Abstract:
A triaxial AC magnetic field analyzer/dosimeter instrument measures the field strength of three mutually orthogonal AC magnetic field components at a plurality of different frequencies in a frequency range of interest and stores corresponding data which may be processed to indicate the field strength at each of the frequencies and/or the sum of the field strengths over the frequency range of interest. The instrument is computer controlled and comprises three measurement channels corresponding to respective magnetic field components, each channel including a sensor coil, a clock controlled, switched capacitor, bandpass filter, and a TRMS detector. The passband of each filter is swept across the desired frequency range by a varying frequency clock.

Description:
SPECIFICATION 
     This invention is concerned with the measurement of AC magnetic fields, and more particularly with the measurement of AC magnetic field spectra. 
     BACKGROUND OF THE INVENTION 
     In recent years there has been increasing concern with the effects of AC magnetic fields on the human body. Such fields are associated with a wide variety of electrical apparatus, including, for example, power lines, transformers, electric blankets, computer monitors, and microwave ovens. However, there has been no practical instrument available for the measurement of such fields. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides, for the first time, a compact, highly portable instrument, capable of fitting a shirt pocket or being clipped on a belt, for analyzing and totalizing an AC magnetic field environment into which the instrument is carried. 
     More particularly, the invention provides a triaxial AC magnetic field spectrum analyzer/dosimeter for measuring and storing an AC magnetic field spectrum over a desired frequency range of, for example, 40 Hz to 1000 Hz. An individual spectrum can be measured and stored in a short period of time, e.g., about 45 seconds, and can later be analyzed for specific magnetic field frequencies, amount of activity at particular frequencies, and the total dose over the frequency range of interest. AC magnetic field strengths of from 0.2 to 375 milligauss, for example, can be measured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described in conjunction with the accompanying drawings, which illustrate a preferred (best mode) embodiment and wherein: 
     FIG. 1 is a block diagram of an instrument in accordance with the invention; 
     FIG. 2 (comprising FIGS. 2A and 2B) is a schematic diagram of a major portion of the instrument; 
     FIG. 3 is a flow chart illustrating data gathering in accordance with the invention; 
     FIGS. 4 and 5 are diagrams illustrating passband characteristics of filters used in the invention, at different portions of a frequency spectrum; and 
     FIGS. 6 and 7 are diagrams illustrating measured AC magnetic field spectra. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, an instrument in accordance with the invention comprises three measurement channels A, B, C and a frequency setting circuit D connected to a small low power data logging computer LC, such as the Tattletale &#34;Lite&#34; Data Logger of the Onset Computer Corp., N. Falmouth, Mass. This computer has 512 KB of RAM memory, an 8 channel analog-to-digital converter, 8 digital I/O ports, and a serial interface for loading programs and unloading data to an outside computer. 
     Each measurement channel includes a sensor coil 10, e.g, 2400 turns of No. 36 gauge wire wound on a PVC cylindrical post (1/8&#34; diameter with 1/2&#34;×1/2&#34; PVC end plates). The three sensor coils are disposed along X, Y and Z axes, respectively, and are mounted orthogonally. In the preferred embodiment of the invention, the entire instrument (including the computer) forms a package that is 2.36×4.71×1.0 in., i.e., slightly larger than a king size pack of cigarettes. The components shown in FIG. 1 to the left of line L are shown in FIG. 2 in greater detail and are mounted on a circuit board housed within the case of the instrument. 
     AC magnetic field signals sensed by the sensor coils 10 are amplified by amplifiers 12 and fed to clock controlled, switched capacitor, bandpass filters 14. Such filters are well known in the art. See, for example, Maxim Engineering Journal, Vol. 2, published by Maxim Integrated Products of Sunnyvale, Calif. The filters shown in FIG. 2 are Linear Technology LTC 1060. These filters transfer &#34;buckets&#34; of charge per clock cycle and amplify and bandpass with a center frequency at 1/100 of the filter clock frequency. 
