PATENT DOCUMENT

Publication Number: US-8565709-B2
Application Number: US-201113023902-A
Country: US
Kind Code: B2

Title: Digital signal filter

Abstract:
A passive filter circuit filters an input signal to attenuate an undesired frequency. The passive filter circuit includes a first stage and a second stage. The input to the first stage is the input signal. The first stage includes a first inductor and a first branch coupled to the output of the first inductor. The first branch includes a first capacitor and a second inductor. The first stage is coupled to the second stage. The second stage includes a third inductor and a second branch coupled to the output of the third inductor. The second branch includes a second capacitor. Other embodiments are also described and claimed.

Claims:
What is claimed is: 
     
       1. A circuit comprising:
 a digital signal generator to produce a digital signal; and 
 a passive filter circuit for filtering the digital signal to attenuate at a radio communications frequency, the passive filter circuit comprising 
 a first stage to receive the digital signal, the first stage having a first inductor and a first branch coupled to the output of the first inductor, the first branch having a first capacitor and a second inductor, and 
 a second stage coupled to the first stage, the second stage having a third inductor and a second branch coupled to the output of the third inductor, the second branch having a second capacitor. 
 
     
     
       2. The circuit of  claim 1 , wherein the radio communications frequency is between 800 MHz and 900 MHz. 
     
     
       3. The circuit of  claim 1 , wherein a capacitance value of the first capacitor substantially equals a capacitance value of the second capacitor. 
     
     
       4. The circuit of  claim 1 , wherein the first capacitor and the second inductor are discrete components, wherein addition of the second inductor to the first branch lowers a resonant frequency of the first branch without having to increase a capacitance value of the first capacitor, which degrades a rise time of an output signal. 
     
     
       5. The circuit of  claim 4 , wherein the rise time of the output signal does not exceed 12 nanoseconds. 
     
     
       6. The circuit of  claim 1 , wherein the inductance value of the second inductor is smaller than that of the first inductor and the third inductor. 
     
     
       7. The circuit of  claim 1 , wherein the second branch provides a rolled-off frequency response at frequencies above the undesired frequency. 
     
     
       8. A circuit comprising:
 a digital signal generator to produce a digital signal; and 
 a passive filter circuit for filtering the digital signal to attenuate at a radio communications frequency, the passive filter circuit comprising 
 an input terminal to receive the digital signal, 
 an output terminal to provide a filtered version of the digital signal, and 
 a first inductor, a second inductor, and a third inductor, the first inductor and the third inductor coupled in series between the input terminal and the output terminal; and 
 a first capacitor and a second capacitor, the first capacitor and the second inductor coupled in series between (1) a node that is between the first inductor and the third inductor and (2) ground, wherein the second capacitor is coupled between the output terminal and ground. 
 
     
     
       9. The circuit of  claim 8 , wherein the inductance and capacitance values of the first inductor, the first capacitor, and the second inductor are selected to attenuate the radio communications frequency. 
     
     
       10. The circuit of  claim 9 , wherein the radio communications frequency is between 800 MHz and 900 MHz and the digital signal&#39;s fundamental frequency is less than 100 MHz. 
     
     
       11. The circuit of  claim 8 , wherein a capacitance value of the first capacitor substantially equals a capacitance value of the second capacitor. 
     
     
       12. The circuit of  claim 8 , wherein the inductance value of the second inductor is smaller than that of the first inductor and the third inductor. 
     
     
       13. The circuit of  claim 8 , wherein the second inductor lowers a resonant frequency of the passive filter circuit without having to increase a capacitance value of the first capacitor, which degrades arise time of the filtered version of the digital signal. 
     
     
       14. The circuit of  claim 13 , wherein the rise time of the filtered version of the digital signal does not exceed 12 nanoseconds. 
     
     
       15. The circuit of  claim 8 , wherein the second capacitor connected between the output terminal and ground provides a rolled-off frequency response at frequencies above the radio communications frequency. 
     
     
       16. A mobile communications device, comprising:
 an antenna coupled to radio communications circuitry to establish a wireless connection through a wireless communications network; 
 camera circuitry to capture an image of a scene; 
 a clock signal generator to produce a clock signal for operation of the camera circuitry; and 
 a passive filter circuit having an input coupled to the clock signal generator, an output coupled to the camera circuitry, a first inductor at the input, a pi filter coupled to the first inductor, the pi filter having a first leg with a first capacitor and a second leg with a second capacitor, and a second inductor coupled in series with the first capacitor in the first leg of the pi filter. 
 
