WO1999057570A1

WO1999057570A1 - Dual funcion field sensor

Info

Publication number: WO1999057570A1
Application number: PCT/US1998/008984
Authority: WO
Inventors: Mark Kettering; Robert Lechter; George Lechter
Original assignee: Techealth Corporation
Priority date: 1998-05-01
Filing date: 1998-05-01
Publication date: 1999-11-11
Also published as: AU7280498A

Abstract

A dual function field strength sensor (10), in accordance with an inventive arrangement, comprises: a alternating current magnetic field detection circuit (30); a first antenna (31) for the AC magnetic field detection circuit; a radio frequency (RF) field detection circuit (20); a second antenna (21) for the RF field detection circuit (20); an amplifier and filter stage (24, 25, 35, 36) for processing output signals from any selected one of the detection circuits; a switch (12) for selecting which one of the output signals is processed by the amplifier and filter stage; an output stage responsive to the amplifier and filter stage and driving at least one field strength indicating device (50) operable for both AC magnetic fields and RF fields detected by the detection circuits. The second antenna comprises a flat conductive path (26) partially encircling and separated from other components on a printed circuit board (90), the path having a dipole configuration and including a serpentine section (27).

Description

DUAL FUNCTION FIELD SENSOR Background of the Invention

1. Field of the Invention

This invention relates to the field of electromagnetic field (EMF) sensors, and in particular, to a dual function EMF sensor which can detect both alternating current (AC) magnetic fields and radio frequency (RF) fields in the frequency range used by cellular telephones .

2. Description of Related Art All persons are subjected to AC magnetic fields from time to time, at levels which vary with proximity to sources of AC magnetic fields, for example high tension power lines. Many persons, and the number keeps growing, are subjected to RF fields generated by cellular telephones . Both of these types of fields are suspected to cause health problems in humans. Single function meters are available to measure these fields respectively, but the meters tend to be large and awkward to use, particularly for persons who are unsophisticated in using such meters and in measuring electromagnetic fields in general. There is a long-felt need for a single meter which can be easily used to measure both of these types of fields, and thus enable consumers to directly measure their exposure and assess their risk.

Summary of the Invention A dual function meter in accordance with the inventive arrangements satisfies this long-felt need. The overall design provides maximum performance, multiple functionality, high reliability, small size and high precision at a minimum cost. Standard parts are used in a new and unique circuit topology which enables both AC magnetic fields and RF fields in the frequency range used by cellular to be measured with a single unit. Although standard components are utilized in the novel topology, a novel antenna for detecting the RF field has also been developed.

A dual function field strength sensor, in accordance with an inventive arrangement, comprises: a alternating current (AC) magnetic field detection circuit; a first antenna for the AC magnetic field detection circuit; a radio frequency (RF) field detection circuit; a second antenna for the RF field detection circuit; an amplifier and filter stage for processing output signals from any selected one of the detection circuits; a switch for selecting which one of the output signals is processed by the amplifier and filter stage; an output stage responsive to the amplifier and filter stage and driving at least one field strength indicating device operable for both AC magnetic fields and RF fields detected by the detection circuits. The switch is responsive to attachment of one of the antennas, and in the presently preferred embodiment, the switch is responsive to attachment of the first antenna to the magnetic field detection circuit.

The first antenna comprises a plug. The magnetic detection circuit comprises a jack for receiving the plug,

-2- the switch being disposed in the jack and being operable for disabling the magnetic field detection circuit when the plug is not in the jack Only one of the AC and RF field detection circuits is active at a time, responsive to the switch.

The common range switching circuit can be user controlled.

The output signals of the detection circuits are combined at a summing junction, the summing junction being an input to the amplifier and filter stage.

The first antenna comprises a wound coil. The second antenna comprises a flat conductive path on a printed circuit board, the path having a dipole configuration and including a serpentine section. The flat conductive path is mostly on a peripheral portion of the printed circuit board, partially encircling and separated from other components grouped on the printed circuit board.

The at least one field strength indicating device comprises at least one visually perceptible device and at least one audibly perceptible device. The at least one field strength indicating device can comprise one or more of a field strength meter, a loudspeaker and a blinking display.

