AU2011101742A4

AU2011101742A4 - A metal detector

Info

Publication number: AU2011101742A4
Application number: AU2011101742A
Authority: AU
Inventors: Alexander Lewis Jones
Original assignee: Minelab Electronics Pty Ltd
Current assignee: Minelab Electronics Pty Ltd
Priority date: 2010-12-22
Filing date: 2011-12-22
Publication date: 2014-08-14
Anticipated expiration: 2019-12-22
Also published as: AU2011265456A1

Abstract

Abstract A configurable handheld metal detector for producing an output signal indicative of a target, including: transmit electronics for generating a repeating transmit signal cycle; a magnetic field transmitter 5 connected to the transmit electronics for receiving the repeating transmit signal cycle to generate a transmit magnetic field; a device for receiving one or more parameters from a source external to the metal detector; a magnetic field receiver; a demodulator for synchronously demodulating a receive signal from the magnetic field receiver using a configurable demodulation function, wherein the configurable demodulation function includes at least three demodulation windows, and at least one characteristic of at 0 least one of the at least three demodulation windows is defined by a variable that is configurable based on the one or more parameters; and a digital storage medium for storing the configurable demodulation function, the digital storage medium is adapted to retain a value of the variable when the metal detector is switched off.

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicants: Minelab Electronics Pty Limited Actual Inventor: Alexander Lewis Jones Address for Service: C/- MADDERNS, GPO Box 2752, Adelaide, South Australia, Australia Invention title: A METAL DETECTOR The following statement is a full description of this invention, including the best method of performing it known to us.

2 Technical field This invention relates to a metal detector. Background 5 The general forms of most metal detectors which interrogate soils are either hand-held battery operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held products include detectors used to locate gold, explosive land mines or ordnance, coins and treasure, Examples of conveyor-mounted units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to locate buried land mines. 0 These electronic metal detectors usually consist of transmit electronics generating a repeating transmit signal cycle of a fundamental period, which is applied to an inductor, for example a transmit coil, which transmits a resulting alternating magnetic field sometimes referred to as a transmit magnetic field. Time domain metal detectors usually include switching electronics within the transmit electronics, that switches 5 various voltages from various power supplies to the transmit coil for various periods in the repeating transmit signal cycle. Time domain metal detectors include pulse-induction ("Pl") or P1-like metal detectors. It is possible for this technology to transmit and receive using the same inductor, often called a 'mono-loop coil". !0 Metal detectors contain receive electronics which processes a receive signal from a measured receive magnetic field to produce an indicator output signal, the indicator output signal at least indicating the presence of at least some metal targets within the influence of the transmit magnetic field. Depending on the operation of a metal detector and the characteristics of the ground a user is prospecting, a user can change the settings of the metal detector to improve its performance. 25 Brief summary of the invention According to an aspect of the present invention, there is provided a configurable handheld metal detector for producing an output signal indicative of a target, including: transmit electronics for generating a repeating transmit signal cycle; a magnetic field transmitter connected to the transmit electronics for 30 receiving the repeating transmit signal cycle to generate a transmit magnetic field; a device for receiving one or more parameters from a source external to the metal detector; a magnetic field receiver; a demodulator for synchronously demodulating a receive signal from the magnetic field receiver using a configurable demodulation function, wherein the configurable demodulation function includes at least three demodulation windows, and at least one characteristic of at least one of the at least three 35 demodulation windows is defined by a variable that is configurable based on the one or more parameters; and a digital storage medium for storing the configurable demodulation function, the digital storage medium is adapted to retain a value of the variable when the metal detector is switched off.

