Frequency Domain and Time Domain relationships

[mikebg]

NARROW BAND METAL DETECTORS

I renamed these detectors SI (sine induction) detectors because there are PI (pulse induction) detectors.

When TX radiates CW - Continous Wave (more suitable term is single frequency) magnetic field, RX receives TGT signal as a narrow band in frequency spectrum. The band appears because of modulation. When search head moves, the TGT signal changes; it has a beginning, a maximum and an end as was shown before. If we stop movement, the TGT signal remains constant, ie the modulation disappears (bandwidth becomes zero) and received TGT signal has the same frequency as AIR signal . As mentioned, the bandwidth does not exceed 16Hz. So TGT signal seems on the locus in complex plane as a point despite in reality it is a very small arc or segment. If the TX circuit is competent designed, the ground can not modulate the AIR signal.

Below is illustrated AM (amplitude modulation) with carrier wave and two side bands. I think the side bands are not two for metal detectors because when AIR signal is zero, we have onlyTGT signal as a SSB (single side band),
I can make the RX of a PI machine to operte as narrow band metal detector. Then the TX (the excitating field will operate very inefficient.

[Ferric Toes]

Lamontagne link:

http://geophysics.kos.net/website/pd...0Advantage.pdf

Many thanks. Had not seen that before.

Eric.

[Qiaozhi]

Hi Qiaozhi and Carl,

Not worth arguing about, but with traditional PI the TX is well and truly off when sampling takes place, particularly at medium and late times. The only time I would consider that the return signal is modulating the TX is if the TX is still ringing.

Eric.

I agree, it's not worth arguing about.
But I will add that I was really referring to a VLF (continuous wave) system when I said that.

The generally accepted definition of modulation, with regard to communication systems, is that a signal is mixed with a sinusoid to produce a new signal. In the case of a VLF detector, a metal target modulates the TX signal both in amplitude and phase. The sample gates are then used to demodulate the RX signal and to provide the in-phase (I) and quadrature (Q) components. The I and Q parts of the demodulated signal can then be used to provide an indication of the possible target type.

In a BFO detector, the metal target modulates the frequency of the TX signal.

For a PI detector, the accepted definition of modulation is somewhat stretched, but you could consider the decay curve as being amplitude modulated by the target.

As with many things in science and engineering, there are multiple ways of looking at a problem.

[Dave J.]

Yep.

If you want wave-particle duality in quantum mechanics to lose its mystique, design a circuit in the frequency domain and then analyze it using an oscilloscope in the time domain.

Don't forget to take into account Nyquist critereon, bandwidth, and noise figure. When you take those things into account, Szilard was right, Maxwell's Demon is not a god but a parasite.

--Dave J.

[Aziz]

You guys have been sticking to TD for a very long time. You all don't see the forest for the trees.

It's even time to leave the FD. Use math to transform the TD/FD domain:
The MD (math domain).

Aziz

[Aziz]

Ooooops, I've forgot the other M domain.
The madness domain (MD). Become first mad before you enter the math domain (MD). You don't need much IQ for it. You need a much higher MQ (madness quotient). And then you see the forest.
*LOL*
Aziz

[Ferric Toes]

Another MadLabs riddle! The proof is what works in the real world.

Eric.

[Aziz]

Another MadLabs riddle! The proof is what works in the real world.

Eric.

Eric,

all the information of the response (if any there) can be processed by state of the art methods. My profession and expertise is just processing the available information (data mining).
Believe it or not. The WBGB is the output of such engineering.
Aziz

[Qiaozhi]

Yep.

If you want wave-particle duality in quantum mechanics to lose its mystique, design a circuit in the frequency domain and then analyze it using an oscilloscope in the time domain.

That's a good analogy.

Don't forget to take into account Nyquist critereon, bandwidth, and noise figure. When you take those things into account, Szilard was right, Maxwell's Demon is not a god but a parasite.

--Dave J.

I would concur with Szilard, it's an absolute certainty that Maxwell's Demon uses more energy to operate the gate (not to mention the actual measurement of the speed of the molecules) than is gained in the overall system. Thus entropy increases and the Second Law of Themodynamics is not violated.
But we digress ...