A dual function field strength sensor, in accordance with another inventive arrangement, comprises: a printed circuit board; a alternating current (AC) magnetic field detection circuit on the board; a first antenna for the AC magnetic field detection circuit; a radio frequency (RF) field detection circuit on the board; a flat conductive path on the board forming a second antenna for the RF field detection circuit, the path having a dipole configuration and including a serpentine section; an amplifier and filter stage on the board for processing output signals from the detection circuits; an output stage on the board responsive to the amplifier and filter stage and driving at least one field strength indicating device operable for both AC magnetic fields and RF fields detected by the detection circuits .

The flat conductive path is mostly on a peripheral portion of the board, partially encircling and separated from components of the other circuits grouped on the board. An antenna arrangement for a printed circuit board, in accordance with yet another inventive arrangement, comprises: a printed circuit board; a radio frequency (RF) field detection circuit mounted on the board; a flat conductive path on the board forming an antenna for the RF field detection circuit, the path having a dipole configuration and including a serpentine section; and, the flat conductive path being mostly on a peripheral portion of the board, partially encircling and separated from components of the RF field detection.

The antenna is a 1/4 wave length antenna tuned to the middle of the cellular telephone band. The dipole configuration comprises two traces, each of the traces being approximately 8.5 cm in length.

•4- Brief Description of the Drawings Figure 1 is a block diagram of a dual function field sensor in accordance with the inventive arrangements.

Figures 2-5 are a schematic of a presently preferred embodiment of the dual function sensor shown in Figure 1. Figure 6 is a plan view, partially cut away, of a removable AC magnetic field coil antenna adapted for use with the field sensors shown in Figures 1-5.

Figure 7 is a plan view, in enlarged scale, of an RF field antenna laid out on a printed circuit board, which can be used with the field sensors shown in Figures 1-5.

Figure 8 is a top plan view of a housing, in full scale, for the field sensors shown in Figures 1-5.

Figure 9 is a left side elevation of Figure 8. Figure 10 is a right side elevation of Figure 8.

-5- Detailed Description of the Preferred Embodiments A dual function field sensor 10 in accordance with the inventive arrangements is shown in block diagram form in Figure 1. The sensor 10 comprises an RF detector circuit 20 and an AC magnetic field detector circuit 30. The outputs of the detector circuits 20 and 30 are summed at junction 14. The output of junction 14, which is the output of the RF detector or the AC magnetic field detector, but not both simultaneously, is an input to an output signal processing circuit 40. The output signal processing circuit 40 drives an output indicator circuit 50. Output indicator circuit 50 comprises one or more visual and audible displays. In the presently preferred embodiment, these displays include a field strength meter 51, an illuminated blinking visual display circuit 60 and an audible indicator circuit 70. It can be appreciated from this high systems level description that the unique topology minimizes the specialized circuitry required for detecting the respective RF and AC magnetic fields and maximizes the remainder of the circuitry used to process both types of detected fields.

The RF circuit 20 uses a fixed printed circuit 1/4 wave length dipole antenna 21 tuned to the middle of the cellular telephone band. The antenna 21 is shown in more detail in Figure 7. A flat conductive path 26 is formed on printed circuit board 90. The path 26 has a dipole configuration and includes a serpentine section 27. The flat conductive path 26 is mostly on a peripheral portion of the board 90, partially encircling and separated from the components of the RF field detection circuit 20, for example integrated circuits, resistors and capacitors. The dipole configuration has a ground terminal 91 and a signal terminal 92. The dipole configuration comprises two traces 28 and 29, each of the traces being approximately 8.5 cm in length. As shown, the traces need not be straight, but should be spaced as far as possible from the rest of the circuit components .

An RF germanium diode detector 23 is AC coupled to the antenna by capacitor 22 and used to demodulate the signal over a broad band. The signal is shaped by low pass filter (LPF) 24 and DC amplified by an operational amplifier 25. The DC amplifier 25 has a user selectable gain to provide two ranges. A range circuit 11 is controlled by a switch. The output of the RF field detector circuit is DC coupled to summing junction 14. The magnetic field is sensed using a detachable assembly 80 shown in Figure 6 and having a coil 31 connected to a plug 81 by a coaxial cable 82. The plug 81 is adapted for insertion into a jack 31. The AC magnetic field signal is coupled by capacitor 34 to an operational amplifier 35. This first stage of amplification provides gain and includes a two pole low pass filter (LPF) 36. The output of the low pass filter 37 is coupled to operational amplifier 38 through capacitor 37. The capacitive coupling is used to provide zero offset error with a high gain circuit . The output of amplifier 38 is coupled to the summing junction 14 through capacitor 39.

