Compensation of magnetic interference

A compensation coil placed at least partially underneath a magnetic field sensor package in an electronic system provides attenuation of electromagnetic interference (EMI). In an embodiment, the compensation coil attenuates EMI in a frequency band which overlaps with an operating frequency band of the magnetic field sensor. This allows the magnetic field sensor to make accurate magnetic field measurements in the presence of system level alternating current (AC) EMI. In an embodiment, a system comprises: a magnetic field sensor; a compensation coil placed at least partially underneath the magnetic field sensor; and a reverse current generator coupled to the compensation coil and to a power supply that is coupled to an electromagnetic interference (EMI) source, the reverse current generator operable to generate a reverse current in the compensation coil to generate a counter magnetic field for compensating the EMI.

TECHNICAL FIELD

This disclosure relates generally to compensating for electromagnetic interference (EMI) in electronic systems.

BACKGROUND

EMI affects an electronic system by electromagnetic induction, electrostatic coupling or conduction. EMI can be problematic in, for example, electronic systems that include several EMI sources integrated within the same housing as a digital magnetic compass. A typical earth magnetic field ranges between 25 μT˜62 μT, depending on the geographical location. To maintain an acceptable heading error (e.g., <10° heading error) for the magnetic compass typically requires EMI to be in the range of 2 μT˜6 μT. In some electronic systems having a small form factor (e.g., a smartphone), a magnetometer sensor package may be located close (e.g., several millimeters) to one or more EMI sources (e.g., a voice coil in a telephone receiver module). The EMI is frequency dependent and can have an amplitude as high as ˜22 μT. This large EMI could result in a large magnetic compass heading error (e.g., up to 90° heading error), impacting the performance of applications, such as gaming and navigation applications, that use multi-axis magnetometers to determine direction or compensate for errors in other sensors (e.g., compensate gyro sensors).

SUMMARY

In an embodiment, a system comprises: a magnetic field sensor; a compensation coil placed at least partially underneath the magnetic field sensor; and a reverse current generator coupled to the compensation coil and to a power supply that is coupled to an EMI source, the reverse current generator operable to generate a reverse current in the compensation coil to generate a counter magnetic field for compensating the EMI.

In an embodiment, a system comprises: a power supply; a voice coil coupled to the power supply for receiving a first current from the power supply; a magnetic field sensor; a compensation coil placed at least partially underneath the magnetic field sensor; and a reverse current generator coupled to the compensation coil and to the power supply, the reverse current generator operable to generate a second current in the compensation coil that is opposite the first current to generate a counter magnetic field for compensating electromagnetic interference (EMI) generated by the voice coil in a frequency band which overlaps with an operating frequency band of the magnetic field sensor.

Particular embodiments disclosed herein provide one or more of the following advantages. A compensation coil placed underneath a sensor package in an electronic system provides attenuation of EMI generated from an EMI source, such as a voice coil of a loudspeaker. The compensation coil is driven by a reverse current generator coupled to a power supply of the EMI source to generate a counter magnetic field that attenuates the EMI in a frequency band (e.g., an audio frequency band) that overlaps with the operating frequency band of a multi-axis magnetometer (e.g., 2 or 3-axis magnetometer). This allows the magnetometer sensor to make accurate magnetic field measurements in the presence of system level alternating current (AC) EMI.

The details of the disclosed embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages are apparent from the description, drawings and claims.

The same reference symbol used in various drawings indicates like elements.

DETAILED DESCRIPTION

Embodiments are disclosed for a system level AC EMI compensation system that includes a compensation coil integrated in multiple layers of a printed circuit board (PCB) placed at least partially underneath a magnetic sensor package.

In an embodiment, the compensation coil generates a counter magnetic field against the EMI source. The direction of the counter magnetic field can be manipulated by utilizing a reverse current generator coupled (e.g., coupled in parallel) to a power supply of the EMI source. The reverse current generator reverses the current flow direction within the compensation coil. The direction of the counter magnetic field can also be manipulated by adjusting the winding direction of the compensation coil.

