MULTI-FREQUENCY TRANSMITTER FOR A METAL DETECTOR

• US 20090318098A1
• Filed: 06/23/2009
• Published: 12/24/2009
• Est. Priority Date: 06/23/2008
• Status: Active Grant

• Current Assignee: Minelab Electronics Pty Limited (Codan Ltd.)
• Original Assignee: Minelab Electronics Pty Limited (Codan Ltd.)
• Inventors: Nesper, Oliver, Stamatescu, Laurentiu
• Granted Patent: US 8,159,225 B2
• Time in Patent Office: Days
• US Class Current: 455/118
• CPC Class Codes: G01V 3/104 using several coupled or uncoupled coils G01V3/101 takes precedence
• CPC Class Codes: G01V 3/30 operating with electromagnetic waves

Abstract

A method for generating a transmit signal for transmission including the steps of a) generating at least two selected rectangular wave signals, each having a different fundamental frequency; b) mixing the selected rectangular wave signals to produce a driving signal; and c) driving a switching circuit using the driving signal for generating a transmit signal for transmission, wherein the Fourier transform of the transmit signal contains frequency components of relatively high magnitude, at frequencies corresponding to the convolution of the fundamental frequencies of the said at least two selected rectangular wave signals, as compared to other frequency components across the frequency spectrum of the Fourier transform, and wherein the rectangular wave signals are selected such that the frequency components of relatively high magnitude are substantially the same in magnitude, and spaced from each other in the frequency spectrum in a predetermined manner.

Claims

1. A method for generating a transmit signal for transmission including the steps of:

a) generating at least two selected rectangular wave signals, each having a different fundamental frequency;

b) mixing the selected rectangular wave signals to produce a driving signal; and

c) driving a switching circuit using the driving signal for generating a transmit signal for transmission, wherein the Fourier transform of the transmit signal contains frequency components of relatively high magnitude, at frequencies corresponding to the convolution of the fundamental frequencies of the said at least two selected rectangular wave signals, as compared to other frequency components across the frequency spectrum of the Fourier transform, and wherein the rectangular wave signals are selected such that the frequency components of relatively high magnitude are substantially the same in magnitude, and spaced from each other in the frequency spectrum in a predetermined manner.

2. A method according to claim 1, wherein the switching circuit is a half-bridge or a full-bridge, the full-bridge including two inputs, a first input for receiving the driving signal, and a second input for receiving the inverted said driving signal;

and wherein the selected rectangular wave signals include;

a first signal having a first fundamental frequency f_A;

a second signal having a second fundamental frequency f_B; and

a third signal having a third fundamental frequency f_C;

wherein the convolution of the fundamental frequencies generates four frequency components of relatively high magnitude at frequencies f₁, f₂, f₃ and f₄, and the four frequency components are substantially linearly-spaced in a logarithmic scaled frequency spectrum.

3. A method according to claim 2, wherein the fundamental frequencies are substantially determined through the relationship of

$f_A=\frac{k+1}{2}$

f 1 , 

f B = k 2 + 1 2 

f 1 , 

and 



f C = k 

( k + 1 ) 2 

f 1 ,

where k is the tribonacci constant, and f₁ is the lowest frequency among f₁, f₂, f₃ and f₄.

4. A method for generating transmit signals for transmission including the steps of:

a) generating a first group of at least two selected rectangular wave signals, and a second group of at least two selected rectangular wave signals, each rectangular wave signal having a different fundamental frequency;

b) mixing the first group to produce a first driving signal, and the second group to produce a second driving signal; and

c) driving a first half-bridge using the first driving signal for generating a first transmit signal, wherein the Fourier transform of the first transmit signal contains frequency components of relatively high magnitude, at frequencies corresponding to the convolution of the fundamental frequencies of the first group, as compared to other frequency components across the frequency spectrum of the Fourier transform, and driving a second half bridge using the second driving signal for generating a second transmit signal, wherein the Fourier transform of the second transmit signal contains frequency components of relatively high magnitude, at frequencies corresponding to the convolution of the fundamental frequencies of the second group, as compared to other frequency components across the frequency spectrum of the Fourier transform, and wherein the rectangular wave signals are selected such that the frequency components of relatively high magnitude are substantially the same in magnitude, and spaced from each other in the frequency spectrum in a predetermined manner.

