X vs R

[Sergey_P]

[Qiaozhi]

Your documentation of phasers/vectors inspires me to try to create some simply documentation on the use of these as my next documents.

Or you could become the master of cryptic replies, like Sergey.

[mikebg]

Or you could become the master of crytic replies, like Sergey.

George, I have a feeling that they would love to join in a new revived (R)EMI group.

(R)EMI means reinvent, rediscover, revise, redesign electromagnetic induction.

If one organize such a group, I am a candidate for spokesman because I have two years work experience as such in the forum. However members of the group should indicate me what I will talking about.

[mikebg]

Sergey, M=0 is an impossible mission.
Despite this, I have analyzed your suggestion before years:
http://www.geotech1.com/forums/showt...629#post102629
To suppress AIR signal, we should analyze how it is generated.

[Sergey_P]

M= zero or constant ... does not matter ... it has no effect on Re, Im target!

[mikebg]

M= zero or constant ... does not matter ... it has no effect on Re, Im target!

It does mater because the AIR signal appears as carrier wave for TGT (target) signal.
1. When the AIR signal is large, the TGT signal has very low modulation index and synchronous demodulator can not detect it.

2. The AIR signal limits possible gain of preamp. When AIR signal is large, we should decrease the gain of preamp to avoid its saturation. That means less sensitivity.

Let we continue with AIR signal at hegative M. At positive M we have phase lead of response 90 deg as if the system predicts the future. At negative M we have phase lag 90 deg and all seems normal. However this is not normal for induction balanced search head. In the next post we will see how IB (induction balance) generates M<0 (negative mutual inductance).

[Sergey_P]

<1…, 2…> - Dynamic range - is reduced.
<M(tx-rx)> - It is associated with target (Re, Im)?? – NO! This - static error, its leads to zero.

[mikebg]

<1…, 2…> - Dynamic range - is reduced.
<M(tx-rx)> - It is associated with target (Re, Im)?? – NO! This - static error, its leads to zero.

The TGT signal is phase associated with AIR signal. AIR signal is EMV induced in RX coil by change of TX current. The change of TX current induces EMV in eddy loop of conductive target. Both EMV have the same shape and differ only in scale.

Synchronous demodulators see the TGT signal as modulation of AIR signal. If we can adjust M=0 (ie the AIR signal is zero), the modulation of TGT signal is SSB (single side band shifted no more than 16Hz). The AIR signal appears as carrier wave and modulation becomes SB (side band modulation with radiated carrier wave).

However we should add third drawback of AIR signal to both mentioned in post #36.
3. The AIR signal can act as additional GND signal with different phase. Then the demodulated AIR signal is not only DC component. When TX coil moves, the ground halfspace acts as a changing core. It causes modulation of TX current and respectively modulation of AIR signal. The genuine GND signal has the same modulation.

If we want to design very sensitive metal detector, we must suppress with automatic control amplitude modulation of TX. Here is a suitable circuit diagram of TX.

http://www.geotech1.com/forums/showt...772#post152772

In practice, when TX coil moves, the demodulated AIR signal at this TX circuit is pure DC.

[Sergey_P]

The TGT signal is phase associated with AIR signal. AIR signal is EMV induced in RX coil by change of TX current. The change of TX current induces EMV in eddy loop of conductive target. Both EMV have the same shape and differ only in scale

more details, please ...
( Modern devices have stable TX (Quasar – Qtx<7, Verator4 – Qtx<1, …, PI - Qtx<1) )

[Davor]

Sergey, you often exaggerate Tx Q as something very relevant, while Low Q it is just an engineering solution to quartz controlled Tx. Nothing more. There is a voltage across a coil, and it's phase is a reference for everything else. High or low Q, it is always that particular voltage that counts. If you remove the tank capacitor and produce the same voltage across the coil it would work the same. Very same.

There is a way of supplying a resonant coil with digital excitation at the tank resonance (thus preserving LOTS of battery current), and it involves a PLL and some extra bits. Once some of the leading detectors manufacturers patents this approach, a new mantra will be Hi-Q. Perhaps I should beat them to it.

One thing I can't fathom with Verator, a rig that I find very advanced in many details, is - why the heck the X and Y phases are not extracted digitally. A pair of flip-flops would do that just beautifully and dead-on.

[mikebg]

SIGNAL OF PERMEABILITY

SIGNAL OF PERMEABILITY

We know that ferrite core increases magnetic flux. Let we see what happens when an induction balanced search head moves over a small core made of lossless ferrite as illustrated below.

Both coils L1 and L2 are in induction balance because the positive flux Ф+ (shown with the red area) is equal to negative flux Ф- (shown with the blue area), When the blue area of search head appears over ferrite core, the search head generates signal as negative mutual inductance M<0 shown in post #36 . When the red area is over the ferrite core, the search head generates signal as positive mutual inductance M>0 shown in post #27 .
The diagram below represents how changes virtual mutual inductance M when search head moves.

CONCLUSIONS:
1. The generated by lossless permeabilty signalis are as AIR signal at positive mutual inductance.
2. The induction balanced search head has zones with reverse readings.

The theory shows that the lossy ferrite has Re component proportional to frequency. That means the phase lead of lossy ferrite is less than 90 deg as is illustrated on the diagram.
Small iron meteorites and small hot rocks generate signal as small ferrite core.
What happens when we have a large ferrite core as halfspace? It increases both Ф- and Ф+. Therefore, the induction balanced search head can suppress the signal genrated from magnetic soil.

[mikebg]

AIR signal at MONOCOIL

Davor, have you read the articles pointed by Ken with links in his post #30?
http://www.geotech1.com/forums/showt...592#post163592
What you mean?

AIR signal at MONOCOIL

MONOCOIL or self-inductance is specific case of mutual inductance when k=1 because Ф2 = Ф1. The formula for M


becomes M = L. Note that M for MONOCOIL is always positive. That means the AIR signal should have positive Im

component as shown in post #27, but we should analyze this with SPICE.

Unlike induction balanced search head, the MONOCOIL has no reverse reading. It has a lot advantages, but a

disadvantage makes MONOCOIL not suitable even for BFO metal detecors. Monocoil generates the largest AIR

signal.

Let we connect a MONOCOIL to NETWORK ANALYZER as shown below.

[Tepco]

So, finally, that elusive AIR signal in IB coil is actually signal due to coil disbalance. But coils can produce disbalance signal in both phase directions referenced to TX, (negative mutual inductance?!?) and ferrite may or may not generate signal of opposite polarity to it. I hate computer sims.

[WM6]

Davor, have you read the articles pointed by Ken with links in his post #30?
http://www.geotech1.com/forums/showt...592#post163592
What you mean?

AIR signal at MONOCOIL

MONOCOIL or self-inductance is specific case of mutual inductance when k=1 because Ф2 = Ф1. The formula for M


becomes M = L. Note that M for MONOCOIL is always positive. That means the AIR signal should have positive Im

component as shown in post #27, but we should analyze this with SPICE.

Unlike induction balanced search head, the MONOCOIL has no reverse reading. It has a lot advantages, but a

disadvantage makes MONOCOIL not suitable even for BFO metal detecors. Monocoil generates the largest AIR

signal.

Let we connect a MONOCOIL to NETWORK ANALYZER as shown below.

You are probably right.
In theory.
In practice mono coil is still the best possible PI coil if you are asking for depth.

[Davor]

What you mean?

For a given coil configuration only a voltage across the Tx coil is relevant for target excitation, and not the Q factor of a tank the Tx coil is placed into. That was only a reflection on Sergey's insisting on low Q for good detection.