Question's re Phase and "True" AM mode

[Qiaozhi]

Rather than talking about a target's inductance, capacitance and resistance as separate things, it may be better to talk in terms of the target's admittance.

Whether you talk about the admittance (i.e. reciprocal of impedance) or inductance, capacitance and resistance, this is only an electrical analogy of the real physical process. In other words, there are no tiny electrical components hidden inside the metal target. It's just a convenient way of modeling the process.

The TX/RX coil arrangement is essentially a transformer with loosely coupled windings, and not an antenna as people think. The target detection all happens in the near field. Without a target present, the search head is an air-cored transformer, with the RX coil nulled to minimize pickup from the TX. When the coil is over a metal target, the coil coupling is unbalanced, and the target acts like a transformer core. There is an associated delay (phase-shift) between the TX and RX signals that is dependent on the target material, and it's shape and orientation.

[Rudy]

I would say that your analysis is correct, even though it is difficult to visualize a metal target as having a physical inductance and capacitance. That's why I simplified the description by saying "the signal sensed by the RX coil is delayed". Of course, this delay could be modeled by an equivalent electrical network.

Depending on the actual physical coil construction and nulling, the received signals can all shift left, shift right, or a combination of both, as a result of the target being ferrous or non-ferrous. The coil tuning also has a significant effect. That's one of the reasons why so many people have problems constructing their own coils.

Rather than talking about a target's inductance, capacitance and resistance as separate things, it may be better to talk in terms of the target's admittance.

[ToddB66]

Question's re Phase and "True" AM mode

Thanks everyone ! I'm "eating this up". Keep it coming !

It helps to have a variety of replies, as each person has a unique way of explaining.

Soon as I digest all of this, I might have some additional questions or need something clarified.....we'll see.

Todd

[Gwil]

Hi Qiaozhi

I agree, a coin can't have much inductance measured in Henries (or even microHenries) but it doesn't have much resistance either, and it's the ratio of inductive reactance to resistance that determines the phase angle. So maybe its inductance could be significant...

Regards,

Gwil

[Qiaozhi]

I too tried to find an explanation of this phase shift business but I couldn't find one I understood, so I tried to analyse what happens using basic electrical theory. Not sure how true my conclusions are but so far they seem to agree with available information. Anyway this is how I see it:

Starting right at the beginning, a metal detector has a transmit coil that has an oscillating current circulating in it (at least most types do, although not pulse induction detectors which, not surprisingly, use pulses). This current sets up an oscillating magnetic field that penetrates the ground and cuts through any metal object there, inducing a similarly oscillating voltage in it. The voltage causes current to flow in the target, and this produces its own magnetic field which generates a signal in the receive coil. Assume for now that the target is non-ferrous.

All metal targets possess resistance, inductance, and capacitance (although capacitance seems not to be significant at the sort of frequencies used in MDs). These qualities depend on the size, shape and material of the target. If the target is entirely resistive, the current in it will be in phase with the induced voltage, that is to say a phase shift of zero degrees. If the target is entirely inductive, the current will lag the voltage by 90 degrees. These are extreme cases, and real life targets have different combinations of resistance and inductance, so the phase of the induced current, and therefore the phase of the signal in the receive coil, varies. For example, thin targets like foil tend to have higher resistance, and thick/low resistance targets like silver coins tend to have higher inductance. This fact can be used by discrimination and target ID circuits.

The extreme case of a "ferrous" target would be a piece of ferrite (a ceramic), which has high magnetic permeability but is a non-conductor so no current flows in it. However, it increases the coupling between the transmit and receive coils, unbalancing the nulled condition so that a signal appears. Normally its phase is 180 degrees from the transmit coil voltage. Real ferrous targets have high permeability, but also varying amounts of resistance and inductance which modify the end result.

Would anyone like to comment?

Regards,

Gwil

I would say that your analysis is correct, even though it is difficult to visualize a metal target as having a physical inductance and capacitance. That's why I simplified the description by saying "the signal sensed by the RX coil is delayed". Of course, this delay could be modeled by an equivalent electrical network.

Depending on the actual physical coil construction and nulling, the received signals can all shift left, shift right, or a combination of both, as a result of the target being ferrous or non-ferrous. The coil tuning also has a significant effect. That's one of the reasons why so many people have problems constructing their own coils.

[Gwil]

I too tried to find an explanation of this phase shift business but I couldn't find one I understood, so I tried to analyse what happens using basic electrical theory. Not sure how true my conclusions are but so far they seem to agree with available information. Anyway this is how I see it:

Starting right at the beginning, a metal detector has a transmit coil that has an oscillating current circulating in it (at least most types do, although not pulse induction detectors which, not surprisingly, use pulses). This current sets up an oscillating magnetic field that penetrates the ground and cuts through any metal object there, inducing a similarly oscillating voltage in it. The voltage causes current to flow in the target, and this produces its own magnetic field which generates a signal in the receive coil. Assume for now that the target is non-ferrous.

All metal targets possess resistance, inductance, and capacitance (although capacitance seems not to be significant at the sort of frequencies used in MDs). These qualities depend on the size, shape and material of the target. If the target is entirely resistive, the current in it will be in phase with the induced voltage, that is to say a phase shift of zero degrees. If the target is entirely inductive, the current will lag the voltage by 90 degrees. These are extreme cases, and real life targets have different combinations of resistance and inductance, so the phase of the induced current, and therefore the phase of the signal in the receive coil, varies. For example, thin targets like foil tend to have higher resistance, and thick/low resistance targets like silver coins tend to have higher inductance. This fact can be used by discrimination and target ID circuits.