     
     
       17. The mobile communications device of  claim 16 , wherein a capacitance value of the first capacitor substantially equals a capacitance value of the second capacitor. 
     
     
       18. The mobile communications device of  claim 16 , wherein the inductance value of the second inductor is smaller than that of the first inductor and a third inductor. 
     
     
       19. The mobile communications device of  claim 16 , wherein the passive filter circuit is designed to attenuate an undesired frequency between 800 MHz and 900 MHz. 
     
     
       20. The mobile communications device of  claim 19 , wherein the second leg provides a rolled-off frequency response at frequencies above the undesired frequency. 
     
     
       21. The mobile communications device of  claim 16 , wherein the second inductor lowers the resonant frequency of the first leg without having to increase a capacitance value of the first capacitor, which degrades a rise time of an output signal of the passive filter circuit. 
     
     
       22. The mobile communications device of  claim 21 , wherein the rise time of the output signal does not exceed 12 nanoseconds.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This present U.S. Non-Provisional Patent Application claims the benefit of the earlier filing date of related U.S. Provisional Application No. 61,428,667, filed Dec. 30, 2010, currently pending. 
    
    
     FIELD OF THE INVENTION 
     An embodiment of the invention relates to a filter circuit for attenuating an undesired frequency in a digital signal. Other embodiments are also described. 
     BACKGROUND 
     In recent years, portable consumer electronic wireless communications devices such as smart phones, tablet computers, and laptop computers have been equipped with a variety of integrated digital subsystems. One example of such a subsystem is an integrated digital camera. The cameras used in such devices are often in the form of a module that includes, for example, a charge coupled device (CCD) imaging sensor or a complementary metal oxide semiconductor (CMOS) imaging sensor that is in a compact package together with its optical system. Camera modules having either type of imaging sensor move photodetected charge as a mass of digital signals sequentially, at a rate synchronized with a clock signal that is typically generated from a source outside the camera module. The clock signal that is input to the camera module and the digital data signal that is output in synchronism with the clock signal contain harmonic components that may interfere with the operation of other integrated subsystems such as the radio communications circuitry. In the case where a harmonic component coincides with the radio communication frequency band, the radio communication would be adversely affected. 
     With the recent trend toward wireless communications devices of increasingly smaller size, the radio communications circuitry, the camera module, and the camera clock signal generator are often arranged within the housing or body of the device in close proximity to each other. For instance, in order to have a thinner body, the camera module is not mounted directly on a main logic board, but rather may be connected to the main logic board and the clock signal generator through a flexible circuit. In this case, the radiation noise of the harmonic components of the clock signal and the digital data signal may be released from the flexible circuit and interfere with radio communications. On the other hand, radio communications frequencies may be picked up by the flexible circuit and interfere with the camera clock signal and the digital data signal that is transmitted to and from the camera module through the flexible circuit. 
     SUMMARY 
     In accordance with an embodiment of the invention, a passive filter circuit may be used to attenuate an undesired frequency component in a digital signal. The passive filter circuit includes a first stage that is to receive the digital signal and a second stage that is coupled to the output of the first stage. The first stage includes a first inductor and a first branch that is coupled to the output of the first inductor. The first branch includes a first capacitor and a second inductor. The second stage includes a third inductor and a second branch that is coupled to the output of the third inductor. The second branch includes a second capacitor. The passive filter circuit may be used within a wireless communications device having a camera module that uses a clock signal, to attenuate a harmonic component in the clock signal that coincides with a radio communications frequency, without significantly degrading the rise time of the clock signal. Other digital signal applications are possible. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the drawings summarized below. The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
         FIG. 1  is a circuit schematic of a passive filter circuit according to an embodiment of the invention 
         FIG. 2  is a waveform diagram of an example digital signal and a filtered version of the digital signal. 
         FIG. 3  is an illustration showing a user holding an example communications device during a call. 
         FIG. 4  is an illustration showing a user holding an example communications device while taking a video of a scene. 
         FIG. 5  is a block diagram of an example mobile device with a built-in camera. 
         FIG. 6  is a block diagram of an example camera circuitry with a passive filter circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments of the invention with reference to the appended drawings are now explained. While numerous details are set forth, it is understood that some embodiments of the invention may be practiced without these details. In other instances, well-known circuits, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description. 