With further reference to Figure 2, a low current voltage divider R18, R19 provides a DC reference voltage for the first and second stage of amplification of the magnetic field detector circuit 30, namely amplifiers 35 and 38

-7- respectively. The DC reference voltage is also supplied to amplifier 25 in the RF field detector circuit 20 by either one of resistor R21 or range switch 11, and resistor R7. The bias voltage saturates amplifier 25 negatively, thus effectively deactivating the RF detector circuit 20 and assuring a zero output to summing junction 14. This arrangement allows a single voltage operation of the whole circuit. The user selectable gain range circuit 11 also provides two ranges for the magnetic field detector circuit. Another unique aspect of the inventive arrangements is the method used to switch between detection modes for magnetic fields and RF fields. In order to use the unit for detecting magnetic fields the user only needs to plug in the magnetic field sensor 80. As noted above, this deactivates the RF detection circuit 20. In order to use the unit for detecting RF fields the user only needs to unplug the magnetic field sensor. The single pole switch 33 contained in the magnetic field jack 32 (Jl in Figure 2) is used to short the low current voltage divider R18, R19 to ground when no magnetic field probe is plugged in. This makes the first two stages of amplification for the magnetic field signal inoperative, and permits normal operation of amplifier 25, allowing only the RF level signal to reach the summing junction 14 and be processed by the rest of the circuitry. This is a low cost and user friendly method of switching between magnetic field readings and RF field readings .

After the summing junction 14, the same components provide all of the signal processing and output signal generation, irrespective of the source. It should be noted that, for purposes of clarity, summing junction 14 and resistor R13 are shown in both Figures 2 and 3. An active precision rectification circuit 42 includes operational amplifier U1C and diodes D3 and D2 is capacitively coupled to the second stage amplification by the capacitor 39. The precision rectification circuit 42 provides extremely low offset, hysteresis, and dead band. A low pass filter (LPF) 43 after the precision rectifier reduces ripple and provides user friendly meter movement and response. The low pass filtered signal is an input to a DC operational amplifier

44, which supplies a signal to the output indicator circuit 50.

The DC amplifier 44 drives a field strength meter 51 directly, and in so doing, isolates the precision rectification circuit 42 from the meter for increased accuracy. The meter provides the user with an accurate and ergonomic measurement of the AC magnetic field or the RF field. In the presently preferred embodiment, the meter will show measurements of milligauss in two scales, namely 0-5 and 0-50. Alternatively, the meter will show measurements in milliwatts/cm² in two scales, namely 0-1 and 0-10.

The DC amplifier 44 also drives a voltage controlled oscillator (VCO) 52, which is also shown in Figure 3. The VCO 52, embodied by operational amplifier U2A, drives a first transistor switch Ql and a second transistor switch Q2. Transistor switch Ql drives a visual flashing light display 60, which can be implemented with light emitting diodes (LEDs) D5 and D6 , which are also shown in Figure 7. The LEDs provide the user with a visual indication of the field strength. Transistor switch Q2 drives a free running multivibrator 71, also shown in Figure 4. The block diagram of Figure 1 shows that the output of multivibrator 71 is amplified by amplifier 73. In fact, multivibrator 71 and amplifier 73 are advantageously both embodied in operational amplifier U2B . The block diagram shows the output of amplifier 73 to be an input to a negative function block 72, the output of which is amplified by operational amplifier 74. In fact, negative function block 72 and amplifier 74 are advantageously both embodied in operational amplifier U2D. The amplifiers U2B and U2D form a push pull configuration 77 which provides maximum drive signal to the piezo electric speaker 76 on the single ended 9 volt power supply (shown in Figure 5) . The loudspeaker 76 provides an audio indication of the field strength. The volume of the speaker 76 is controlled by potentiometer 75, responsive to a volume dial 53. Both the audio and visual signals are calibrated to start indicating at mid scale on the meter and increase in frequency and duty cycle with increasing field strength. The additional indicators provide the user with useful information on the field strength without the need to read the field strength meter itself.

