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Chet

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Chet last won the day on June 23

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  1. Most EMI is picked up by the coil. The coil receives EMI similar to the antenna of an AM radio. The larger the coil surface the more EMI is detected. Proper EMI shielding of the coil is the most critical factor. For a low impedance mono coil the approximate three foot of lead can be a twisted pair of unshielded wires and be very effective in canceling EMI. It does this by the EMI on each wire being balanced and of opposite voltages being introduced by the twisted wires receiving equal and opposing EMI signals. With a Double D coil the low impedance transmit wires are often a unshielded twisted pair. The higher impedance sensitive receive coil is normally connected by a small diameter shielded cable.
  2. The shielding quality and effectiveness of different coil manufactures could explain some of the differences in EMI levels from different coils. Shielding must provide a critical balance between EMI, Ground effects and best overall detector performance. Most coils are shielded with a conductive paint which varies in resistance by design and by quality control. Too low of a resistance paint applied incorrectly will reduce EMI as well as Detector performance. Too high of a resistance paint will allow more EMI in and more ground noise. Shielding method and materials used in the approximately three feet of transmit and receive leads is also important to preventing EMI problems. Some coils may have quality control problems during the manufacturing process. As with many products performance reports from the users generally sorts out the better products. Have a good day, Chet
  3. Shortly after I got my GPZ 7000 I tested the WM12 delay and found it to be approximately 20ms which is close to the Garrett Axiom Z-Lynk 17ms delay. Reference; https://www.detectorprospector.com/forums/topic/2854-testing-the-minelab-pro-sonic-response-time/#comments Have a good day, Chet
  4. “So just having some fairly decent shallow discrimination on a prospecting machine would be pretty useful in many cases and save a lot of time just being able to concentrate on digging the targets that are higher probability of being gold,“ My preference is to incorporate a separate VLF circuit board or integrated VLF circuity within a Pulse Induction detector with a fast switchover from one mode to the other. The switchover could be as simple as turn off/on the appropriate transmitter. The coil would require special or additional winding/s but is doable. The display and control functions would change with the mode change. For years when in trashy areas I have carried a modified VLF detector for discriminating and pinpointing. Currently I prefer the Equinox 800.
  5. First some background on discrimination; VLF discrimination is normally dealing with measuring phase shift referenced between a transmitted sine wave and a relatively strong sine wave caused by a target that unbalances a null between the transmitter and receiver coils. This works quite well for small shallow targets that are expected to cause phase shifts within reasonable design limits. The design limits might range from iron nails to large rings or large coins with aluminum, lead and gold in the middle of the range within 12 inches of depth. Also the targets are expected to be within a size range that allows the phase shift to be within limits of the expected possible targets. They do not work well for shallow large targets that exceed the capability of the phase shift measuring circuitry. Nor do they work well for weak deep targets. There are a lot of unknown variables encountered with a Pulse Induction detector to incorporate into the design of discrimination processing. There are so many false variables to deal with unknown target sizes, shapes, wires, rusted metal shapes, and mineralized soil from oxidized metals. Some pulse induction detectors have been built that have some discrimination capabilities. My GPX 5000 with a Double D coil has that capability. It worked on some targets such as nails but overall I found that running the more sensitive Mono coil and digging all targets was faster and produced more gold. I have built a couple of prototype pulse induction discrimination circuits (based on target time constant) which worked on the bench with good solid targets. But failed when junk targets out the ground were tried they failed miserably. Measuring phase shift of the transmitter power waveform similar to what you suggest works well with good targets but falls apart when various questionable or weak targets are tested. A GPZ 7000 or GPX 6000 like detector could be designed from the ground up with a lot of software processing to make some reasonable target evaluations of phase shift and/or target time constant and display a result along with a percentage of probability number. It will take a lot of work and testing with an engineering expenditure to match the task. Hopefully Minelab will find a way to do it. But they may not appreciate the complaints from us users when we are unhappy with some of the limitations.
  6. "a real world square-like waveform that has ringing and finite sloped rise/fall edges due to real world impedance factors" Yes, the amount of ringing and slope of the ringing waveform is shaped by damping resisters (impedance loading). The slope and length of the sloped wave will be altered/strecthed by Eddy Curents emitted from metal targets that were energized by the transmit pulse. Target detection is determined by sampling for changes in the sloped wave with high speed gating/timing circuits of different timings/mode selections. Examples; High Yield timing favoring short Time Constant targets (small nuggets); General timing favoring Long Time Constant targets (large deeper nuggets).