       FIG. 1  shows a circuit schematic of a filter circuit  2  that may be used to attenuate an undesired frequency component that may be present in an input signal  5 . This is achieved by inserting the filter circuit  2  in line with the input signal  5 . The input signal  5  may be any digital signal such as a clock signal, a data signal, a control signal, etc. For instance, the filter circuit  2  may receive the input signal  5  from a signal generator  3  and provide a filtered signal  6  to a signal destination  4 . 
     The filter circuit  2  is formed using a combination of capacitors and inductors, both of which are passive components (i.e., components that do not depend on an external power supply). Because the filter circuit  2  is formed using passive components, the filter circuit  2  is unlikely to be impacted by radiated interference from other nearby subsystems. The filter circuit  2  includes inductors L 1 , L 2 , and L 3 , and capacitors C 1  and C 2 . 
     The filter circuit  2  may be described as having a first stage and a second stage. An input of the first stage of the filter circuit  2  receives the input signal  5  from the signal generator  3 . The first stage is an LC stage that includes the inductor L 1  and a first branch (also referred to as a first leg) that is coupled to the output of the inductor L 1  (i.e., connected to a node between the series inductors L 1  and L 3 ). The first branch includes the capacitor C 1  and the inductor L 2  which is added in series to the capacitor C 1  and connected to ground. The added inductor L 2  to the first branch reduces the resonant frequency of the first branch (which is attenuated), without having to increase the value of the capacitor C 1 , which would significantly degrade the rise time of the filtered signal  6  (and lead to a larger component and hence an undesired result especially when implemented with a handheld mobile communications device such as a smartphone). The rise time of the filtered signal  6  is sufficiently maintained when its rise time is sufficiently fast (i.e., short) as dictated by the requirements of the signal destination  4 . In one embodiment, the rise time of the filtered signal  6  may be considered sufficiently fast when the time interval of the rise time does not exceed 12 ns at a voltage swing of 1.8 V. In such an embodiment, a frequency component may be deemed attenuated by the filter circuit  2  if it is reduced by at least 20 dB. 
     The first stage of the filter circuit  2  is connected to a second stage. The second stage is also an LC stage that includes the inductor L 3  and a second branch (also referred to as a second leg) that is coupled to the output of the inductor L 3  (i.e., the output of the filter circuit  2 ). The second branch includes the capacitor C 2  which is connected to ground. The second stage provides a rolled-off frequency response at or above the undesired frequency component. An output of the second stage (i.e., the output of the filter circuit  2 ) is connected to the signal destination  4  to provide the filtered signal  6  to the signal destination  4 . 
     The inductor L 3  and the capacitors C 1  and C 2  form an LC pi filter (also referred to as a capacitor-input filter). The inductor L 3  forms the series component of the LC pi filter and the capacitors C 1  and C 2  form the parallel or shunt components of the LC pi filter. The inductor L 1  is coupled to the input of the LC pi filter. The inductor L 2  is added to the first leg of the LC pi filter in series with the capacitor C 1 . The first and second legs of the LC pi filter are connected to ground (which may not be at zero volts but rather at any power supply return node voltage). 
     The filter circuit  2  may be configured to attenuate an undesired frequency by selecting inductance and capacitance values for the first stage such that the resonant frequency of the first stage equals (i.e., is within a practical tolerance range of) the undesired frequency. When the resonant frequency of the first stage equals the undesired frequency, the filter circuit  2  attenuates the undesired frequency in the input signal  5 . The amount of attenuation depends on the values of the dominant contributing components in the filter circuit  2 , namely inductors L 1  and L 2  and capacitor C 1 . In an embodiment of the invention, the inductor L 1  may have a value greater than the inductor L 3 , and the inductor L 3  may have a value greater than the inductor L 2 . The capacitors C 1  and C 2  may have substantially the same capacitance value, allowing for practical tolerances (e.g., a nominal capacitance value ±5%) assigned to each discrete component by its manufacturer. For example, the component values for the capacitor C 1  and the inductors L 1  and L 2  may be chosen so that the resonant frequency of the first stage is reduced from 3 GHz to a narrow band, for example, around 880 MHz (or other similar fundamental RF radiated carrier signal frequency) and attenuated by a minimum of 20 dB. The component values for the capacitor C 2  and inductor L 3  may be chosen to provide a rolled-off frequency response above 880 MHz. 