3 In one form, the synchronous demodulation of the receive signal produces a post-demodulation signal which is further processed to produce the output signal. According to another aspect of the present invention, there is provided a configurable handheld metal 5 detector for producing an output signal indicative of a target, including: transmit electronics for generating a repeating transmit signal cycle; a magnetic field transmitter connected to the transmit electronics for receiving the repeating transmit signal cycle to generate a transmit magnetic field; a device for receiving one or more parameters from a source external to the metal detector; a magnetic field receiver; a demodulator for synchronously demodulating a receive signal from the magnetic field receiver 0 using at least two configurable demodulation functions, wherein each of the at least two configurable demodulation functions includes at least four demodulation windows, and at least one characteristic of at least one of the at least four demodulation windows is defined by a variable that is configurable based on the one or more parameters; and a digital storage medium for storing the at least two configurable demodulation functions, wherein the digital storage medium is adapted to retain a value of the variable 5 when the metal detector is switched off. In one form, the synchronous demodulation of the receive signal produces at least two post-demodulation signals which are further processed to produce the output signal indicative of a target. !0 In one form, the digital storage medium is a non-volatile random access memory. In one form, the digital storage medium retains a predetermined value corresponding to the variable. In one form, if the one or more parameters fail a predetermined rule, the value of the variable is set to the 25 predetermined value. According to another aspect of the present invention, there is provided a method for configuring a handheld metal detector, including: receiving one or more parameters from a source external to the metal detector for configuring a configurable demodulation function, wherein the configurable demodulation 30 function includes at least three demodulation windows, and at least one characteristic of at least one of the at least three demodulation windows is defined by a variable that is configurable based on the one or more parameters; and retaining a value of the variable when the metal detector is switched off. To assist with the understanding of this invention, reference will now be made to the drawings: 35 Brief description of the drawings Figure 1 depicts a block diagram showing a functional block diagram of a metal detector which is configurable; 4 Figure 2 depicts examples of configurable demodulation functions; and Figure 3 depicts an example of a demodulator which can use configurable demodulation functions. Detailed description of the invention 5 Figure 1 is a block diagram showing a functional block diagram of one embodiment of a metal detector which is configurable. The term "configurable" means that at least a characteristic of at least one function in relation to the processing of a receive signal (for example, a demodulation function, a timing for sampling, a gain) is changeable/modifiable based on an external input. 0 The metal detector includes a sensing head I, which includes a magnetic field transmitter and a magnetic field receiver (not shown). The magnetic field transmitter receives a transmit signal from transmit electronics 3 to transmit a transmit magnetic field 10. In one embodiment, the transmit signal is a repeating transmit signal cycle. The magnetic field receiver receives a receive magnetic field 11. The magnetic field transmitter and receiver can be separate coils, or can be the same coil, within the 5 sensing head I. The magnetic field transmitter and the receiver can also be in other forms and shapes of magnetic field transmitters and receivers known to a person skilled in the art. Receive signal generated by the receive magnetic field I I received by the sensing head I is synchronously demodulated by demodulator 5 using one or more demodulation functions whose parameters are stored in a digital storage medium 7. The received signal may be filtered by a filtering !0 module, may be converted to digital form by Analogue-to-Digital Convertor (ADC) prior to the synchronous demodulation. The synchronous demodulation is timed to be synchronous with respect to the repeating transmit signal cycle. The outputs of the demodulator 5 are further processed, by processing unit 9, to generate an indicator output signal 13 indicating a