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Frequency Shift
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| 3355658 |
Differentiating Metal Detector for Detecting Metal Objects and Distinguishing Between Detected Diamagnetic and Non-Diamagnetic Objects |
Gardiner, VLF BFO |
| 3467855 |
Object Detector and Method for Distinguishing Between Objects Detected Including a Pair of Radio Oscillators |
Basic BFO |
| 3546628 |
Oscillating Metal Object Detector |
Simplest detector on earth |
| 3601691 |
Metal Detector Responsive to Small Metallic Objects for Differentiating Between Ferrous and Non-Ferrous Objects |
Gardiner, Z-response |
| 3662255 |
Apparatus for Locating Concealed or Buried Metal Bodies and a Stable Inductor Usable in Such Detectors |
Garrett Zero-Drift BFO |
| 3823365 |
Metal Detecting Apparatus Having Improved Ground-Effect Immunity |
Uses induction balance |
| 3875498 |
Metal Detector for Distinquishing Between Precious Metal Objects and Other Metals |
D-Tex patent, your real basic BFO. |
| 3896371 |
Metal Detector With a Resonating Circuit Being Driven by a Frequency Higher Than Its Natural Resonance Frequency |
A.H. Electronics, off-resonance type (AH Pro) |
| 3961238 |
Selective Metal Detector Circuit Having Dual Tuned Resonant Circuits |
Gardiner, very basic Z-response |
| 3986104 |
Dual Frequency Metal Detector System |
Gardiner |
| 4130792 |
Metal Detector With Feedback Tuning |
Z-response with feedback |
| 4196391 |
Metal Locator with Stereotonic Indication of Translateral Position |
Stereo BFO. |
| 4204160 |
Metal Detector With Automatic Optimum Sensitivity Adjustment |
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| 4255710 |
Plural Search Frequency Directional Metal Detector Apparatus Having Enhanced Sensitivity |
Uses two gated oscillator frequencies (I think). |
| 4263553 |
Discriminating Metal Detector With Compensation for Ground Minerals |
A.H. Electronics, off-resonance detector |
| 4321539 |
Digital BFO Metal Detecting Device with Improved Sensitivities at Near-Zero Beat Frequencies |
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| 4439734 |
Metal Object Locator Including Frequency Shift Detector |
Induction balance and ground balance. |
| 4678992 |
Electronic Metal Detector |
A.H. Electronics, off-resonance type (Super Pro?) |
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Induction Balance
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| 3405354 |
Apparatus for Limiting Phase-Angle Response Range, Particularly in Eddy Current Testing Apparatus |
Uses syncronous demodulation to determine target phase, see 3848182. |
| 3471772 |
Instrument for Measuring the Range and Approximate Size of Buried or Hidden Metal Objects |
Synchronous detectors, lots of fundamental theory & equations. |
| 3471773 |
Metal Detecting Device with Inductively Coupled Coaxial Transmitter and Receiver Coils |
Basic TR with 4B-style coil (NOT coaxial!). |
| 3826973 |
Electronic Gradiometer |
Actually, a metal detector. |
| 3848182 |
Apparatus for Limiting Phase-Angle Response Range, Particularly in Eddy Current Testing Apparatus |
See 3405354. |
| 3872380 |
Metal Detector Distinguishing Between Different Metals by Using a Bias Circuit Actuated by the Phase Shifts Caused by the Metals |
Gardiner, basic phase response IB. |
| 4030026 |
Sampling Metal Detector |
White's, the basis of most of their early-80's analog detectors. |
| 4053828 |
Metal Detector with First and Second Nested Rectangular Coils |
Not too practical for a handheld. |
| 4096432 |
Metal Detector for Discriminatory Detection of Buried Metal Objects |
Basic phase response IB. |
| 4099116 |
Metal Detector With Phase Related Selective Discrimination Circuit |
Nautilus, feedback method for ground balance and discrimination. |
| 4110679 |
Ferrous/Non-ferrous Metal Detector Using Sampling |
White's |
| 4128803 |
Metal Detector System With Ground Effect Rejection |
PNI (the old Bounty Hunter), probably the Red Baron series (RB3/5/7). |
| 4249128 |
Wide Pulse Gated Metal Detector With Improved Noise Rejection |
White's |
| 4300097 |
Induction Balance Metal Detector with Ferrous and Non-ferrous Metal Identification |
Techna, now First Texas Mfg. (Bounty Hunter) |
| 4303879 |
Metal Detector Circuit with Mode Selection and Automatic Tuning |
1979 Garrett patent (ADS?) |
| 4325027 |
Metal Detector for Locating Objects with Full Sensitivity in the Presence of Distributed Mineral Material |
Compass |
| 4334192 |
Metal Detector Circuit Having Automatic Tuning With Multiple Rates |
Garrett, probably their Master Hunter VLF. |
| 4334191 |
Metal Detector Circuit Having Momentary Disabled Output |
Garrett |
| 4344034 |
Selective Ground Neutralizing Metal Detector |
Gardiner patent for phase discriminator. |
| 4348639 |
Transmitter-Receiver Loop Buried Metal Object Locator with Switch Controlled Reference Vooltage |
Discovery Electronics' two-box detector. |
| 4423377 |
Compact Metal Detector of the Balanced Induction Type |
Garrett, handheld with integrated double-D coil. See also 4488115. |
| 4470015 |
Metal Detector System With Undesirable Target and Mineralized Ground Discrimination |