       FIG. 2  shows a waveform diagram of an example digital signal  22  that is produced by a signal generator and a filtered version of the digital signal  24  that is provided by a filter circuit of  FIG. 1 . The term digital signal as used here refers to a signal that has at least two or more discrete and stable levels. The digital signal  22  may be, for example, a clock signal that has a square wave with a 50% duty cycle with a fixed constant fundamental frequency that is, for example, less than 100 MHz. The term “square wave” is used here generically to also refer to rectangular shaped waveforms, i.e. pulse signals whose duty cycle may be different than 50%. The filtered version  24  has an undesired frequency (that may be a harmonic component of the unfiltered version  22 ) attenuated, without significantly degrading the rise time of the filtered clock signal  24 . In one embodiment, proper selection of the inductance value of the inductor L 2 , in view of L 1  and C 1 , may enable the time interval of the rise time of the filtered clock signal  24  to not exceed 12 ns at a voltage swing of 1.8 V. 
       FIGS. 3-6  show an example of a camera-equipped mobile communications device  30  in which the passive filter circuit may be used.  FIG. 3  shows a user holding a handheld version of the multi-function device  30  in his hand against his ear (during a call).  FIG. 4  shows a user holding the device  30  up to take a video of a scene. The device  30  may be any camera-equipped portable communications device, such as a cellular phone, a smart phone, a notebook personal computer, or a tablet-like computer, all of which may have a built-in digital camera in addition to wireless communication capabilities (e.g., cellular network connectivity, WLAN connectivity). The device  30  may have an exterior back face in which there is a rear-facing camera  32  and an exterior front face in which there is a display screen  34 . 
       FIG. 5  is an example block diagram of the camera-equipped mobile communications device  30 . The mobile device  30  includes a main processor  40  that interacts with a GPS receiver  46 , a Wi-Fi and Bluetooth transceiver  48 , memory  50 , storage  52 , a speaker  54 , a microphone  56 , a display  58 , and a user interface  60 . The various components of the mobile device  30  may be digitally interconnected and used or managed by a software stack being executed by the main processor  40 . Many of the components shown or described here may be implemented as one or more dedicated hardware units and/or a programmed processor (software being executed by a processor, e.g., the main processor  40 ). 
     The main processor  40  controls the overall operation of the mobile device  30  by performing some or all of the operations of one or more applications implemented on the mobile device  30 , by executing instructions for it (software code and data) that may be found in the storage  52 . The processor may, for example, drive the display  58  and receive user inputs through the user interface  60  (which may be integrated with the display  58  as part of a single, touch sensitive display panel  34  on the front face of the mobile device  30 ). The main processor  40  may also control the reception of forward channel signals and the transmission of reverse channel signals by an RF transceiver  42 . 
     Storage  52  provides a relatively large amount of “permanent” data storage, using nonvolatile solid state memory (e.g., flash storage) and/or a kinetic nonvolatile storage device (e.g., rotating magnetic disk drive). Storage  52  may include both local storage and storage space on a remote server. Storage  52  may store data, such as image data taken with the mobile device  30 , and software components that control and manage, at a higher level, the different functions of the mobile device  30 . For instance, there may be telephony application that configures a built-in touch sensitive display to look like the keypad of a traditional telephony handset, and allows the user to enter a telephone number to be called, or select a previously stored number from a telephone address book. The telephony application then proceeds to allow the user to make a call, and controls the built-in microphone  56  and earpiece speaker  54  to enable the user to experience a two-way conversation during the call. Another application may be a camera application that would allow the user to take pictures and video through the user interface  60  and display  58 . 
     In addition to storage  52 , there may be memory  50 , also referred to as main memory or program memory, which provides relatively fast access to stored code and data that is being executed by the main processor  40 . Memory  50  may include solid state random access memory (RAM), e.g., static RAM or dynamic RAM. There may be one or more processors, e.g., main processor  40 , that run or execute various software programs, modules, or sets of instructions (e.g., applications) that, while stored permanently in the storage  52 , have been transferred to the memory  50  for execution, to perform the various functions described above. It should be noted that these modules or instructions need not be implemented as separate programs, but rather may be combined or otherwise rearranged in various combinations. In addition, the enablement of certain functions could be distributed amongst two or more modules, and perhaps in combination with certain hardware. 
     The mobile device  30  includes components that are required for wireless communications, such as the RF transceiver  42  and associated antenna  44 . The device  30  may also include other wireless communications components, such as the Wi-Fi and Bluetooth transceiver  48  and its associated antenna  62 , and the GPS receiver  46  and its associated antenna  64 . Accordingly, there is the likelihood that one or more of these components will be adversely affected by harmonic components in a clock signal used by a digital camera in the device  30 . 