target within the influence of the transmit magnetic field 10. Further processing may include filtering, averaging and linear combination of the 25 outputs of the demodulator 5. In one embodiment, the receive signal is sampled according to timing information stored in the digital storage medium 7, with gains, whose values are also stored in the digital storage medium 7, applied to the sampled signals. Such a process has a same effect with the receive signal being synchronously demodulated using one or more demodulation functions. The processing unit 9 receives input 12 from a source (not shown), which can be external to the metal 30 detector, to change the parameters stored in digital storage medium 7. The external source can be a user of the metal detector, changing the demodulation function via one of, or a combination of, conventional controls such as buttons, keypad, menus, switches, rotary encoders, knobs, touch-screen etc. A parameter can be changed by first identifying the parameter intended to be changed through one of, or a combination of, the conventional controls above, and manipulating the one of, or a combination of, the 35 conventional controls above. For example, by pushing buttons, a parameter indicating a starting time of 5 the demodulation function can be delayed or brought forward. Similarly, by turning a knob clockwise or anti-clockwise, a gain can be decreased or increased. Alternatively, the external source can be a computer. For example, a metal detector can be connected to a computer, through a physical connection or a wireless connection, and instructions or data can be loaded 5 as input 12 to change one or more of the demodulation functions stored in digital storage medium 7. Alternatively, the external source can be a portable digital storage medium, for example, a USB memory device. The portable digital storage medium, containing data or instructions, can be connected to the metal detector to change one or more of the demodulation functions in the digital storage medium 7 either directly, or through the processing unit 9. 0 In one embodiment, the digital storage medium 7 is a non-volatile random access memory which is capable of retaining the current values of the configurable settings for a demodulation function of the metal detector when the metal detector is switched off. This allows for the configurable demodulation functions to be the same even after a metal detector is switched off and on again, and prevents the configurable demodulation function from reverting back to default factory-set demodulation functions. 5 This is important as without such capability, a user needs to re-configure the configurable settings everytime the metal detector is switched off and on again. For example, if a user discovered that a slight delay of a starting time of one demodulation window (from the original factory-set starting time) can increase a detection performance of a metal detector when used on a particular ground type, the user can delay, through one of, or a combination of the conventional controls above, the starting time. Once the 0 starting time has been set and stored in the digital storage medium, the digital storage medium retain the new starting time, even if the detector is switched off. The new starting time is used when the detector is switched back on again, so that the user does not need to waste time configuring the starting time again. In one embodiment, the factory set starting time is also retained in the digital storage medium with the new starting time, so that if the new value for the starting time fails a predetermined rule, the starting time 25 will revert to the factory-set value. Predetermined rules are discussed further in a later part of this specification. It is often desirable to detect fast time constant metal targets such as fine gold nuggets, small bits of metal in low-metal-content landmines, fine gold chains etc. In PI detectors, this requires that synchronous 30 demodulation commence as soon as possible following a termination of a "back-emf' or transmit high voltage period, and the commencement of a zero-transmit and receive period. During synchronous demodulation, a demodulation function is applied to a received signal. A demodulation function can include one or more demodulation windows; during each of the demodulation windows, the receive signal is processed in accordance with the characteristics of the corresponding 35 demodulation window. A demodulation window is sometimes also known as a demodulation period. Characteristics of a demodulation window include the beginning time and ending time of the window.