In an embodiment, the placement of the compensation coil underneath a multi-axis magnetic sensor package takes advantage of coil symmetry to focus the counter magnetic field along a desired sense axis while substantially cancelling components of the counter magnetic field along other sense axes. The disclosed embodiments are suitable for use in any electronic systems (e.g., smartphones, tablet computers, gaming devices, wearable devices, navigation devices) that include magnetometer sensors or other sensors that are susceptible to EMI.

Example EMI Compensation System

FIG. 1illustrates an example electronic system100that could benefit from an EMI compensation system, according to an embodiment. In the example embodiment shown, system100is a smartphone that includes magnetic sensor101and EMI source102. For example, magnetic sensor101can be a multi-axis magnetometer (e.g., 2 or 3-axis magnetometer) and EMI source102can be a telephone receiver module with a coil that radiates EMI in the audio frequency band. Although the example electronic system100described is a smartphone, the disclosed embodiments are applicable to any electronic system or device that could benefit from compensation of EMI, including but not limited to notebook computers, tablet computers, gaming devices, digital cameras/camcorders, vehicle navigation systems and wearable devices (e.g., smart watches, smart glasses, fitness devices). EMI source102can be any component that generates EMI in the audio frequency band.

FIG. 2is an example Bode plot illustrating EMI as a function of frequency without EMI compensation, according to an embodiment. The vertical axis of the plot shows magnetic interference (μT) and the horizontal axis is the logarithm of frequency (Hz). As can be observed from the plot the level of EMI is significantly higher than the example system requirement of 5 μT in the audio frequency band. For example, if the user is engaged in a telephone call the voice coil in the receiver module can generate significant EMI in the audio frequency band. Because the EMI is in the operating frequency range of the magnetometer it is sensed by the magnetometer sensors, resulting in a large heading error in a magnetic compass (e.g., as high as 90°). One solution would be for the sensor to take magnetic measurements only when the EMI source is inactive. Deactivating an EMI source, however, is not desirable for many applications such as gaming or virtual reality applications that rely on a digital compass or motion sensors. In the description that follows, a compensation coil is described that compensates for EMI in the audio frequency band without compromising the performance of applications that rely on magnetometer measurements.

FIG. 3is a conceptual block diagram illustrating an example EMI compensation system300, according to an embodiment. System300includes power supply301, EMI source302, reverse current generator303and compensation coil304. Reverse current generator303and compensation coil304are coupled in parallel to power supply301.

Power supply301provides current to EMI source302(e.g., a voice or speaker coil) and reverse current generator303reverses the current in compensation coil304so that the direction of current flow in compensation coil304is opposite to the direction of current flow into EMI source302. In an embodiment, reverse current generator303includes one or more switches operable to reverse the connection to the power supply. In other embodiments, other circuitry can be used to generate an opposite current (180° out of phase with power supply current) in compensation coil304. When supplied with current compensation coil304generates a counter magnetic field that substantially cancels out the EMI generate by EMI source302.

FIG. 4is a top view of compensation coil400, according to an embodiment. Compensation coil400is a three-dimensional (3D) solenoid structure that is placed underneath magnetometer package401and can be integrated in multiple layers of a PCB. A solenoid is a coil of wire designed to create a strong magnetic field inside the coil. The number of turns of the coil of wire refers to the number of loops of the solenoid. More loops will bring about a stronger magnetic field.

In this example, magnetometer package401houses three sensors402a-402cfor sensing in three different sense axes (x, y, z). Note that the positive +z direction is into the page. To reduce the foot print of compensation coil400and to take advantage of symmetry, the width W of compensation coil400is made coextensive with the width of package401, as shown inFIG. 4. Additionally, center403of compensation coil400is offset from center404of package401to ensure maximum compensation along the z-axis while minimizing unwanted interference along the x and y axes. The number of turns of wire per layer is selected to reduce current consumption. For example, compensation coil400can have a total of 16 turns of wire integrated into 4 PCB layers, where each PCB layer has 4 turns of wire. Other compensation coil designs can have more or fewer layers and more or fewer turns of wire per layer.

