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Date Posted: 15:18:05 03/22/02 Fri
Author: Ralph
Subject: Understanding VLF Detectors - General Information

As a first in this series of articles on VLF metal detectors, and more particularly gold-specific detectors, I wanted to just quickly mention a few things that might better help in understanding these types of machines.

First of all, the nomenclature used these days to identify certain types of metal detectors as "VLF" is a little misleading, and not entirely accurate.

All metal detectors operate somewhere within the radio spectrum within a certain range of frequency or frequencies, depending on how that particular machine is designed to operate. While most "VLF" detectors operate within a very narrow range or a specific frequency, others, such as pulse induction or "PI" machines, work off of a somewhat wider range of combined frequencies rather than just one or two (or in the case of some Minelabs, several).

Most single frequency detectors these days are grouped into the class of "VLF", although these detectors may operate within the true VLF range or have a higher frequency range up into the LF spectrum. To put things on a level playing field in detector descriptions, you would have to call a PI type detector a "multi-frequency" machine, and the corresponding types either VLF or LF (for very low frequency or low frequency).

But to help end the confusion over which is which, perhaps the VLF/LF class should be more accurately called "INDUCTION BALANCE" or IB rather than just referring to their frequency range. That would give us two primary categories of

1. Pulse Induction
2. Induction Balance

Now, without getting into the principles of pulse induction, which we will do at a later time, let's look briefly at how an induction balance metal detector actually works.

In very simple terms, an induction balance detector creates an artificial electromagnetic field from it's coil at a known and very specific frequency. The transmitted signal frequency (TX) of that field is then received back into the detector through it's receiver circuits (RX), and the TX frequency and the RX frequency are effectively "balanced" so that they are the same coming out of the detector and again re-entering the detector. Then, when a metal object is entered into the field of the coil, there is a slight variation or distortion created between the RX frequency and the TX frequency which is read by the receiver circuits as a "target" within that field. A very simple concept actually.

In order for the detector to "see" a metal object, that object has to exhibit some level of magnetic field of it's own. This is where the artificially "induced" electromagnetic field of the detector comes into play.

While ferrous metals exhibit a natural, "self-radiating" magnetic field of their own (as a result of the magnetic properties of the earth itself), "most" non-ferrous metals have no way of producing their own magnetic field in nature. This is one reason why magnetometers operate as they do, being able to locate only ferrous objects while totally ignoring most non-ferrous metals. Magnetometers are "receiver only" type detectors, and detect the distortions in the earth's natural magnetic field that the ferrous metals create within their surroundings. What an induction balance or pulse induction detector does is to "inject" any metal object within the field of the coil with an artificial electromagnetic field of it's own. While this is a "constant induction" process for an induction balance detector, it could be described as a "period induction" process for the PI type machine. In pulse induction, the transmitted field through the coil is turned on and off, allowing for a remaining or "eddy" current to discipate in the metal object, where with an induction balance machine, the induction process is constant with the distortions in the field between the transmitted signal and received signal being used to detect the presence of metal.

This is just a very basic description of how an induction balance type metal detector works, and later we will delve into the areas of ground balancing, discrimination of different metal conductivities, and other operating functions and features of the different machines.

Ralph

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