     The RF transceiver  42  may communicate with a base station of a wireless communication network using a cellular band such as an 800 MHz band. Other RF or even microwave communication bands are possible. RF transceiver  42  receives, from antenna  44 , an incoming RF signal transmitted by the base station. RF transceiver  42  may down-convert the incoming RF signal to produce a baseband signal. The baseband signal is sent to a baseband processor  70  that produces a processed baseband signal by filtering, decoding, and/or digitizing the baseband signal. The baseband processor  70  provides the processed baseband signal to the main processor  40 . The main processor  40  may then transmit the processed baseband signal to the speaker  54  (e.g., voice communications) or perform further processing of the processed baseband signal for display to the user (e.g., data communications such as text messaging and web browsing). The baseband processor  70  may also receive voice data or other outgoing baseband data (e.g., web data, email, text message) from the main processor  40 . The baseband processor  70  may encode, multiplex, and/or digitize the outgoing baseband data to produce a processed baseband signal. RF transceiver  42  receives the outgoing processed baseband signal and may up-convert the baseband signal to an RF signal that is transmitted via the antenna  44 . 
     The mobile device  30  has a built-in camera that includes camera circuitry  80 . The camera circuitry  80  implements the digital camera functionality, which is shown in detail in the block diagram of  FIG. 6 , of the device  30 . The camera circuitry  80  may include a microelectronic image sensor array  90  and a camera controller  92 . The camera controller  92  (e.g., implemented as a combination of programmed data processing components and hardwired logic circuitry) manages the process of capturing images. It does so by generating various commands that are signaled to the sensor array  90 , which captures light from a scene. The sensor array  90  responds to the incident light and collects photo-generated charge or forms image signals during an integration interval (or exposure interval). At the end of the integration interval, the image signals are read (including being digitized) and transferred as a captured digital image or “exposure” to an image storage, which may be memory  50  or storage  52 , that stores many digital images captured by the sensor array  90 . 
     The operation of the camera circuitry  80 , including the timing of image capture by the sensor array  90 , is driven by a clock signal of a predetermined frequency, which is provided by a clock signal generator  82 . The clock signal generator  82  may be a function that is integrated into the main processor  40 , or it may be a separate circuit (e.g., a voltage controlled oscillator) that generates the clock signal. In one embodiment, the clock signal fundamental frequency is between 10 MHz and 100 MHz. Because the clock signal may radiate frequencies that can interfere with wireless communications components that are integrated in the device  30 , the clock signal is filtered by a filter circuit  2  (shown in  FIG. 1 ) prior to it being received by the camera circuitry  80 . The filter circuit  2  attenuates an undesired frequency in the clock signal, such as between 800 MHz and 900 MHz, and more particularly at about 880 MHz that the device  30  uses for RF wireless communications. Of course, the filter circuit  2  may alternatively be designed to attenuate other RF bands (which may be at least several times higher in frequency than the clock signal fundamental). Because the sensor array  90  captures and transfers its image data at a high rate, the filter circuit  2  should attenuate the undesired frequency without significantly impacting the rise time of the clock signal. This may be achieved, as explained above, through the addition of the inductor L 2  (to the “front” leg of the pi filter) having a selected inductance value that lowers a resonant frequency of the front leg to equal that of the undesired frequency (e.g., a fundamental frequency of an RF wireless communications radiated carrier signal.) 
     To conclude, various aspects of a passive filter circuit for attenuating an undesired frequency in a clock signal is described. For purposes of explanation, specific embodiments were described to provide a thorough understanding of the present invention. These should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed in detail above. Various other modifications, changes, and variations which will be apparent to those skilled in the art may be made in the arrangement, operation, and details of the systems and methods of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims. For instance, while the device  30  is depicted in  FIG. 1  as a mobile telephony device with wireless call capability and a built-in camera (e.g., a smart phone handset), it may alternatively be a digital still or video camera (e.g., a handheld camcorder) with wireless networking capability. Therefore, the scope of the invention should be determined by the claims and their legal equivalents. Such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. Furthermore, no element, component, or method step is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims.

Metadata:
Filing Date: 20110209
Publication Date: 20131022
Grant Date: 20131022
Priority Date: 20101230
Inventors: MULLINS SCOTT P.
JOHNSON TIMOTHY M.
PEREZ YEHONATAN
Assignee: APPLE INC
CPC Classifications: [{"code": "H03H7/0115", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03H7/1758", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03H7/0115", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03H7/1758", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 46380246