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Metal Detectors & Emi - Electro Magnetic Interference


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For background on electrical interference in VLF detectors here is a great essay on electrical interference by Dave Johnson of First Texas Products. You can also find a more detailed discussion that includes PI detectors in section 2.1 of this Minelab document by Bruce Candy.

OK, so I am bench testing my new Teknetics G2 at home recently. The 19 kHz models are renowned for being immune to EMI (ElectroMagnetic Interference). No electrical interference ever (well, almost never) even at highest gain levels.

As a rule, the lower the frequency, the more issues you have with EMI. It is especially bad under 10 kHz. DEUS owners may see significant EMI at 4 kHz, only a little at 8 kHz, and none at 12 and 18 kHz. Another reason why manufacturers favor mid frequency over low frequency detectors these days.

I have been bench testing detectors in my house for years so think I know the EMI levels. No issues with previous 19 kHz units like the Gold Bug Pro and F19. Yet this new G2 chatters like crazy! I fire off an email to First Texas asking if some change I was unaware of. Nope. And could not be a bad coil because both coils I have did it.

Then on another go I noticed the EMI was bad on one end of house but not the other. I walk around and the detector leads me to a new LED bulb I installed recently. This thing is pumping out 19 kHz EMI like crazy! Not long ago I installed a number of these cheap LED bulbs in my house. https://www.amazon.com/Feit-Electric-Replacement-CEOM60-927/dp/B01BJ0Y1MC Looks like my mission to upgade my house to all LED just ran into a snag!

More on LED bulb interference - https://www.google.com/search?num=30&q=led+bulb+electrical+interference&oq=led+bulb+electrical+interference 

Just shows how more and more we are surrounded by new forms of electrical interference to make life harder for detector engineers.

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Another issue with the constant bombardment of EMI is regarding the health risks associated with it.

We live in a society that is awash in EMI some benign and some potentially hazardous. As to the level of risk and exactly what levels produce these risks I don't know but it is something to think about. Or not...... We're all gonna end up worm food at some point.

Remember the old saying..... Eat right, stay fit.... Die anyway. 

Have a great day :biggrin:

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Yesterday I connected the new wireless headphones to my MX Sport but nothing sounded right. You move the detector around it made all kind of noise and the sound coming through the headphones was bad too. I disconnected the the wireless but the detector done the same. It showed the battery level was good but I put new ones anyway. Then what was said on EMI may be my problem. So outside with the MXS I go and now not one oddball sound came out of it.

What I was in was a small room that I play around with my junk. I have a light,fan and a small TV not over three feet away. With the electrical field around me I should light up too.

Chuck

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9 hours ago, Glenn in CO said:

Question Steve: How does EMI effect a PI detector at the frequency they operate at? If you could put in layman terms, the Minelab info is over my head.

 

It actually is very simple. Metal detectors have a transmit circuit (TX) and a receiver circuit (RX). You see marketing focus all about what the detectors transmit, but that is actually kind of beside the point. VLFs are transmitting usually on a primary frequency plus all kinds of harmonic frequencies. PI detectors are like frequency shotguns, just blasting all over the place. Just ask any VLF operator trying to work near a PI. Detectors are transmitting on far more frequencies than are actually used, except for marketing purposes. More is better, right? Well, not really. Seen from another perspective throwing frequencies off all over the place is a waste of energy and focusing all the power into a single frequency gets the most punch. And if you get really technical about it metal detectors are not actually intentionally transmitting anything except an electromagnetic field. So forget all that.

What really matter most is that the detector is tuned to receive and process only certain frequencies. The electromagnetic field generated by a metal detector coil induces eddy currents into conductive targets, that in turn can be sensed by the metal detector. The signals deep in the ground from tiny targets are like NASA trying to hear the Voyager spacecraft in interstellar space. Most of the work in a detector goes into trying to build a sensitive receiver that wants to pick up everything in the world, and then tuning and filtering it to remove the vast amount of interference from EMI, the ground, and trash targets. Part of that effort may include receiving and comparing two or more frequencies.

Minelab PI detectors have incredibly sensitive receivers and do actually have to compensate for the constant lighting strikes around the world (sferics) and the earths magnetic field.

My first metal detector was actually just a little square printed circuit board with a few components and a 9V battery plugged into it. The coil was printed on the board. The receiver was any AM radio you had. You just set the radio at the right frequency, and it heard what the little board was doing. See, the DEUS is not as revolutionary as people think - my very first detector in the 1970s was wireless.

So forget all the transmitting stuff and just think in terms of ultra sensitive radio receiving devices and you can easily see where the problems arise. In a nutshell, how to pick up the signal generated by a current induced into a 1 grain gold nugget buried in mineralized soil with power lines nearby while I am carrying a cell phone and a lightning storm looms in the distance!

The basics of the transmitter end are generally old school. Induction balance VLF type detectors, or pulse induction detectors, with simple or modified pulse trains. Where most of the action has been going on as of late is in the area of signal processing and how detectors sort out the good stuff from the bad using microprocessors and sophisticated algorithms all acting all the receiving side of things.

Typical, I started out intending to give a simple answer and then made it complex by loading up on details......

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The problem is mostly in the detector though adding wireless does just add another interference source that has to be dealt with. This can be seen in the Minelab WM10 and WM12 wireless modules and the fact that the CTX and GPZ versions ended up different due to interference issues. Anyone designing a wireless system, be it Bluetooth, etc has to deal with the interference issue. And look at the problems we face with pinpointers and detectors interfering with each other. It just keeps getting worse.

 

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Steve,

Excellent ... I'll reread and let it soak in several times.

The receiver ... I have always 'tested' my sensitivity of coil/detector by how close (2-3 feet) you can get to an aluminum can (for example) from the SIDE.  The transmit has always lit up the target but the receiver often can't 'see' it.  When hearing from the side I will move toward the target like a moth to a flame.  Of course the target is louder under the coil and it is one of the reasons why we have to remove the trash with a boot scrape or rake away the hot rocks on a good/gold hill.

The little or faint target can be masked by so many things.  Dig most things ...

Mitchel

 

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