[mikebg]

SYNCHRONOUS DEMODULATOR IN FREQUENCY DOMAIN

SYNCHRONOUS DEMODULATOR IN FD

Frequency domain is handy tool for visual analysis.
Let we use it to analyze how SD (synchronous demodulator) eliminates GND signal and what happens with TGT signal
at ground balance .
As you know, signals are represented in FD as vectors.
The synchronous demodulator operates as vector multiplicator. It makes scalar multiplication of two vectors named

signal and (phase) reference. The output is DC voltage as illustrated below.

Scalar product of two vectors is constructed by taking the component of one vector in the direction of the other and multiplying it times the magnitude of the other vector.

More understandable is the visual representation of this definion. Let we use the phase reference as scalar axis
shown below as straight line S- . . . 0 . . . S+ . The DC output appears as positive voltage OD on the scalar axis.
The operator can change the phase angle between received signal and phase reference with GEB or DISC control.

When phase difference is quadrature, then output is zero because cos90°=0. This is illustrated In the right side
where SD receives two signals: vector OG (GND signal) and vector OA (TGT signal). The operator can not change
angle ß between TGT and GND signal, but he can set the GND signal in quadrature.
This simple visual analysis of SD allows us to make important
CONCLUSIONS:
1. The SD can operate as amplitude demodulator (because when magnitude OA changes, output OD changes).
2. The SD can operate as angle demodulator (because when angle changes, output OD changes).
3. The SD can suppres a signal when is set in quadrature (90°) relative to phase reference.
4. For best results, the angle ß between GND and TGT signal should be quadrature (90°). However 45° is not bad. As
shown in the drawing, the output is 0.707 ODmax at 45°. This is only -3dB.

[mikebg]

INCOMPETENT DESIGNED SEARCH HEAD

The circuit diagram in left shows an incompetent designed search head for PI or wideband metal detector. RX and TX windings have equal number of turns N1=N2. The windings are placed in close proximity, which makes coefficient of coupling almost K=1. This generates the maximal possible AIR signal in RX winding and RX receves strong GND signal. To avoid saturation of RX preamp, we should reduce its amplification (gain). This reduces sensitivity. Despite drawbacks, the incompetent designed search head has two advantages:
1. The form of received EMV is as derivative of TX current.
2. The phase of received signal is constant independent on frequency,
http://www.geotech1.com/forums/showt...552#post163552

The circuit diagram in right shows the worst case - both windings are combined to a self - inductance called MONOCOIL. Then voltage across coil resistance r is added to AIR signal. A Re component is added to received signal at all frequencies and the phase angle depends on frequency. The RX can not sample when flows current in TX coil and we have not correct reference voltage for synchronous demodulation:
http://www.geotech1.com/forums/showt...729#post163729

CONCLUSION: For best results, QED and all other (more primitive :-) metal detectors should use induction balanced search head.

[Tinkerer]

metal detectors should use induction balanced search head. From above.

Why are people afraid of IB coils?

What makes IB coils more difficult to build?

Where lie the advantages of IB coils?

I will start a new thread to discuss Induction Balanced coils. For myself, I much prefer them to mono coils.

Tinkerer

[mikebg]

metal detectors should use induction balanced search head. From above.

Why are people afraid of IB coils?

What makes IB coils more difficult to build?

Where lie the advantages of IB coils?

I will start a new thread to discuss Induction Balanced coils. For myself, I much prefer them to mono coils.

Tinkerer

This is a good idea because Aziz also prefers IB.

[Ferric Toes]

I'll just pop this into the melting pot. Geonics Technical Note TN-30 (part of).

Eric.

Geonics MF004.pdf

[Carl-NC]

The circuit diagram in left shows an incompetent designed search head for PI or wideband metal detector. ... Despite drawbacks, the incompetent designed search head has two advantages:

There is another advantage: it greatly simplifies power supply design. So is it really an incompetent design, or just another way to do things that works perfectly fine in some applications? Can you name a popular commercial detector that has this kind of search coil?