Patent US3875498A successfully refreshed from Google Patents.
US3875498A
Expired / Lapsed
US
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Metal detector for distinguishing between precious metal objects and other metal objects
Assignee: D TEX ELECTRONICS
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Filing: 1973-09-20
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Granted: 1975-04-01
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Expires: 1993-09-20
Abstract
Abstract The circuit for a portable metal detector includes a search coil oscillator which includes the search coil in the foot or probe of the detector and a tunable reference oscillator, with the oscillators being connected in a differential circuit whereby the difference or beat frequency of the oscillators is detected and amplified, with corresponding signals being fed to audio devices and to a milliammeter. The circuit provides for a shift in relative phase angle between the oscillators when the reference oscillator is tuned to a frequency below that of the search coil oscillator. When the field of the search coil approaches a ferrous object, the phase angle shifts in one direction to produce one type of change in the aural and meter signals; and when the search coil field approaches a precious metal object, the relative phase angle of the two oscillators shifts in the other direction to produce a different type of change in the aural and meter signals.
Inventors
- 👤 William A Mahan
- 👤 Lester A Williams
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-28 04:29
CPC Classifications
G; G01; G01V; G01V3/00; G01V3/08; G01V3/10; G01V3/101
Claims Analysis
Total Claims: 8
Claim 1
Independent Claim
253 words
A detector including first transistor oscillator means including a search inductor radiating radio frequency energy and adapted to be passed in proximity to an object to be detected; second transistor oscillator means; said first and second oscillator means each including a tank circuit tuned to a common resonant radio frequency; the transistors of said first and second oscillators being connected in a common base bias resistive network, and the output electrodes thereof being connected to a common mixer network; said first oscillator including a resonant tank circuit consisting of a parallel connected inductor and capacitor; said tank circuit capacitor including a feedback capacitor connected in parallel with said transistor between said transistor emitter and collector electrodes; a variable resistor connected in parallel with said feedback capacitor; and said feedback capacitor and variable resistor providing a phase adjustment for said first oscillator; a transistor detector means includIng a base bias resistive network independent from the common base bias resistive network for said oscillators; means including a capacitor connecting the output of said mixer network to said detector means base bias resistive network; means coupling the output of said detector means to a transducer to produce a corresponding aural signal; means coupling the output of said detector means to a transducer for producing a corresponding visual signal; and tuning means connected in one oscillator tank circuit for changing the phase angle of said one oscillator relative to the other oscillator, whereby said first and second oscillators unlock to produce a difference frequency at the mixer.
Claim 2
Dependent Claim
253 words
A detector including first transistor oscillator means including a search inductor radiating radio frequency energy and adapted to be passed in proximity to an object to be detected; second transistor oscillator means; said first and second oscillator means each including a tank circuit tuned to a common resonant radio frequency; the transistors of said first and second oscillators being connected in a common base bias resistive network, and the output electrodes thereof being connected to a common mixer network; said first oscillator including a resonant tank circuit consisting of a parallel connected inductor and capacitor; said tank circuit capacitor including a feedback capacitor connected in parallel with said transistor between said transistor emitter and collector electrodes; a variable resistor connected in parallel with said feedback capacitor; and said feedback capacitor and variable resistor providing a phase adjustment for said first oscillator; a transistor detector means includIng a base bias resistive network independent from the common base bias resistive network for said oscillators; means including a capacitor connecting the output of said mixer network to said detector means base bias resistive network; means coupling the output of said detector means to a transducer to produce a corresponding aural signal; means coupling the output of said detector means to a transducer for producing a corresponding visual signal; and tuning means connected in one oscillator tank circuit for changing the phase angle of said one oscillator relative to the other oscillator, whereby said first and second oscillators unlock to produce a difference frequency at the mixer.
Claim 3
Dependent Claim
51 words
A detector as set forth in claim 1 audio amplifier means connected between said detector means and said aural transducer; the output of said detector means being coupled to the input of said audio amplifier means through a differentiating network consisting of a series connected capacitor and a parallel connected resistor.
Claim 4
Dependent Claim
51 words
A detector as set forth in claim 1 audio amplifier means connected between said detector means and said visual transducer; the output of said detector means being coupled to the input of said audio amplifier means through a differentiating network consisting of a series connected capacitor and a parallel connected resistor.
Claim 5
Dependent Claim
45 words
A detector as set forth in claim 1 said second oscillator including a resonant tank circuit consisting of a parallel connected inductor and capacitor; said tank circuit capacitor including a feedback capacitor connected in parallel with said transistor between said transistor emitter and collector electrodes.
Claim 6
Dependent Claim
27 words
A detector as set forth in claim 4 a variable capacitor connected in parallel with said inductor of said second oscillator tank circuit, defining said tuning means.
Claim 7
Dependent Claim
47 words
A detector as set forth in claim 4 said capacitor for each oscillator tank circuit including a respective first capacitor and said respective feedback capacitor connected in series; and the respective capacitance of each said first capacitor and said feedback capacitor providing a feedback ratio of 3:1.
Claim 8
Dependent Claim
81 words
A detector as set forth in claim 6 said inductors for both oscillator resonant tank circuits having an inductance of 2.33 millihenrys, said first capacitors for both of said resonant tank circuits having a capacitance of 15, 000 pf, and the resonant frequency for both oscillators being 54 KHZ; the feedback capacitors for both oscillators each having a capacitance of 5,000 pf, thereby defining feedback ratios of 3:1; and said inductor for said first oscillator resonant tank circuit having 99 turns.
References Cited (Backward Citations)
US3467855A
Cited
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1978-03-31 | 1978-09-30 | 1979-04-01 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1982-03-31 | 1982-09-30 | 1983-04-01 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1986-03-31 | 1986-09-30 | 1987-04-01 | $7700.00 | Window Closed / Lapsed |