The extreme case of a "ferrous" target would be a piece of ferrite (a ceramic), which has high magnetic permeability but is a non-conductor so no current flows in it. However, it increases the coupling between the transmit and receive coils, unbalancing the nulled condition so that a signal appears. Normally its phase is 180 degrees from the transmit coil voltage. Real ferrous targets have high permeability, but also varying amounts of resistance and inductance which modify the end result.

Would anyone like to comment?

Regards,

Gwil

[Qiaozhi]

... but what causes the phase itself ? I assume it has to do with something at the atomic level, but what is the cause for the occurrence of "phase" in various materials ?

I assume this is the main question.

When a metal detector TX coil passes over a target there are eddy currents induced in that target. These eddy currents in turn create their own electromagnetic field, which opposes the original TX field. However, it takes time for the eddy currents to become established in the target, and consequently the signal sensed by the RX coil is delayed. This is the phase-shift that you see between the TX and RX signals. Different metal targets give different phase-shifts that depend on such things as conductivity, shape and orientation.

[Carl-NC]

#1..Wikipedia gives information on "Phase shift", but what causes the phase itself ? I assume it has to do with something at the atomic level, but what is the cause for the occurrence of "phase" in various materials ?

Phase is just a property of existence, sorta like color or time. A sinusoidal signal can be generally described by its amplitude and frequency, and its phase usually comes in when comparing it to another signal, or looking at instantaneous portions. Like time, phase can be relative (lunch occurs 6 hours after breakfast), or it can be instantaneous (lunch is at 1pm), but like time there really is no such thing as absolute phase... both depend on defining a reference system. It is 9:45 am as I type this, but only because someone once defined our system of time such that it's now considered to be 9:45 am.

#2..What is the difference between "True" All-Metal mode and regular (not true) All Metal mode ?

Depends on who you ask! IMO, true AM mode is zero-motion all metal. Some detectors still apply disc to their AM mode so that it knocks out some iron, so it's really not all metal. Some apply SAT so it's not zero motion.

#2.1..Is there a simple test I can do to determine if my metal detector's are operating in "True" AM mode ?

Hold the loop motionless over a nail. It should continuously detect the nail and not tune it out.

Re question #2.1, If you don't know of a simple test that can be performed, then perhaps you know for certain if the following models have "True" AM mode. I have a Garrett Ace250, White's Classic ll and Tesoro Bandido ll uMax.

The Bandido II uMax is definitely true AM, the Classic definitely has SAT so it is not zero motion, and I don't know about the Ace 250.

- Carl

[ivconic]

"...#1..Wikipedia gives information on "Phase shift", but what causes the phase itself ? I assume it has to do with something at the atomic level, but what is the cause for the occurrence of "phase" in various materials ?.."

There is not "phase in various materials" !
Phase can be understood as just another preference to closely describe some signal. Each signal has certain phase - relative to some (previously chosen) reference. Usual reference is "zero phase" or origin which we can take as primary reference.
When talking about phase shift in metal detectors, usual reference (origin) is phase of TX-ed signal. RX-ed signal is delayed in time relative to TX-ed signal and logically has certain shift in phase comparing to TX-ed signal phase.
Various metals in vicinity of that em field do cause various shifts in phase.
Let's say that RX-ed signal is shifted just few degrees from origin (TX-ed) signal when no metal detected. Than when some metal detected - phase of RX is shifted more from previous position on scale. How much will some metal shift the phase of RX signal will depend on metal preferences. Phase shift caused by metal in detection area is excel preference to be used further in distinguishing the type of that metal.
Phase shift can be also understood as delay in time of some signal. Delay from origin position in some period of time.
Best way to understood phase and phase shift is to use 2 channel oscilloscope. On channel 1 TX-ed signal and on channel 2 RX-ed signal. You will notice small time shift between signals. Than when some metal object closed to coil, you will see more shift in phase of RX signal.
Conventional oscilloscopes without phase and phase shift readings are not suitable for precise phase measurements. However there are newer and more advanced digital scopes with phase angle reading features. Or there were special devices in the past like phase angle meters, usually with 2 inputs for reference signal and observed signal. That devices can display exact phase shift angle in degrees.
However...you may simply understand term "phase" as just another signal preference used to closely describe that signal. Phase is just position in time of that signal.
Regards!

[ToddB66]

I have no formal education in the science of electronics, but am interested in learning about some of the aspects that relate to metal detectors.

Where can I find explanation's (preferably website or alternatively book) on the following questions ? If you want to give a direct answer here, that's even better !!

#1..Wikipedia gives information on "Phase shift", but what causes the phase itself ? I assume it has to do with something at the atomic level, but what is the cause for the occurrence of "phase" in various materials ?

#2..What is the difference between "True" All-Metal mode and regular (not true) All Metal mode ?

#2.1..Is there a simple test I can do to determine if my metal detector's are operating in "True" AM mode ?

Re question #2.1, If you don't know of a simple test that can be performed, then perhaps you know for certain if the following models have "True" AM mode. I have a Garrett Ace250, White's Classic ll and Tesoro Bandido ll uMax.

Thanks everybody !
Todd