Minelab Multi-frequency

[Steve Herschbach]

21 hours ago, Dubious said:

Geotech, who apparently is an engineer with the right equipment, determined that for each Equinox mode, only two frequencies are used (i.e., current at those frequencies that energizes the coil in a meaningful sense).  Unless someone can explain how he's wrong, that is the reality.

Geotech is Carl Moreland, founder of the famed Geotech Forum. Later a senior engineer at White’s Electronics, where he had a major hand getting the V3i out the door. He is now a senior engineer at First Texas. The people who make Bounty Hunter, Fisher, and Teknetics detectors. You can very much rely on his opinions.

[Geotech]

19 hours ago, Chase Goldman said:

While we are both factually correct in using the radio analogy because (the metal detector meets both the definition of radio and magnetometer) because it is emitting electromagnetic energy in the radio frequency part of the electromagnetic spectrum.  But the electric field energy (the radio part) is being lost in the ground while the accompanying magnetic field is doing all the work, including inducing the current in the target that results in magnetic field detected by the receive portion of the coil (the magnetometer part), as you described.  It is probably more correctly referred to as a radio frequency magnetometer (i.e., what you were describing) than a radio.

It's nether radio-like nor magnetometer-like. It's transformer-like.

5 hours ago, dewcon4414 said:

All you electronic gurus........ which im surely not, when it sends ...... lets just say 15khz single into a ring.... does it receive 15khz back or is it converted to something else entirely based on the conductivity/magnetic field of the metal and everything around it?   Meaning ..... does the khz used only LIGHT UP the metal which the length of this time is a known for various metals?   On multi freqs....... does one sustain a longer period that it lights up the metal...... in other words..... 5khz... is transmitted first then followed by 15khz.... giving it a longer period to be seen/read than a single freq might?

It's complicated. With a sinusoid TX (most SF-VLFs) the return signal from eddy current targets (non-ferrous) is a sinusoid at the same frequency. No distortion, no harmonics. Ferrous targets can distort the signal according to their B-H curves, but a SF-VLF doesn't consider the distortion.

MF detectors don't use sinusoids, they transmit triangle/ramp looking waveforms which have a lot of harmonics. Non-ferrous eddy responses are exponentials which also have harmonics, and ferrous responses are distorted triangle/ramps. These can be processed in the raw (time-domain: BBS/FBS) or filtered (freq domain: CZ, DFX, V3). Equinox is direct-sampling so it's hard to say which method is used, but freq domain seems easier.

Not sure what else you're asking.

3 hours ago, Alluminati said:

With the exception of Field1/2, I don't see much "weighting" at the coil, it could be done on the receive side. Field 2 is interesting, it kinda looks like a squarish wave, like a ghost with the right shoulder constantly moving up and down, while the ghost beside it has it's left shoulder moving up and down.

Beach mode does not have much of a sawtooth, almost sinusoidal looking, but it too has a little bit of the dancing shoulders.

IMO this dancing might imply some mild shifting of frequencies going on. I recall older FBS/BBS machines being steady images.

If you're using an oscope, then this is a triggering issue. MF waveforms are difficult to trigger on. Also, it's critical to look at the current waveform, not the voltage.

[Chase Goldman]

1 hour ago, Geotech said:

 It's nether radio-like nor magnetometer-like. It's transformer-like.

Yeah, I had the transformer analogy in the long form version of my reply that I subsequently redacted for length.

I recognize the transformer coupling effect in the process but isn't the detector effectively sensing the magnetic field by measuring the current induced in the receive coil?  Isn't  that a type of AC magnetometer using transformer coupling? 

Legitimately just trying to improve my geek level of understanding here, Carl, so let me know what I am missing conceptually from your expertise as a MD engineer/designer which I certainly am not.  Thanks.

[dewcon4414]

Thanks Carl.........I appreciate your effort to educate.... the uneducated.... ME.

[Geotech]

The RX coil in a detector is almost* always used in voltage mode, so that the magnetic field induces a voltage (EMF) across the coil, not a current through it. See Faraday's Law. Mags may or may not use an RX coil in a similar manner, and may otherwise have similarities (some mags are designed in a similar way as a PI detector), but they are really different animals.

* At least I've never seen an RX coil set up in current mode, outside of stuff I've done.

 

[Alluminati]

Chase if I may, at least the way I picture it.

The 2 halves of the coil are tuned so that they null out. If one half is oscillating at 10kHz, the second half is receiving @10kHz (effectively for this discussion)

When you introduce a coin into the field, the coin has to wind up it's induction like a small fly wheel. This slight delay causes the signal to phase shift meaning the coin is not broadcasting in sync with the transmit side of the machine, it's delayed. That is what the machine is measuring.

The machine says hey, this 10kHz is a little bit behind the 10kHz transmit signal. The amount that the signal is delayed equates to the amount of phase shift. This phase "angle" is part of how the discrimination works. (A full 360° phase shift would have the peaks and troughs of the signal line up again, just the receive side is one hump behind the transmit side. "Group delay" at that point I think.)

In reality the receive side probably has a couple other phase de modulators, perhaps 90° apart to garner it's info. It probably accounts for permeability, angular freq and conductivity to come up with a number. 

[Chase Goldman]

Thanks Alluminati, I previously understood the phase angle shift detection discrimination piece but you explained it differently than I have seen it before.  Helps with understanding to have it explained in different ways.

I wanted to know the physical principle used on the receive coil end and Carl cleared that up for me that the detector receive coil is sensing induced voltage vice induced current in the coil.

One more question for Carl,  you mentioned that it was important to measure current vice voltage waveform to get an accurate picture of the transmit coil output frequency components?  But is the detector itself is filtering, processing, comparing voltage, presumably frequency domain information from the receive coil based on an output voltage reference from the transmit coil (vice output current reference)?  I know this is mainly speculation.  But it threw me that you emphasized measuring the transmit coil current waveform while the detector is using the voltage waveform induced on the receive coil for detection.  Why is it critical to measure current vice voltage on the transmit coil?

[Geotech]

If you want to understand frequency content, TX current gives a much clearer picture than TX voltage.

[Mark Gillespie]

Always enjoy Carl's knowledge inputs.

Now I'm waiting on a true PI detector with a good discriminator or a time function to eliminate unwanted metal targets.  "If man can think it, he can make it" with time.

My TDI has found a lot of good targets masked with iron.  And to add just a little, even gold that has eluded even the very best VLF's in the world.

 

[Cal_Cobra]

Carl how's that slow ship turning? 

I hope it's going to dock some time in 2019?