Geotech

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Geotech last won the day on December 9 2016

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  1. Not in a VLF, the "ground zone" (ferrite) is a pure magnetic response (no eddy response) and has a 0° phase shift. Everything from there up to 90° represents ferrous content (includes some amount of magnetic response), and from 90° to 180° represents non-ferrous. By definition, there are no eddy targets in the "ground zone." A traditional PI is different, as it doesn't give a reactive response. The "ground zone" is usually a GB point set for magnetically viscous material, which can mimic a certain eddy response. There seems to be some confusion over whether you're describing a VLF or PI.
  2. Fisher CZ and White's DFX/V3 all simultaneously transmit & receive 2 (or 3) frequencies at the same time, and use continuous-time (so-called frequency domain) demodulators. What I call "Concurrent MultiFrequency," or CMF. Minelab BBS/FBS/FBS2 all sequentially transmit & receive 2 frequencies, and use discrete-time demodulators. What I call "Sequential MultiFrequency," or SMF.
  3. Hahahaha I ask David that every day.
  4. I'm not feeling near as pissy as I was when I learned that my wireless pinpointer patent was dropped and XP would own that technology, so I won't comment on all the behind-the-scenes stuff other than to say, official statements rarely reflected reality. One bit of truth was that the Cypress module has a low transfer rate (62kbits/sec as I recall) which is good enough for audio packets, but transferring programs turned out to be slow and prone to drops. New modules are 15-30x faster and can easily do the job. And it's only with Bluetooth 4 that BT audio has gotten fast enough to be usable for detectors, but still tends to be more power-hungry than using a proprietary protocol.
  5. Deus uses a PIC24F processor, which is simple to hack and almost impossible to code-protect. By taking the Deus to the African market XP was begging to be hacked.
  6. This idea has been batted around for years but, to my knowledge, has ever been implemented. Several reasons why come to mind: 1. Target responses don't vary a whole lot until you get beyond a 2:1 frequency ratio. That is, the difference between 10k and 12k isn't enough to make both of those frequencies worthwhile. So we could sweep the following: 1k, 2k, 5k, 10k, 20k, 50k, & 100kHz. 7 frequencies would give you all the information you need, maybe even as little as 4. 2. Most detectors use zero-IF demodulators and for good SNR require multiple cycles of the frequency. The more frequencies you have, the less time the demods have to accumulate data. Beyond 3 or 4 frequencies you'll see this as a lag in responsivity. On the upside, it would be a good method for pinpointing where lag is not important, and you could even do full non-motion (static) disc. In fact, White's has a patent on a "zero motion 3F disc method" that was supposed to end up in the V3 but did not. Ideally, you would want to use direct sampling in a sweep-frequency detector, but this requires a really fast high-precision ADC. Direct sampling detectors (X-Terra, Go-Find, Prizm 6T, Deus) currently max out at less than 20kHz. 3. Sweeping the TX across a wide range of frequencies is easy, a DDS can do it. But as the frequency increases the coil current decreases, so a 100kHz TX signal would be very weak. You can see this in Minelab BBS/FBS detectors, where the 25kHz TX signal has 1/8th the strength of the 3.125kHz signal; they go deep on silver, but don't do well on small gold. Solving this means increasing the TX voltage with frequency which sounds easy but it's not. Designing a coil to work well enough over a 100:1 frequency range that you can easily swap coils without having to recalibrate the detector is also a huge challenge. 4. Until about 10 years ago the processing horsepower just wasn't there. Now it is, but it takes a while for detector companies to catch up to what the rest of the mass electronics market is widely doing. So a sweep-frequency detector is doable, but I'm not sure it would offer the benefits people imagine it would. Probably a wide-band 3F detector would offer 99% of the target ID benefits while being overall more usable and deeper.
  7. Nothing was ever changed for EMI issues. The PC interface was done and working, really nicely. I pushed hard to challenge the ML patent but was alone in that effort. I expect XP will win.
  8. Well, it's more complicated than that. Instead of a sinusoid, you could run a square wave drive which generates a triangle wave current, which does have a fundamental and harmonics. This is exactly what a Fisher CZ does. A SF detector could demodulate only the fundamental (ignore the harmonics), and vary the square wave frequency to whatever it wants, the advantage of which no coil capacitor needs to be selected. The V3 does it this way (in SF mode), and I'd guess (but don't know) that the Alter 71 does it this way, too. But I expect most of the switchable frequency detectors on your list run sinusoidal transmitters.
  9. Good article Steve, but wrong on the harmonic info. Switchable frequency machines switch to different fundamental frequencies, not harmonics of the base frequency. Most of them run sinusoidal transmitters so there are no harmonics, and they switch in different capacitors to do so. Also, the DFX doesn't really run in single frequency mode. It always transmits/receives 2 frequencies, just ignores one of them in "single" mode.
  10. Yes, nuggets often have multi-domain eddy responses.
  11. Both polar and cartesian plots can have XY axes, what they represent depends on what they represent. Technically, for a metal detector they should be the IQ axes, for the "in-phase" and "quadrature" signals. Sometimes they are called the XR axes, for reactive (X) and resistive (R). Sometimes the ferrite is on the left, sometimes on the right, and sometimes at the 12 o'clock position, depending n the preference of the engineer. Yes, kind of. Bent and twisted metal ends up having eddy responses with different phase angles (the metal thickness appears to vary), so the response is not homogeneous. A bent nail, or can slaw, are good examples. Jewelry can also be all over the place.
  12. Deus looks to be the same as V3, maybe a little more real time. The V3 attempts to plot a single sweep over the target, which is hard to figure out without a motion sensor in the coil. A perfectly straight tight line represents a strong single-domain eddy response. The tilt of the line is the raw phase. Close to 3 o'clock (180°) is high silver; 2 o'clock might be copper cent; 1 o'clock nickel; 12 o'clock would be salt; between 9 and 12 o'clock is iron; 9 o'clock (0°) is pure ferrite. The distance from any point on the line to the origin is the signal strength. Yes, this is a simple vector response. Most (all?) detectors map the raw phase (0-180°) into a VDI range (i.e. 0-100); there is no standard for this. As the response balloons out into a tilted ellipse, the ballooning represents a changing phase as the coil sweeps over. Often this is simply due to a weak signal. So you can read the tightness of the response as a confidence level. The wild responses can be due to 2 things: multi-domain eddy responses, as in the silver chains; and combined magnetic & eddy responses, as with the bottle caps. As a bottle cap enters the coil field it initially looks ferrous due to the iron content. As it goes past the center of the coil the eddy response of the flat steel dominates and so the phase at the peak amplitude of the response looks non-ferrous. Many detectors report target VDI as read at the signal peak, which is why bottle caps fool them. With the XY plot you can visually see the whole response, so bottle caps become easy to discern.
  13. X=reactive and Y=resistive; together they give you the phase. So for a straight line response, the angle of the line is the phase, and the length of the line is the amplitude.
  14. This excuse is greatly overused by companies. Custom & semi-custom parts (keypad, display, etc) and sole-source parts, yes. But most electronic parts are either multi-sourced, or easily substituted with something close enough. The actual reality is, most companies (and not just detector companies) don't repair at the component level, they repair at the board level. That is, they swap out a whole board. When a product is discontinued, they normally set aside a stock of replacement boards sufficient for x-years of repair. Once those boards are gone, no more repair.
  15. That was pointed out by customers over & over on the Prizms. Before I left, the MX Sport package had reached proto stage, and I tried to convince mgmt that the h/p jack was on the wrong side, and why. I think my effort actually persuaded them to leave it where it was. In a nutshell, that's why there were so many missed opportunities.