting in '78, we gained the pleasure of not hearing the ground signal and still being able to reject a lot of the common trash using the VLF-Disc. search mode. However, to accomplish that task, the search coil had to be swept at an amazingly brisk sweep speed. I mean, it almost looked like a golf club swing, except keeping the coil in uniform relationship with the ground.

To accomplish that task it involved 'filtering' in order to reject or cancel the ground signal. The search coil was swept very fast to feed in all the information, the ground signal and target signal, and then the detector's circuitry would 'filter-out' the low-reading ground signal and pass along the target signal. Of course it could be a trash target so the circuitry was still busy 'filtering out' the Discriminated trash signal as well.

The cell phone you use, the computer you use, the TV you watch and all sorts of appliances and electronic contraptions around your home are made up using a LOT of 'filters,' but thy are not talked about, referred to or discussed. They are just there and do their job ... hopefully.

Back to the fast-motion Discriminating detectors. Slow but sure the engineers managed to slow that very brisk sweep-speed to a little more comfortable moderate rate of speed and that made these detectors more appealing. So much that many detector makers were about to simply eliminate the conventional TR-Disc. mode and simply market a detector that always cancelled the ground, and the operator could pick between a VLF, or All Metal search mode, or a VLF-Disc. or motion-based, ground cancelling Discriminate mode

It was the early '80s and we're slowly entering these mode transitions and everyone who tried a VLF-Disc., aka motion-Disc., model had to learn to use a sufficient sweep speed to get performance.desired. Some newcomers to the hobby, or some older folks who had enjoyed it with earlier models just didn't seem to adapt or learn that a faster sweep speed was required. Those who hunting wide-open grassy parks or sports fields, kind of enjoyed those detectors, and that's fine. I know I did for some applications, bt I spent most of my detecting time in brushy, weedy, old town sites or other out-of-the-way locations, and with building rubble and a lot of debris, fast-sweeping detectors were a disadvantage, to be sure. I wanted a slower-sweep detector because the fast-motion units didn't work well in the heavily littered and brushy sites I sent most of my time.

Up to that point we did start to hear the term '4-Filter' occasionally when they tried to describe how the VLF-Disc. models worked and we able to process (filter) all the ground signals, ground noise, target signals, EMI, and all sorts of things. Then along came 1982 and'83. Fisher introduced their 1260X and in mid '83 Tesoro introduced their Inca, and both of those models became the first 'popular' use detectors that worked with a very slow-motion sweep speed.

The difference between those two detectors and almost every other motion-based Discriminator on the market was everyone body's unit needed a fast sweep and many had a long "ring time" before they restored to handle a nearby target. In the Fisher 1260-X literature they described it as being a "Double Derivative" design and people asked, so to simplify that tongue-tangle it means a 2-Filter design and Tesoro started to use that description, and the detecting industry started to toss it around a lot. faster-motion sweep speed and usually a little better in higher mineralized ground would related to a 4-Filter circuitry, and a slow-motion sweep speed with a quick-response and fast-recover was associated with a 2-Filter design.

None of those models use only 4 Filters or only 2-Filters, but even though that was mentioned in print, folks have had a difficult time understanding it. In '87, at Compass, John Earle designed the 'Vari-Filter circuitry which, as he explained, it might be considered a 3-Filter design because it offers the benefits of both a 2-Filter and 4-Filter concept. About a dozen years later, White's brought out the MXT, a Dave Johnson design, which was also a 3-Filter type and ever since I see the 'filter' topic come up a lot.

Since then, a couple of decades ago, we have seen a very pronounced move from analog detectors, or the analog - digital blend like the MXT series, move on into today's major market where most everything is more digitally-designed circuitry. What does that mean to this topic? Well, there is far more filtering' going on in the many different modes and functions that's done in software, that there really isn't any discussion or reference to 'filtering 'like we used to use. And that was mainly used to sort detectors into one of two categories. Slow-motion or Fast-motion sweep requirement.

The vast majority of everything offered today is more digital design and not analog or analog-digital like the MXT series. We usually don't come across references to 2 or 3 or 4 'filters' in discussions today because there are actually way too many filters or filtering involved in any decent detector. Besides, so much is done in the circuitry, either by hardware components or in software, that it would bee too much for engineers to explain so everyone listening would understand.

Hi Monte !

Does the All Metal mode circuit of VLF metal detectors have filters to separate, exclude and prevent various types of interference from masking target signals, or prevent falsing ?


As for this question, Todd, I'm at a loss. Filters are used for many things, and yes, most All Metal modes used today have at least some amount of 'filtering' going on for them to work functionally, but I guess I'm not clear on what you are asking with regard to an All Metal mode.

An All Metal mode of a VLF or LF frequency detector will have some filters used in the circuitry. Mainly those are to deal with 'noise' or possibly the filtering needed in an auto-retune circuitry. All Metal mode should respond to all metals so there wouldn't be any issues with good-target masking. The All Metal mode should respond to it all and you simply recover what causes a beep, take a look and see what you found.


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Edited 1 time(s). Last edit at 01/11/2020 05:35PM by Monte.
Subject Author Views Posted

VLF Metal Detector Filters

ToddB64 65 January 09, 2020 05:52PM

Todd, an answer about 'Metal Detector Filters' regardless of operating frequency range.

Monte 74 January 10, 2020 11:20AM

Re: Todd, an answer about 'Metal Detector Filters' regardless of operating frequency range.

ToddB64 39 January 10, 2020 07:28PM

Re: An error I didn't catch. Meant to type the 'b' and not the neighboring 'n'.

ToddB64 31 January 13, 2020 02:28AM

Re: Todd, an answer about 'Metal Detector Filters' regardless of operating frequency range.

ToddB64 40 January 10, 2020 02:35PM

Rob's write-up is interesting, and points out some of what I remind folks quite often.

Monte 41 January 11, 2020 09:22AM

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