6 The characteristics may also include gain, or multiplication factor. The gain within a window need not be constant. In one embodiment, the gain may be presented in sine-wave, triangular, rectangular shape or any other form/shape deemed appropriate by a person skilled in the art. The gain can be positive or negative. A demodulation window may temporally overlap another demodulation window. 5 It is possible to configure the number of demodulation windows directly, by having input 12 instruct the processing unit 9 to set up a certain number of demodulation windows, one or more of which are configurable. Alternatively, the number of demodulation windows may be reduced by setting to zero the gain of one or more demodulation windows. In one embodiment, the metal detector includes a screen indicating the one or more current characteristics 0 of one or more of the demodulation windows to allow a user to select which of the characteristics to be changed. For example, the screen can show the width and the gain of the first demodulation windows of a demodulation function and a user can select one or more of the gain and the width or the starting time to be changed. Figure 2 depicts exemplars of configurable demodulation functions with reference to receive signal 17. 15 Demodulation function 21 includes four demodulation windows 23, 24, 25 and 26. Demodulation function 31 includes four demodulation windows 32, 33, 34 and 35. Receive signal 17 is an example of a receive signal illustrating a decaying nature of a receive signal. It is from such a receive signal that information is extracted through synchronous demodulation or sampling process to produce the indicator output signal 13. Z0 Each demodulation window produces an output signal. The output signals generated by the demodulation windows are further processed by the processing unit 9 to produce an indicator output signal 13. The characteristics of each demodulation window include one or more of the gain, the beginning time, the ending time of a demodulation window. According to the present invention, for at least one demodulation window of interest, at least one of the gain, the beginning time, the ending time of a demodulation 25 window is a variable, in that it can be configured based on parameters from input 12. For example, a demodulation window of interest may be defined by a variable for the gain and one or more constants for the beginning time and the ending time. In one embodiment, the gain of each demodulation window has the same fixed value, with only the beginning time and the ending time of one or more of the demodulation windows, of the demodulation 30 function, configurable. The magnitudes of the output signals of demodulation windows can thereby be manipulated by a discrete circuit or through software. In another embodiment, the gain of each demodulation window is fixed value, but not necessarily with the same value as that of another demodulation window; the beginning time and the ending time of one or 7 more of the demodulation windows, of the demodulation function, is configurable. The magnitudes of the output signals of demodulation windows can thereby be manipulated by a discrete circuit or through software. As the one or more demodulation functions are stored in digital storage medium 7, one or more of the 5 characteristics of a demodulation function can be changed through input 12, which include one or more parameters. In one embodiment, input 12 change only the beginning time and/or ending time of a demodulation window within a demodulation function. In another embodiment, input 12 change only the gain (or multiplication factor) of a demodulation 0 window within a demodulation function. Input 12 may change more than one characteristic of more than one demodulation window. In one embodiment, input 12 may contain sufficient input information to change more than one characteristic of more than one demodulation window at the same time. Alternatively, input 12 change the characteristics of repeating transmit signal cycle 15, including the 5 number of transmit periods during the repeating transmit signal cycle, the beginning and ending time of each transmit period, the amplitude of the transmit waveform during each transmit period and/or the shape of the transmit waveform during each transmit period. In one embodiment, input 12 change the characteristics of one or more transmit periods and of one or more demodulation windows. In one embodiment, input 12 may contain sufficient input information to change the characteristics of one or 0 more transmit periods and of one or more demodulation windows at the same time. In another embodiment, input 12 changes the characteristics of one or more transmit periods, and the processing unit 9 changes one or more demodulation windows accordingly. This is important, as discussed above in relation to PI detectors, that synchronous demodulation process begins as soon as possible after the end of a high-voltage period or a pulse. Thus, if, for example, the ending time (or 25 starting time or duration) of the high-voltage period or pulse is changed by input 12, the timing of the one or more demodulation windows is changed accordingly automatically to preserve the delay between the high-voltage period or pulse and the one or more demodulation windows. Thereafter, input 12 may fine tune further one or more demodulation windows. Allowing a user to configure one or more characteristics of one or more transmit periods within a 30 repeating transmit signal cycle, or configure one or more characteristics of one or more demodulation windows, may damage a metal detector. For example, if the start time of demodulation window 23 is set too near to the end time of the high-voltage period 16 of repeating transmit signal cycle 15, a huge voltage spike may be induced in the receive electronics (which includes one or more demodulators, filters 8 and amplifiers), thus potentially damaging the receive electronics. In another example, the duration of a transmit period may be made long enough that the magnitude of the voltage during the high-voltage period may be too high, or that the current through the transmitter is over the operational limit. Hence, in one embodiment, the processing unit 9 first checks input 12 to ensure that the inputs adhere to 5 one or more predetermined rules before allowing the input 12 to change the characteristics of transmit periods and demodulation windows stored in the digital storage medium 7. For example, the beginning time of demodulation window 23 must be after a predetermined delay after the high-voltage period 16, or the voltage level during the high-voltage period 16 must be below a predetermined threshold value, with the predetermined delay and the predetermined threshold value set to prevent input 12 from damaging the 0 metal detector. The predetermined delay and threshold value are often factory set, based on experience and/or experimentation. Alternatively, they can be set by an experienced user. In one embodiment, the input 12 are ignored if the inputs fail one or more predetermined rules. In another embodiment, the input 12 are set to one or more predetermined values when the input 12 fail one or more predetermined rules. In another embodiment, some parameters have range limits beyond which it is not 5 possible to set respective values of those parameters. A metal detector may perform more than one synchronous demodulations using more than one demodulation functions simultaneously. In such cases, the more than one demodulation functions are different from one another. The characteristics of any one of the more than one demodulation functions is configurable based on input 12. In one embodiment, one or more demodulation functions may be !0 predetermined and not configurable, or alternatively, modifications are the results of predetermined parameters being applied. The more than one demodulations, each using a demodulation function, produce more than one post-demodulation signal. In one embodiment, a post-demodulation signal with larger amplitude is used as the indicator output signal. For example, that post-demodulation signal with larger amplitude is fed 25 into an audio output for the indication to a user of a metal detector. In another embodiment, more than one post-demodulation signals are weighted based upon their respective signal-to-noise ratios and combined. This linear combination is fed into an audio output for the indication to a user of a metal detector. Many soils can be classified as saline soils having salt components, and many soils also have components 30 with the property of viscous superparamagnetism. The effects of both of these soil components, when the soil is illuminated by the transmitted magnetic field, may produce large unwanted signals in metal detectors. To maximise metal target detection capability, the signals from both of these soil components are ideally substantially minimised or cancelled, revealing more clearly the typically weaker signals from desirable targets which may be located deep in the soil medium.