Teknetics, lots of diagrams and waveforms. |
| 4486712 |
Frequency Dependent Pulsed Gain Modulated Metallic Object Detector |
| 4488115 |
Low Battery Voltage Indicator Circuit for a Metal Detector |
Garrett, see also 4423377 |
| 4677384 |
Target-Identifying Metal Detector |
Teknetics |
| 4700139 |
Metal Detector Circuit Having Selectable Exclusion Range For Unwanted Objects |
Garrett patent with a pretty good explanation of I&Q signal processing. |
| 4709213 |
Metal Detector Having Digital Signal Processing |
White's |
| 4868910 |
Metal Detector with Microprocessor Control and Analysis |
White's, ton's of info on their target ID. |
| 4894618 |
Metal Detector Using Cross-Correlation Between Components of Received Signals |
Minelab, good explanatory text. |
| 5506506 |
Metal Detector for Detecting and Discriminating Between Ferrous and Non-ferrous Targets in Ground |
Minelab, combines synchronous demodulators with PI, calls it multi-frequency. Compare with US5576624 |
| 5523690 |
Metal Detector with Bivariate Display |
White's patent detailing VDI, includes 6805 assembly code. (see also 5596277) |
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Magnetometers
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| 3919630 |
Flux Detection Device Using a Parametrically Excited Second Harmonic Oscillator |
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| 4068164 |
Harmonic Flux Gate Magnetometers and Gradiometers and Windings |
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| 4277751 |
Low-Power Magnetometer Circuit with Constant Current Drive |
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| 4293815 |
Fluxgate Magnetometer Circuit with Earth's Field Elimination |
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| 4439732 |
Electrically Balanced Fluxgate Gradiometers |
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| 4677381 |
Flux-gate Electrical Drive Method and Circuit |
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| 4887033 |
Magnetometer Having Coil Formed by Two Spiral Shaped Conductors |
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| 5168223 |
High Sensitivity Saturable Core Magnetic Field Sensor with Symmetrical Structure |
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| 5670882 |
Ferrous Target Identification System with Magnetic Field Polarity and Gradient Indication |
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| 5939881 |
High Dynamic Range Digital Fluxgate Magnetometer |
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Other Geophysical
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| 3500175 |
Electromagnetic Prospecting Apparatus Utilizing VLF Radio Field |
VLF Receiver. |
| 3614600 |
Electromagnetic Prospecting Apparatus for Detecting Electrically or Magnetically Responsive Ore Bodies |
VLF Receiver. |
| 3617866 |
Geophysical Surveying with Audio Frequency Electromagnetic Fields and Orthogonal Receiver Coils |
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| 3636435 |
Method of Electromagnetic Prospecting by Measuring Relative Gradient of a Resultant Electromagnetic Field |
VLF Receiver. |
| 3763419 |
Geophysical Using the Vertical Electric Component of a VLF Field as a Reference |
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| 3852659 |
Geophysical Prospecting Method and Apparatus Utilizing Correlation of Received Waveforms with Stored Reference Waveforms |
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| 3950695 |
Geophysical Prospecting Method Utilizing Correlation of Received Waveforms with Stored Reference Waveforms |
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| 3967190 |
Method Using Induced Polarization for Ore Discrimination in Dissiminated Earth Deposits |
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| 4070612 |
Method and Apparatus for Measuring Terrain Resistivity |
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| 4114086 |
Inductive Source Method of Induced Polarization Prospecting |
Prospecting method. |
| 4258321 |
Radio Geophysical Surveying Method and Apparatus |
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Misc.
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| 3882374 |
Transmitting-Receiving Coil Configuration |
IB |
| 3941690 |
Portable Sluice Pan |
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| 4255711 |
Coil Arrangement for Search Head of a Metal Detector |
Compass, concentric IB |
| 4293816 |
Balanced Search Loop for Metal Detector |
White's concentric loop |
| 4345208 |
Anti-falsing and Zer Nulling Search Head for a Metal Detector |
Daytona search coil |
| 4472269 |
Sluice Box Classifier for Gold Mining Dredge |
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| 4862316 |
Static Charge Dissipating Housing for Metal Detector Search Loop Assembly |
White's concentric loop |
| 5247257 |
Electronic Metal Detector Return Signal Phase Changer |
A nail and two coils, weird. I don't know what to make of this one. |
| 5696490 |
FM (VHF) Infrared Wireless Digital Metal Detector |
Places the control box near the coil and uses IR to transfer info to the meter, pretty useless. |
| 5863445 |
Etched Coil Unibody Digital Detector |
Handheld wand. |