     The filter clock frequency is supplied from the frequency setting circuit D, that includes a ramp generator 16 and a voltage controlled oscillator 18. The voltage controlled oscillator is a voltage-to-frequency converter which outputs a frequency range of 100 KHz to 4 KHz during the rundown time of the ramp generator, which is an RC network charged at the beginning of each spectrum sweep. The output of the voltage controlled oscillator is connected to a counter input of the computer LC via a divide-by-ten circuit 20 and is monitored by the computer. 
     The output of each bandpass filter 14 is fed to a TRMS (total root mean square) detector 22, which converts the output of the bandpass filter to DC. The output of each TRMS detector is supplied, through an inverting DC amplifier 24, to an analog-to-digital converter input of the computer LC. 
     As shown in FIG. 3, the instrument of the invention performs a spectrum gathering process as follows: 
     A command to take a spectrum (step S1) is given either by pushing an initiate spectrum button PB shown in FIG. 2 or by a time driven command from a computer program that has been loaded into the computer LC via the serial input/output port. The computer then supplies power (step S2), e.g., 5 volts DC, to the spectrum analyzer components shown to the left of line L in FIG. 1 and shown in greater detail in FIG. 2. Then the computer initiates a ramp capacitor charge (step S3), charging the capacitors of the ramp generator 16, the voltage of which is converted to a frequency by the voltage controlled oscillator 18, which is monitored by the computer (step S4). When the frequency reaches a programmed point, e.g., 1100 Hz, the RC charging stops (step S5), and an RC rundown begins. When the rundown reaches 1000 Hz, the instrument starts the measurement of a spectrum (step S6). At that point, the computer stores the frequency of the oscillator 18 divided by ten (step S7), samples the output of the three measurement channels and stores these measurements in computer memory, along with the date/time of the sample, provided by an internal computer date/time generator (step S8). 
     The RC discharge is continuous, and the corresponding frequency is monitored by the computer. At a frequency which corresponds to one-half of the filter bandwidth (3db points) for the sample just measured, another sample is taken of the output of all three measurement channels and is stored in computer memory, along with the date/time. This process is continued until the lower end of the spectrum is reached (step S9). As indicated in step S9, the frequency at which each sample (subsequent to the first) is to be taken is determined by dividing by ten the frequency of the voltage controlled oscillator at which the previous sample was taken and subtracting the answer from the previous frequency. Then the computer turns off the power to the instrument and awaits a command to measure the next spectrum (step S10). 
     Each spectrum measurement takes 30 frequency samples across a range of 40 Hz to 1000 Hz, for example. Each frequency &#34;bin&#34; is aligned with the adjacent frequency &#34;bin&#34; so that their passbands intersect at their 3db rolloff points. The filter &#34;Q&#34;, which is set by the resistors R1 and R2 in FIG. 2, is constant across the frequency spectrum, so that the bandwidth is narrower at the lower frequencies (Q=f/bw). See FIGS. 4 and 5. The filter &#34;Q&#34; may be set to about 9. Thus, at the high frequency end of the spectrum, the bandwidth (1000/9) is about 111 Hz at the 3db points, and at the low frequency end of the spectrum the bandwidth (40/9) is about 4 Hz. This permits a faster sweep at the high frequencies because of the increased bandwidth. The frequency sweep ramp from the ramp generator, being an RC discharge, is non-linear. The sweep is conducted as rapidly as possible with acceptable frequency spectrum information. 
     The data stored in the computer may be offloaded and processed in an outside computer, such as a PC (personal computer), for example. Using a program such as LOTUS 123, all pertinent data related to the magnetic field frequencies and energies may be displayed individually and in their respective frequency doses. The total magnetic field strength may be computed as the square root of the sum of the squares of the X, Y and Z axis field strength components. The PC may be programmed to subtract a background spectrum from all other spectra. The PC may be programmed to display, graphically, an entire AC magnetic field spectrum, such as the spectrum shown in FIG. 6 displaying AC magnetic fields in front of a computer monitor, and the spectrum shown in FIG. 7 displaying AC magnetic fields close to a microwave oven. 
     While a preferred embodiment of the invention has been shown and described, it will be apparent to those skilled in the art that changes can be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.