The single ended power supply is shown in Figure 5. The power supply runs on a 9 volt battery and includes an on/off switch S2 and a power on indicator in the form of an LED D7. Polarity reversal protection is provided by a reverse biased diode D8.

Figures 2-6 are, taken together, a schematic of a presently preferred embodiment for the sensor shown in block

-10- form in Figure 1. A number of the components have been referred to, and except as noted, it is the topology and not the specific components which are of importance. Although not all of the components have been specifically referred to in the description as necessary to explain the inventive arrangements, it is anticipated that descriptions of other parts of the circuitry may become necessary, and to that end, each of the components has been labeled for such future reference. It can be further noted that all NPN transistors are type 2N3904 or similar and that all diodes are type

1N4148 or similar, unless otherwise noted. Moreover, in the presently preferred embodiment, a ground plane for the printed circuit board preferably extends under as much of the circuit as possible, but nevertheless spaced at least 2 cm. from the antenna traces 28, 29.

Figure 10 is a top plan of a housing 100 for the sensor in accordance with the presently preferred embodiment. With further reference to the side views in Figures 9 and 10, the housing 100 has a front member 101, a rear member 102, a battery cover 103 and a translucent cap 104. With reference to Figure 6, which is in enlarged scale relative to the full scale of Figures 8-10, it can be seen that a substantial portion of the dipole antenna extends beneath the cap 104, as do the diodes D5 and D6. The placement of the on/off switch S2, the power on indicator D7, the range control switch 12, the volume control dial 53, the jack 32 for the AC magnetic field antenna assembly 80 and the field strength meter 51 can also be easily ascertained from the drawings.

-11-

Claims

What is claimed is:

1. A dual function field strength sensor, comprising: a alternating current (AC) magnetic field detection circuit; a first antenna for said AC magnetic field detection circuit; a radio frequency (RF) field detection circuit; a second antenna for said RF field detection circuit; an amplifier and filter stage for processing output signals from any selected one of said detection circuits; a switch for selecting which one of said output signals is processed by said amplifier and filter stage; an output stage responsive to said amplifier and filter stage and driving at least one field strength indicating device operable for both AC magnetic fields and RF fields detected by said detection circuits.

2. The sensor of claim 1, wherein said switch is responsive to attachment of one of said antennas to said magnetic field detection circuit.

3. The sensor of claim 2, wherein said switch is responsive to attachment of said first antenna to said magnetic field detection circuit.

4. The sensor of claim 1, wherein only one of said AC and RF field detection circuits is active at a time, responsive to said switch.

^■12-

5. The sensor of claim 3, wherein said switch is responsive to attachment of one of said antennas.

6. The sensor of claim 5, wherein said switch is responsive to attachment of said first antenna to said magnetic field detection circuit.

7. The sensor of claim 1, wherein: each of said detection circuits comprises a respective amplifier having selectable ranges of amplification; and, a common range switching circuit operable for controlling both said respective amplifiers.

8. The sensor of claim 7, wherein said common range switching circuit is user controlled.

9. The sensor of claim 8, wherein said output signals of said detection circuits are combined at a resistive summing junction, said summing junction being an input to said amplifier and filter stage.

10. The sensor of claim 1, wherein: said first antenna comprises a plug; and, said magnetic detection circuit comprises a jack for receiving said plug, said switch being disposed in said jack and being operable for disabling said magnetic field detection circuit when said plug is not in said jack.

11. The sensor of claim 1, wherein said first antenna comprises a wound coil .

-13-

12. The sensor of claim 11, wherein said second antenna comprises a flat conductive path on a printed circuit board, said path having a dipole configuration and including a serpentine section.

13. The sensor of claim 11, wherein: said coil has a plug; and, said magnetic detection circuit comprises a jack for receiving said plug, said switch being disposed in said jack and being operable for disabling said magnetic field detection circuit when said plug is not in said jack.

14. The sensor of claim 1, wherein said second antenna comprises a flat conductive path on a printed circuit board, said path having a dipole configuration and including a serpentine section.

15. The sensor of claim 1, wherein said flat conductive path is mostly on a peripheral portion of said printed circuit board, partially encircling and separated from other components grouped on said printed circuit board.