9 With a configurable metal detector, a user can configure the characteristics of one or more of the one or more demodulation windows of a demodulation function such that the linear combination of the outputs of the demodulation windows produces a signal substantially free of a signal due to perpendicular components of a uniform conducting half-space as approximated by a large volume of saline soil. An 5 example of such a function can be found in AU2011200516. A user also can configure the characteristics of one or more of the one or more demodulation windows such that the combination of the outputs of the demodulation windows produces a signal substantially free of a signal due to components of substantially log-uniform viscous superparamagnetic soil. An example of such a function can be found in AU2011200515. 0 A demodulation function that produces a signal substantially free of a signal due to perpendicular components of a uniform conducting half-space in the soil requires at least three demodulation windows, as does one that produces a signal substantially free of a signal due to components of substantially log uniform viscous superparamagnetic soil. Thus, to have a metal detector that could be tuned to produce either one of the signal would require that the at least three demodulation windows be configurable to 5 some extent. In one embodiment, there is a fourth demodulation window and that window can be fixed (non-configurable) or configurable. Alternatively, a user can configure the characteristics of one or more of the one or more demodulation windows such that the linear combination of the outputs of the demodulation windows produces a signal substantially free of a signal due to perpendicular components of a uniform conducting half-space in the !0 soil and from signal due to components of substantially log-uniform viscous superparamagnetic soil. For such purposes, in one embodiment, a demodulation function includes four demodulation windows, one or more of which might be configurable. In one embodiment, the gain of the first demodulation window after the ending of a transmit pulse has the same polarity as the gain of the third window, but is of different polarity from the gain of the second 25 window. Alternatively, the gains of the demodulation windows are of the same polarity but the outputs of the demodulation windows are multiplied by +1, -1, +1 (or -1, +1, -1) respectively. The multiplication can be performed in circuit (for example, by feeding outputs into the corresponding one of the two inputs of a differential integrator). When there is a fourth demodulation window, its polarity can be set to be the same with that of the second demodulation window. 30 Figure 3 depicts an exemplar of a demodulator which can use configurable demodulation functions to synchronously demodulate a receive signal. The demodulator includes an array of switches 60, 61, 62 and 63. Each of the switches 60, 61, 62 and 63 is controlled to be in a switched-on state or in a switched-off state in accordance with demodulation functions retrieved from a digital storage medium. For example, with reference to demodulation function 21, switch 60 is controlled by control signal 55 to be in a 10 switched-on state during demodulation window 23 and in a switched-off state otherwise, switch 61 is controlled by control signal 56 to be in a switched-on state during demodulation window 24 and in a switched-off state otherwise, switch 62 is controlled by control signal 57 to be in a switched-on state during demodulation window 25 and in a switched-off state otherwise, and switch 63 is controlled by 5 control signal 58 to be in a switched-on state during demodulation window 26 and in a switched-off state otherwise. The outputs of switches 60, 61, 62 and 63 are combined at processing unit 65. The output 66 from processing unit 65 is further processed to produce an indicator output signal. In one embodiment, processing unit 65 is an integrator. Alternatively, processing unit 65 is a differential integrator and each of the outputs from switches 60, 61, 0 62 and 63 can be controlled to be fed into either the non-inverting input or the inverting input of the differential integrator. For example, in one case the demodulation windows 23, 24, 25 and 26 are controlled to have polarity of +, -, +, - respectively, the outputs from switches 60 and 62 are fed into the non-inverting input of the differential integrator and the outputs from switches 61 and 63 are fed into the inverting input of the differential integrator. 15 In one embodiment, with processing unit 65 including an amplifier (not shown), resistors 45, 46, 47 and 48 are selected to affect the gains applied to signals through switches 60, 61, 62 and 63. For example, consider that resistor 45 is selected to be R and the reference resistor (the feedback element applied between the output and the inverting input of the amplifier) is set at some value. If resistor 46 is selected to be 2R, the effect upon the magnitude of the output 66 of a signal applied through resistor 46 and switch Z0 61 will be half that of the same signal applied through resistor 45 and switch 60, given that switches 61 and 60 are switched on for the same period, simultaneously. In one embodiment, if processing unit 65 is capable of digital processing, the gain of the demodulation windows to be applied to the outputs of switches 60, 61, 62 and 63 can be included in the combination at processing unit 65. For example, if the resistors 45, 46, 47 and 48 are all of the same value, a linear 25 combination may take the form of a x output of switch 60 + b x output of switch 61 + c x output of switch 62 + d x output of switch 63, where a, b, c and d are numbers indicating the desired relative gain. For examples, a, b, c and d are 3, -2, 1.5 and -1.5 respectively. It is also possible to use outputs from 60, 61, 62 and 63 in a mathematical function to produce an indicative output signal, for example, using a multiplication and/or division of such outputs at processing 30 unit 65. In one embodiment, the input 41 can be fed to an inverter (not shown) to produce an inverted copy of the input 41. In this case, input 41 can be fed to switches 55 and 57 directly and to the inverter, while the output from the inverter is fed to the inputs of switches 56 and 58.