5. A method according to claim 4, wherein

the first group includes:

a first signal having a first fundamental frequency f_A; and

a second signal having a second fundamental frequency f_B;

the second group includes;

a third signal having a third fundamental frequency f_C; and

a fourth signal having a fourth fundamental frequency f_D;

and wherein the convolution of the first and second fundamental frequencies generates two frequency components of relatively high magnitude at frequencies f₁ and f₂; and

the convolution of the third and fourth fundamental frequencies generates two frequency components of relatively high magnitude at frequencies f₃ and f₄, and all four frequency components are substantially linearly-spaced in a logarithmic scaled frequency spectrum;

and wherein the fundamental frequencies are substantially determined through the relationship of

$f_A=\frac{k-1}{2}<br><br>f_1,<br><br>f_B=\frac{k+1}{2}<br><br>f_1,<br><br><br><br>f_C=\frac{k^2<br><br>\left(k-1\right)}{2}<br><br>f_1,<br><br>\mathrm{and}<br><br><br><br>f_D=\frac{k^2<br><br>\left(k+1\right)}{2}<br><br>f_1,$

where k is any numerical value, and f₁ is the lowest frequency among f₁, f₂, f₃ and f₄.

6. A method according to claim 4, wherein

the first group includes:

a first signal having a first fundamental frequency f_A; and

a second signal having a second fundamental frequency f_B;

the second group includes;

a third signal having a third fundamental frequency f_C; and

a fourth signal having a fourth fundamental frequency f_D;

and wherein the convolution of the first and second fundamental frequencies generates two frequency components of relatively high magnitude at frequencies f₂ and f₃; and

the convolution of the third and fourth fundamental frequencies generates two frequency components of relatively high magnitude at frequencies f₁ and f₄, and all four frequency components are substantially linearly-spaced in a logarithmic scaled frequency spectrum;

and wherein the fundamental frequencies are substantially determined through the relationship of

$f_A=\frac{k<br><br>\left(k-1\right)}{2}<br><br>f_1,<br><br>f_B=\frac{k<br><br>\left(k+1\right)}{2}<br><br>f_1,<br><br><br><br>f_C=\frac{k^3-1}{2}<br><br>f_1,<br><br>\mathrm{and}<br><br><br><br>f_D=\frac{k^3+1}{2}<br><br>f_1,$

where k is any numerical value, and f₁ is the lowest frequency among f₁, f₂, f₃ and f₄.

7. A method according to claim 4, wherein

the first group includes:

a first signal having a first fundamental frequency f_A; and

a second signal having a second fundamental frequency f_B;

the second group includes;

a third signal having a third fundamental frequency f_C; and

a fourth signal having a fourth fundamental frequency f_D;

and wherein the convolution of the first and second fundamental frequencies generates two frequency components of relatively high magnitude at frequencies f₁ and f₃; and

the convolution of the third and fourth fundamental frequencies generates two frequency components of relatively high magnitude at frequencies f₂ and f₄, and all four frequency components are substantially linearly-spaced in a logarithmic scaled frequency spectrum;

and wherein the fundamental frequencies are substantially determined through the relationship of

$f_A=\frac{k^2-1}{2}<br><br>f_1,<br><br>f_B=\frac{k^2+1}{2}<br><br>f_1,<br><br><br><br>f_C=\frac{k<br><br>\left(k^2-1\right)}{2}<br><br>f_1,<br><br>\mathrm{and}<br><br><br><br>f_D=\frac{k<br><br>\left(k^2+1\right)}{2}<br><br>f_1,$

where k is any numerical value, and f₁ is the lowest frequency among f₁, f₂, f₃ and f₄.