16. The sensor of claim 1, wherein said at least one field strength indicating device comprises at least one visually perceptible device and at least one audibly perceptible device.

17. The sensor of claim 1, wherein said at least one field strength indicating device comprises a field strength

-14- meter .

18. The sensor of claim 1, wherein said at least one field strength indicating device comprises a loudspeaker.

19. The sensor of claim 1, wherein said at least one field strength indicating device comprises a blinking display.

20. The sensor of claim 1, wherein said at least one field strength indicating device comprises: a field strength meter; a loudspeaker; and, a blinking display.

21. The sensor of claim 1, wherein: said output signals of said detection circuits are combined at a resistive summing junction, said summing junction being an input to said amplifier and filter stage said first antenna comprises a plug; said magnetic detection circuit comprises a jack for receiving said plug, said switch being disposed in said jack and being operable for disabling said magnetic field detection circuit when said plug is not in said jack; each of said detection circuits comprises a respective amplifier having selectable ranges of amplification, a common range switching circuit controlling both said respective amplifiers, said common range switching circuit being user controlled and, said at least one field strength indicating device

-15- comprises at least one visually perceptible device and at least one audibly perceptible device.

22. A dual function field strength sensor, comprising: a printed circuit board; a alternating current (AC) magnetic field detection circuit on said board; a first antenna for said AC magnetic field detection circuit; a radio frequency (RF) field detection circuit on said board; a flat conductive path on said board forming a second antenna for said RF field detection circuit, said path having a dipole configuration and including a serpentine section; an amplifier and filter stage on said board for processing output signals from said detection circuits; an output stage on said board responsive to said amplifier and filter stage and driving at least one field strength indicating device operable for both AC magnetic fields and RF fields detected by said detection circuits.

23. The sensor of claim 22, wherein said flat conductive path is mostly on a peripheral portion of said board, partially encircling and separated from components of said other circuits grouped on said board.

24. An antenna arrangement for a printed circuit board, comprising:

-16- a printed circuit board; a radio frequency (RF) field detection circuit mounted on said board; a flat conductive path on said board forming an antenna for said RF field detection circuit, said path having a dipole configuration and including a serpentine section; and, said flat conductive path being mostly on a peripheral portion of said board, partially encircling and separated from components of said RF field detection.

25. The arrangement of claim 24, wherein said antenna is a 1/4 wave length antenna tuned to the middle of the cellular telephone band.

26. The arrangement of claim 24, wherein said dipole configuration comprises two traces, each of said traces being approximately 8.5 cm in length.

-17- AMENDED CLAIMS

[received by the International Bureau on 13 October 1998 (13.10.98); original claims 1, 2, 4, 5, 8-10, 13-15, 17-19, 21 and 24 amended; remaining claims unchanged (7 pages)]

ι 1. A dual function field strength sensor, comprising:

2 a alternating current (AC) magnetic field detection

3 circuit ,- a first antenna for said AC magnetic field detection

5 circuit ;

6 a radio frequency (RF) field detection circuit;

7 a second antenna for said RF field detection circui ,-

8 an amplifier and filter stage always coupled to both

9 said detection circuits for processing output signals from o either one of said detection circuits; i a switch for selecting which one of said output signals 2 is processed by said amplifier and filter stage; ₃ an output stage responsive to said amplifier and filter 4 stage and driving at least one field strength indicating s device operable for both AC magnetic fields and RF fields 6 detected by said detection circuits.

i 2. A dual function field strength sensor, comprising:

2 a alternating current (AC) magnetic field detection

3 circuit ; a first antenna for said AC magnetic field detection

5 circuit ; s a radio frequency (RF) field detection circuit ; 7 a second antenna for said RF field detection circuit; B an amplifier and filter stage for processing output

9 signals from any selected one of said detection circuits; o a switch responsive to attachment of one of said first 1 and second antennas for selecting which one of said output 2 signals is processed by said amplifier and filter stage; 3 an output stage responsive to said amplifier and filter 4 stage and driving at least one field strength indicating 5 device operable for both AC magnetic fields and RF fields 6 detected by said detection circuits.

i 3. The sensor of claim 2, wherein said switch is

2 responsive to attachment of said first antenna to said

3 magnetic field detection circuit.