The placement of compensation coil400underneath sensors402a-402ctakes advantage of the symmetry of compensation coil400to maximize the z-axis component of the counter magnetic field and substantially cancel the x-axis and y-axis components of the counter magnetic field. Compensation coil400can be divided into two regions by imaginary line405. By aligning sensor402b(y-axis sensor) so it is symmetric (evenly divided) along imaginary line405the magnetic fluxes flowing in the y-axis direction from the two regions substantially cancel out. In some cases, sensor402c(x-axis sensor) is not located at center404of package401. To create symmetry in the x-axis direction, a length L of compensation coil400is extended beyond the package401boundary in the x-axis so that center403of compensation coil400overlies sensor402c. This arrangement allows the magnetic fluxes flowing in the x-axis direction to substantially cancel out.

Note that the desired to maximize compensation on the z-axis component is due to the layout of electronic system100. In a smartphone, for example, the EMI source is a coil that lies in the same plane as the compensation coil and generates a magnetic field in the z direction as expected for a solenoid structure. For other electronic systems it may be desirable to focus the maximum compensation in the x direction or y direction depending on the position and orientation of the EMI source relative to the position and orientation of the compensation coil. Note that the x, y, z coordinate system disclosed herein is only an example and other reference coordinate frames can be used.

FIG. 5is a example Bodie plot illustrating EMI as a function of frequency with EMI compensation, according to an embodiment. The plot fromFIG. 2is included as baseline (without compensation coil) for comparison purposes. As can be observed from the plot inFIG. 5, the magnetic interference in the lower part of the audio frequency band has been attenuated by about 80%, confirming the expected benefits of EMI compensation system300.

FIG. 6Ais a front view of compensation coil400from the y-axis, according to an embodiment. In this view the 4 PCB layers601-604of the compensation coil400are shown including coil wires606. Vias605a-605care for connecting PCB traces from different PCB layers.FIG. 6Bis a side view of compensation coil400from the x-axis, according to an embodiment. In this view, the internal coil layers are shown connected through vias605a-605c.

Example Device Architecture

FIG. 7is a block diagram of an electronic system architecture700. Architecture700may include memory interface702, data processor(s), image processor(s) or central processing unit(s)704, and peripherals interface706. Memory interface702, processor(s)704or peripherals interface706may be separate components or may be integrated in one or more integrated circuits. One or more communication buses or signal lines may couple the various components.

Sensors, devices, and subsystems may be coupled to peripherals interface706to facilitate multiple functionalities. For example, motion sensor(s)710, light sensor712, and proximity sensor714may be coupled to peripherals interface706to facilitate orientation, lighting, and proximity functions of the device. For example, in some embodiments, light sensor712may be utilized to facilitate adjusting the brightness of touch surface746. In some embodiments, motion sensor(s)710(e.g., an accelerometer, rate gyroscope) may be utilized to detect movement and orientation of the device. Accordingly, display objects or media may be presented according to a detected orientation (e.g., portrait or landscape). Haptic feedback system717, under the control of haptic feedback instructions772, provides haptic feedback in the form of vibration.

Other sensors may also be connected to peripherals interface706, such as a temperature sensor, a barometer, a biometric sensor, or other sensing device, to facilitate related functionalities. For example, a biometric sensor can detect fingerprints and monitor heart rate and other fitness parameters.

Location processor715(e.g., GNSS receiver chip) may be connected to peripherals interface706to provide geo-referencing. Electronic magnetometer716(e.g., an integrated circuit chip) may also be connected to peripherals interface706to provide data that may be used to determine the direction of magnetic North. Thus, electronic magnetometer716may be used as an electronic compass. Electronic magnetometer can be a multi-axis magnetometer that includes a compensation coil as described in reference toFIGS. 1-6.

Camera subsystem720and an optical sensor722, e.g., a charged coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) optical sensor, may be utilized to facilitate camera functions, such as recording photographs and video clips.

Communication functions may be facilitated through one or more communication subsystems724. Communication subsystem(s)724may include one or more wireless communication subsystems. Wireless communication subsystems724may include radio frequency receivers and transmitters and/or optical (e.g., infrared) receivers and transmitters. Wired communication systems may include a port device, e.g., a Universal Serial Bus (USB) port or some other wired port connection that may be used to establish a wired connection to other computing devices, such as other communication devices, network access devices, a personal computer, a printer, a display screen, or other processing devices capable of receiving or transmitting data.