8. A method according to claim 4, wherein

the first group includes:

a first signal having a first fundamental frequency f_A;

a second signal having a second fundamental frequency f_B; and

a third signal having a third fundamental frequency f_C;

the second group includes;

a fourth signal having a fourth fundamental frequency f_D;

a fifth signal having a fifth fundamental frequency f_E; and

a sixth signal having a sixth fundamental frequency f_F;

and wherein the convolution of the first, second and third fundamental frequencies generates four frequency components of relatively high magnitude at four different frequencies;

and the convolution of the fourth, fifth and sixth fundamental frequencies generates four frequency components of relatively high magnitude at four other frequencies, and all eight frequency components are substantially linearly-spaced in a logarithmic scaled frequency spectrum.

9. A method according to claim 4 wherein

the first group includes:

a first signal having a first fundamental frequency f_A;

a second signal having a second fundamental frequency f_B; and

a third signal having a third fundamental frequency f_C;

the second group includes;

a fourth signal having a fourth fundamental frequency f_D; and

a fifth signal having a fifth fundamental frequency f_E;

and wherein the convolution of the first, second and third fundamental frequencies generates four frequency components of relatively high magnitude at four different frequencies; and

the convolution of the fourth and fifth fundamental frequencies generates two frequency components of relatively high magnitude at two other frequencies, and all six frequency components are substantially linearly-spaced in a logarithmic scaled frequency spectrum.

10. An apparatus for generating a transmit signal for transmission including

at least one generator to generate at least two selected rectangular wave signals, each rectangular wave signal having a different fundamental frequency;

at least one mixer to mix the selected rectangular wave signals to produce a driving signal; and

at least one switching circuit for receiving the driving signal to generate a transmit signal for transmission, wherein the Fourier transform of the transmit signal contains frequency components of relatively high magnitude at frequencies corresponding to the convolution of the fundamental frequencies of the at least two selected rectangular wave signals as compared to other frequency components across the frequency spectrum of the Fourier transform, and wherein the rectangular wave signals are selected such that the frequency components of relatively high magnitude are substantially the same in magnitude, and spaced from each other in the frequency spectrum in a predetermined manner.

11. A metal detector used for detecting metallic targets including:

a) transmit electronics having a plurality of switches for generating a transmit signal;

b) a transmit coil connected to the transmit electronics for receiving the transmit signal and generating a transmitted magnetic field for transmission;

c) at least one receive coil for receiving a received magnetic field and providing a received signal induced by the received magnetic field; and

d) receive electronics connected to the at least one receive coil for processing the received signal to produce an indicator output signal, the indicator output signal including a signal indicative of the presence of a metallic target in the soil;

wherein the transmit electronics generates at least two selected rectangular wave signals, each having a different fundamental frequency;

the transmit electronics further mixes the selected rectangular wave signals to produce a driving signal for driving a switching circuit to generate transmit signal for transmission, the Fourier transform of the transmit signal contains frequency components of relatively high magnitude, at frequencies corresponding to the convolution of the fundamental frequencies of the at least two selected rectangular wave signals, as compared to other frequency components across the frequency spectrum of the Fourier transform, and wherein the rectangular wave signals are selected such that the frequency components of relatively high magnitude are substantially the same in magnitude, and spaced from each other in the frequency spectrum in a predetermined manner.

12. A metal detector according to claim 11, wherein the transmit electronics further having a module including a look-up table and a timer, wherein the module selects data points in the look-up table to generate at least one driving signal by setting the timer to appropriate frequencies.

13. A metal detector according to claim 11, the receive electronics further including band pass filters, wherein the frequency resolution of the rectangular wave generators is selected such that any spurious components present in the driving signal do not fall in either the pass band or the transition band of the receiver filters.