  7. GPZ 7000 ZVT The GPZ 7000 Zero Voltage Transmission (ZVT) technology provides a stable processing period throughout the entire receive period. It also provides a stable magnetic field that reduces the amount of undesirable soil that is detected. This along with better signal processing provides improved detection of small nuggets and larger deeper nuggets. Bipolar (positive and negative) pulses can be generated in standard Pulse Induction detectors. But between pulses the transmit energy starts at zero voltage and builds until it peaks and discharges energy then it must start at zero again in the opposite polarity. This creates current and voltage variations on the power wiring that can adversely affect the receiver and processing circuits. Thus is not as stable as the GPZ 7000. In the attached oscilloscope GPZ 7000 Transmit Waveform displays; the zero volt level is across the vertical center of the display. Both the positive and negative excursions of the waveform pass up and down through the zero voltage level rapidly. In a standard Pulse Induction detector the waveform would stop at zero volts for a short period while the receiver timings and processing completed then start recharging the transmit coil for the next cycle. In the GPZ 7000 the receiver coil timings and signal processing does not require the transmit function to return to zero volts to recharge for a new transmit cycle. In the time stretched displays; the squidgy somewhat sine shaped forms at the top and bottom is time periods where the receiver timings and signal processing can be applied to the receiver coil signals to determine if a target is present. Note; only the Transmit waveform is shown in these displays. The receiver displays are a distorted mess of EMI and ground noise. That is where good engineers really excel in signal processing to extract target information that best fits the Time Constant curves that are displayed in a previous posting. In my opinion reference to the Bipolar power and high voltage pulse with coinciding receive operation as being similar to a VLF detector is not false but somewhat of a stretch. But maybe a good sales pitch. Have a good day, Chet
  8. The GPX 6000 11” and 17” x 13” Mono coils have the same electrical specifications as most previous GPX series coils. That is; the inductance is close to 300 microhenrys (uH) and the direct current resistance is close to 0.3-0.5 ohms. And shielding appears to be similar. The GPX 6000 Double D receiver coil is different from previous GPX series Double D coils. I could not get a DC ohms measurement across the receiver coil. It appears to have a capacitor in series with the winding. Small existing GPX aftermarket coils using some type of adapter/dongle will probably work great. The GPX 6000 works great on small shallow gold by having a fast recovery and processing time for short time constant targets. There may be limitations on large deep gold using large existing aftermarket coils. The GPX may be designed with overall power and sensitivity limited near the GPX 5000 performance. Concentric coils could certainly be developed for the GPX 6000. It would require much development and testing of various sizes to hit sweet spots in performance for different nugget sizes and different depths. Concentric coils for the GPX 6000 will need to demonstrate an overall performance improvement or specific niche in the midst of the other coils to make profitable sales.
  9. Some years ago a friend brought his GPX 4500 to me to see if I could fix it. He had taken a hard fall on a large rock pile when it quit working. It had been through multiple owners and long past its warranty period. Attached is a photo of a broken ribbon cable that resulted from his fall. Evidently a rock impacted the battery cable and connector flexing the aluminum faceplate far enough to tear the thin plastic ribbon cable. At the time it occurred to me that that the short span between the 5 pin battery connector and the adjacent toggle switch did not have enough flex loop. The ribbon cable may have been weakened by many years of repeated flexing from connecting and disconnecting the battery cable and sideways straining of the cable in normal use. In our detecting areas the cable gets tangled in a lot of brush and suffers many stretch events. With a lot of patience I bypassed the five broken circuit paths with thin flexible jumpers. In Jennifer’s photos the blobs of solder may have come from replacing this ribbon cable?
  10. My choices for Elliptical coil sizes are; 9” x 6”, 14” x 9”, and 24” x 15”. For Very Low Frequency (VLF) detectors with high length to width ratio elliptical coils such as Big Foot coils work well for coin and relic hunting. In my opinion Pulse Induction (PI) gold detector elliptical coils should have more conservative ratios of less than 1.60 : 1. I think conservative ratios provide a good balance between detecting small shallow nuggets and larger deeper nuggets. If the goal is to detect mostly very small shallow nuggets then higher elliptical ratios of 2:1 or 3:1 in small coil sizes may be desired. Have a good day, Chet
  11. Tony When detecting in trashy areas, gullies and desert washes I frequently carry a VLF along with the GPX 5000, GPZ 7000 and now with the GPX 6000. If roaming large areas of desert with only an occasional bullet or boot tack to contend with the VLF is left in the Jeep. Over many years as VLF detectors improved the models for this purpose were upgraded from the early Goldmaster series to MXT to GMT to Gold Monster to now to a Equinox 800. All equipped with the smallest coil available. When a target is detected in a trashy area with the Large PI coil it is quickly pinpointed and discriminated with the VLF. If is too deep to detect with the VLF it gets you pumped up a little thinking that this is something good. As the hole gets bigger and deeper the small coil on the VLF pinpoints the direction to dig. Sometimes a dozen plastic knives with bright colored surveyors ribbons tied around them are used as markers. Detect the area with the PI. If a shallow loud target is detected lay a knife over it. If it is a weak low broader signal place the blade into an upright position. Lay the PI down and detect each marked target with the VLF. The shallow loud ones are fast and easy. The deeper ones require some aggressive digging to get the VLF close enough to detect, pinpoint and discriminate. From years of experience with many detectors has led to this determination. The Equinox 800 with the small coil is the best, simplest automatic detector for this work and/or for stand-a-lone gold detecting. It has a build in pinpoint mode that immediately gets you on top of the target. Once on top you punch it back to discriminate mode and one or two sweeps you know whether it is ferrous, nonferrous or a hot rock. The target number read-out is so much better than the Gold Monster bar graph. Like most modern highly sensitive VLFs built for gold detecting it will sound off on many hot rocks. But it will have a repeatable number read-out that will correlate to the normal hot rocks produced in that area. Have a good day, Chet
  12. I spent 6 weeks camping in Canada and Alaska in the summer of 1989. It was a great road trip except for the mosquitoes and flat tires. I did master the Canadian salute; the action of slapping them flat on my forehead.
  13. All detectors are of little use at 1m depth unless it is that rare multi-ounce retirement nugget. As previously stated you must remove the overburden. Watch Rob’s video on this forum at; https://www.detectorprospector.com/forums/topic/16593-part-2-dig-shovel-rake-metal-detecting-for-gold-nuggets-video/#comments Have a good day, Chet
  14. The camera operators and TV directors are probably wearing them lol.
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