1 4. The sensor of claim 2 , wherein only one of said AC

2 and RF field detection circuits is active at a time,

3 responsive to said switch.

1 5. The sensor of claim 1, wherein said switch is

2 responsive to attachment of one of said antennas.

1 6. The sensor of claim 5, wherein said switch is

2 responsive to attachment of said first antenna to said

3 magnetic field detection circuit.

1 7. The sensor of claim 1, wherein:

2 each of said detection circuits comprises a respective

3 amplifier having selectable ranges of amplification; and, a common range switching circuit operable for

5 controlling both said respective amplifiers.

l B. The sensor of claim 2, wherein:

19 - 2 each of said detection circuits comprises a respective

5 controlling both said respective amplifiers.

1 3 . The sensor of claim 1, wherein said output signals

2 of said detection circuits are combined at a resistive

3 summing junction, said summing junction being an input to

4 said amplifier and filter stage.

1 10. The sensor of claim 2, wherein:

2 said first antenna comprises a plug; and,

3 said magnetic detection circuit comprises a jack for receiving said plug, said switch being disposed in said jack s and being operable for disabling said magnetic field

6 detection circuit when said plug is not in said jack.

1 11. The sensor of claim 1, wherein said first antenna

2 comprises a wound coil.

i 12. The sensor of claim 11, wherein said second antenna

2 comprises a flat conductive path on a printed circuit board,

3 said path having a dipole configuration and including a

4 serpentine section.

1 13. The sensor of claim 2, wherein said first antenna

2 comprises a wound coil .

l 14. A dual function field strength sensor, comprising:

- 20 - a alternating current (AC) magnetic field detection circuit ; a first antenna for said AC magnetic field detection circuit; a radio frequency (RF) field detection circuit; a second antenna for said RF field detection circuit, said second antenna being formed by a flat conductive path on a printed circuit board, said path having a dipole configuration and including a serpentine section; an amplifier and filter stage for processing output signals from any selected one of said detection circuits; a switch for selecting which one of said output signals is processed by said amplifier and filter stage ,- an output stage responsive to said amplifier and filter stage and driving at least one field strength indicating device operable for both AC magnetic fields and RF fields detected by said detection circuits. The sensor of claim 13, wherein said second antenna comprises a flat conductive path on a printed circuit board, said path having a dipole configuration and including a serpentine section.

15. The sensor of claim 14, wherein said flat conductive path is mostly on a peripheral portion of said printed circuit board, partially encircling and separated from other components grouped on said printed circuit board.

16. The sensor of claim 1, wherein said at least one field strength indicating device comprises at least one visually perceptible device and at least one audibly

21 perceptible device.

17. The sensor of claim 2, wherein said at least one field strength indicating device comprises at least one visually perceptible device and at least one audibly perceptible device.

18. The sensor of claim 1, wherein said at least one field strength indicating device comprises at least one of. a field strength mete ; a loudspeaker; and, a blinking display.

13. The sensor of claim 2, wherein said at least one field strength indicating device comprises at least one of; a field strength meter; a loudspeaker; and, a blinking display.

21. The sensor of claim 2, wherein said at least one field strength indicating device comprises: a field strength meter; a loudspeaker; and.

22 a blinking display.

22. A dual function field strength sensor, comprising: a printed circuit board; a alternating current (AC) magnetic field detection circuit on said board; a first antenna for said AC magnetic field detection circuit ; a radio frequency (RF) field detection circuit on said board,- a flat conductive path on said board forming a second antenna for said RF field detection circuit, said path having a dipole configuration and including a serpentine section; an amplifier and filter stage on said board for processing output signals from said detection circuits; an output stage on said board responsive to said amplifier and filter stage and driving at least one field strength indicating device operable for both AC magnetic fields and RF fields detected by said detection circuits.

24. An antenna arrangement for a printed circuit board, comprising: a printed circuit board;

- 23 a radio frequency (RF) field detection circuit mounted on said board; a flat conductive path on said board forming an antenna for said RF field detection circuit, said path having a dipole configuration and including a serpentine section; and, said flat conductive path being mostly on a peripheral portion of said board, partially encircling and separated from components of said RF field detection circuit.

26. The arrangement of claim 24, wherein said dipole configuration comprises two traces, each of said traces being approximately 8.5 cm in length .

24