The specific design and embodiment of the communication subsystem724may depend on the communication network(s) or medium(s) over which the device is intended to operate. For example, a device may include wireless communication subsystems designed to operate over a global system for mobile communications (GSM) network, a GPRS network, an enhanced data GSM environment (EDGE) network, IEEE802.xx communication networks (e.g., Wi-Fi, Wi-Max, ZigBee™), 3G, 4G, 4G LTE, code division multiple access (CDMA) networks, near field communication (NFC), Wi-Fi Direct and a Bluetooth™ network. Wireless communication subsystems724may include hosting protocols such that the device may be configured as a base station for other wireless devices. As another example, the communication subsystems may allow the device to synchronize with a host device using one or more protocols or communication technologies, such as, for example, TCP/IP protocol, HTTP protocol, UDP protocol, ICMP protocol, POP protocol, FTP protocol, IMAP protocol, DCOM protocol, DDE protocol, SOAP protocol, HTTP Live Streaming, MPEG Dash and any other known communication protocol or technology.

Audio subsystem726may be coupled to a speaker728and one or more microphones730to facilitate voice-enabled functions, such as voice recognition, voice replication, digital recording, and telephony functions. Speaker728includes a voice/speaker coil that generates EMI as described in reference toFIGS. 1-8, and is therefore an example of an EMI source.

I/O subsystem740may include touch controller742and/or other input controller(s)744. Touch controller742may be coupled to a touch surface746. Touch surface746and touch controller742may, for example, detect contact and movement or break thereof using any of a number of touch sensitivity technologies, including but not limited to, capacitive, resistive, infrared, and surface acoustic wave technologies, as well as other proximity sensor arrays or other elements for determining one or more points of contact with touch surface746. In one embodiment, touch surface746may display virtual or soft buttons and a virtual keyboard, which may be used as an input/output device by the user.

Other input controller(s)744may be coupled to other input/control devices748, such as one or more buttons, rocker switches, thumb-wheel, infrared port, USB port, and/or a pointer device such as a stylus. The one or more buttons (not shown) may include an up/down button for volume control of speaker728and/or microphone730.

In some embodiments, device700may present recorded audio and/or video files, such as MP3, AAC, and MPEG video files. In some embodiments, device700may include the functionality of an MP3 player and may include a pin connector for tethering to other devices. Other input/output and control devices may be used.

Memory interface702may be coupled to memory750. Memory750may include high-speed random access memory or non-volatile memory, such as one or more magnetic disk storage devices, one or more optical storage devices, or flash memory (e.g., NAND, NOR). Memory750may store operating system752, such as Darwin, RTXC, LINUX, UNIX, OS X, iOS, WINDOWS, or an embedded operating system such as VxWorks. Operating system752may include instructions for handling basic system services and for performing hardware dependent tasks. In some embodiments, operating system752may include a kernel (e.g., UNIX kernel).

Memory750may also store communication instructions754to facilitate communicating with one or more additional devices, one or more computers or servers, including peer-to-peer communications. Communication instructions754may also be used to select an operational mode or communication medium for use by the device, based on a geographic location (obtained by the GPS/Navigation instructions768) of the device.

Memory750may include graphical user interface instructions756to facilitate graphic user interface processing, including a touch model for interpreting touch inputs and gestures; sensor processing instructions758to facilitate sensor-related processing and functions; phone instructions760to facilitate phone-related processes and functions; electronic messaging instructions762to facilitate electronic-messaging related processes and functions; web browsing instructions764to facilitate web browsing-related processes and functions; media processing instructions766to facilitate media processing-related processes and functions; GNSS/Navigation instructions768to facilitate GNSS (e.g., GPS, GLOSSNAS) and navigation-related processes and functions; camera instructions770to facilitate camera-related processes and functions; and other instructions772for implementing one or more applications including, for example, a virtual reality application, gaming application, navigation application or an electronic compass application.

Each of the above identified instructions and applications may correspond to a set of instructions for performing one or more functions described above. These instructions need not be implemented as separate software programs, procedures, or modules. Memory750may include additional instructions or fewer instructions. Furthermore, various functions of the device may be implemented in hardware and/or in software, including in one or more signal processing and/or application specific integrated circuits (ASICs).

A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made. Elements of one or more embodiments may be combined, deleted, modified, or supplemented to form further embodiments. In yet another example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other embodiments are within the scope of the following claims.