I I In another embodiment, there is an array of more than four switches. More than two switches can be controlled to be in a switched-on state at a same time to affect the gain. For example, with processing unit 65 including an amplifier (not shown), if resistors 46 and 47 are of the value R and the reference resistor of the amplifier is also R, when switches 61 and 62 are controlled to be in a switched-on state at the same 5 time, the gain of the combined signals through switches 61 and 62 is 2. A detailed description of one or more preferred embodiments of the invention is provided above along with accompanying figures that illustrate by way of example the principles of the invention. While the invention is described in connection with such embodiments, it should be understood that the invention is not limited to any of the described embodiments. On the contrary, the scope of the invention is limited 0 only by the appended claims and the invention encompasses numerous alternatives, modifications, and equivalents. For the purpose of example, numerous specific details are set forth in the description above in order to provide a thorough understanding of the present invention. The present invention may be practised according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in 5 detail so that the present invention is not unnecessarily obscured. Throughout this specification and the claims that follow unless the context requires otherwise, the words 'comprise' and 'include' and variations such as 'comprising' and 'including' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 0 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge of the technical field.

Claims

1. A configurable handheld metal detector for producing an output signal indicative of a target, including: transmit electronics for generating a repeating transmit signal cycle; 5 a magnetic field transmitter connected to the transmit electronics for receiving the repeating transmit signal cycle to generate a transmit magnetic field; a device for receiving one or more parameters from a source external to the metal detector; a magnetic field receiver; a demodulator for synchronously demodulating a receive signal from the magnetic field receiver .0 using a configurable demodulation function, wherein the configurable demodulation function includes at least three demodulation windows, and at least one characteristic of at least one of the at least three demodulation windows is defined by a variable that is configurable based on the one or more parameters; and a digital storage medium for storing the configurable demodulation function, the digital storage l5 medium is adapted to retain a value of the variable when the metal detector is switched off.

2. The configurable handheld metal detector of claim 1, wherein the synchronous demodulation of the receive signal produces a post-demodulation signal which is further processed to produce the output signal. 20

3. A configurable handheld metal detector for producing an output signal indicative of a target, including: transmit electronics for generating a repeating transmit signal cycle; a magnetic field transmitter connected to the transmit electronics for receiving the repeating transmit signal cycle to generate a transmit magnetic field; 25 a device for receiving one or more parameters from a source external to the metal detector; a magnetic field receiver; a demodulator for synchronously demodulating a receive signal from the magnetic field receiver using at least two configurable demodulation functions, wherein each of the at least two configurable demodulation functions includes at least four demodulation windows, and at least one characteristic of at 30 least one of the at least four demodulation windows is defined by a variable that is configurable based on the one or more parameters; and a digital storage medium for storing the at least two configurable demodulation functions, wherein the digital storage medium is adapted to retain a value of the variable when the metal detector is switched off. 35

4. The configurable handheld metal detector of claim 3, wherein the synchronous demodulation of the receive signal produces at least two post-demodulation signals which are further processed to produce the output signal indicative of a target. 13

5. The configurable handheld metal detector of any one of claims I to 4, wherein the digital storage medium is a non-volatile random access memory. 5

6. The configurable handheld metal detector of any one of claims I to 5, wherein the digital storage medium retains a predetermined value corresponding to the variable.

7. The configurable handheld metal detector of claim 6, wherein if the one or more parameters fail a predetermined rule, the value of the variable is set to the predetermined value. 0

8. A method for configuring a handheld metal detector, including: receiving one or more parameters from a source external to the metal detector for configuring a configurable demodulation function, wherein the configurable demodulation function includes at least three demodulation windows, and at least one characteristic of at least one of the at least three .5 demodulation windows is defined by a variable that is configurable based on the one or more parameters; and retaining a value of the variable when the metal detector is switched off.

9. A configurable handheld metal detector substantially as herein described with reference to any one of !0 the embodiments of the invention illustrated in the accompanying drawings.

10. A method substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings.