Abstract:
An antenna for a wireless device may be kept dynamically tuned to a desired center frequency to compensate for detuning which may be caused by environmental influences. A sensor provides a feedback signal to a controller to select an appropriate capacitance value from a variable capacitor to tune the antenna for the wireless device. The variable capacitor may comprise a plurality of fixed capacitors and MEMS switches arranged in parallel or may comprise a variable MEMS capacitor having a fixed lower plate and a flexible upper plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to U.S. Provisional Application Ser. No. 60/502,466, filed on Sep. 11, 2003, the contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     Embodiments of the present invention relate to antennas and, more particularly, to MEMS (micro-electromechanical systems) tunable antennas.  
       BACKGROUND INFORMATION  
       [0003]     No longer are electronic communication devices necessarily tethered by wires. In recent times wireless communications has become a popular and often an economical and convenient method by which to communicate both analog and digital information. In particular, cellular phones and other mobile communication devices such as personal digital assistants (PDAs), palm, and lap-top computing devices connect to service providers via wireless links.  
         [0004]     Such wireless devices rely on antenna technology to radiate radio frequency (RF) signals for transmission (Tx) as well as to gather RF broadcast signals for reception (Rx). Often the same antenna or antenna array performs both of these transmit and receive functions. While antennas may be one of the most vital elements in a wireless system, they may be one of the most inefficient elements, typically accounting for a large portion of energy loss.  
         [0005]     As the effort to shrink the size of mobile devices such as cell phones continues, efforts are being made to reduce the size of the antenna. To further compound this effort, modern cell phones may be designed to work with dual/triple/quad bands for Tx/Rx. Given the size considerations, it may be difficult to have a dedicated antenna to operate in each frequency band. For example,  FIG. 1  shows the frequency range of the Global System for Mobile Communication (GSM) 850 and GSM 900 bands which together span about 824 MHz to 960 MHz (megahertz). GSM is currently the dominant digital mobile phone standard for much of the world. As shown, GSM 850 utilizes 25 MHz each for Tx and Rx, and GSM 900 utilizes 35 MHz each for Tx and Rx. The GSM protocol dictates the way that mobile phones communicate with the land-based network of cell towers. Modern mobile communication devices thus call for a small antenna that can efficiently operate over such a broad range.  
         [0006]     The efficiency of so called microstrip or “patch antennas”, which are often used in cellular phone applications can fluctuate dramatically depending on its usage. For example, the radiation efficiency can fluctuate from 80% down to 15% or lower depending the positioning of the antenna and surrounding environment. Environmental considerations include not only geographical terrain, but also more dynamic factors such as the phone is sitting on a table, held the user&#39;s hand, near the user&#39;s head, inside of a car, etc. Further, for any given wireless session the antenna may encounter all of these obstacles as the user constantly repositions the phone and thus repositions the antenna. A major cause of these fluctuations may be due to detuning of the center frequency of the antenna caused by additional capacitive loading from the environment.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is an illustrative representation of the frequency response for a wireless device antenna tuned to operate across the GSM 850 and GSM 900 range;  
         [0008]      FIG. 2  is a plan view of a patch antenna for a for a wireless device such as a cellular telephone;  
         [0009]      FIG. 3  is an illustrative representation of the frequency response for a wireless device antenna dynamically tunable to the center frequencies of the Tx and Rx bands within the GSM 850 and GSM 900 range; and  
         [0010]      FIG. 4  is a block diagram of a dynamically tunable antenna.  
     
    
     DETAILED DESCRIPTION  
       [0011]     Referring now to  FIG. 2 , there is shown a patch antenna  200  that may be used in a mobile device, such as a cellular phone. A patch antenna  200  comprises two conducting plates,  10  and  12 , sandwiching a dielectric material  14 , and may be built in a similar way as a parallel plate capacitor. In the case of an antenna, the bottom conducting plate  10  may be referred to as the “ground plate”, and the top conducting plate  12  may be referred to as the “patch”. The patch  12  may comprise a thin metal foil such as copper or aluminum and may be smaller than, and centered over, the ground plate  10 . An antenna feed  16  may connect to one side of the patch  12 . The ground plate  10 , the patch  12 , and feed  16  may be made of the same conducting material. The dielectric material  14  may be, for example silicon, alumina, or a printed circuit board laminate such as FR-4.  
         [0012]     While the patch may be any shape, for simplicity of illustration it is shown as a square or rectangular. The size of the patch  12  may be chosen relative to the frequency in which the antenna is to operate where antenna bandwidth is proportional to the antenna volume, length (L)×width (W)×height (H), (L×W×H). Antenna efficiency and quality or “Q-factor” are two metrics for qualifying the antenna design. Antenna efficiency may be designated by the symbol “μl”, where q equals power radiated/input power. The Q-factor is generally understood to mean the ratio of the stored energy to the energy dissipated per radian of oscillation and may be used to describe antennas and other inductive or capacitive devices. For patch antennas the Q-factor depends on several factors which are determined not only by the materials in the antenna (metals and dielectrics) but also geometry of the antenna and its surrounding environment.  
         [0013]     According to embodiments of the invention, the center frequency of an antenna may be tuned such as by using a variable MEMS capacitor or varactor. As shown in  FIG. 3 , an antenna may be tuned to the center frequencies of the Tx and Rx ranges for either the GSM 850 or GSM 900 bands. As shown, for GMS 850 the center frequency for Tx is 836.5 MHz and the center frequency for Rx is 881.5 MHz. Similarly, for the GSM 900 band, the center frequency for Tx is 897.5 MHz and the center frequency for Rx is 942.5 MHz. For example, by changing a capacitive load, a single antenna may be tuned to a variety of center frequencies even in different bands. Further, the tuning may be adjusted dynamically to maintain tuning locked on the center frequency even as the capacitive loading due to the environment changes (e.g., as the antenna is moved and repositioned during use).  
         [0014]     Referring now to  FIG. 4 , there is shown an exemplary tunable antenna design in accordance with an embodiment of the invention. The antenna  20  may be a patch antenna as discussed above. The antenna  20  may include the bottom plate or “ground” plate  10  and a top conducting plate or “patch”  12 . The patch  12  may comprise a thin metal foil such as copper or aluminum and may be smaller than, and centered over, the ground plate  10 . An antenna feed  16  may connect to one side of the patch  12 .  
         [0015]     An antenna switch filter (ASF) module  22  switches the antenna  20  between a low noise amplifier (LNA)  24  for transmission (Tx) and a power amplifier (PA)  26  for reception (Rx). The ASF module  22 , LNA  24 , and PA  26  may comprise a front end module of a cell phone for example or other wireless device.  
         [0016]     As previously noted, the antenna  20  may be initially tuned to various center frequencies as well as adjusted in real time to maintain a desired center frequency by adjusting the capacitive load to compensate for environmental loading.  FIG. 4  shows two types of variable capacitive modules  30  and  32 , discussed in greater detail below, for altering the capacitive load to the antenna. The variable capacitive module,  30  or  32 , connects between the ground plate  10  and patch  12  of the antenna  20 .  
         [0017]     A controller  34  connects to the capacitive module,  30  or  32 , to select a proper capacitance to initially tune the antenna  20  to a desired center frequency such as, for example, those shown in  FIG. 3 . A feedback loop  36  comprising a sensor  38  that measures the radiated power, which may be a pick-up coil or directional coupler, and a power detector  40 , continuously measures the near field radiated power from the antenna  20  to provide the appropriate tuning corrections. The controller  34  may use a Fourier transform to correlate the detected near field to a far field measurement to closely approximate the current tuning frequency of the antenna  20 . Alternatively the power delivered to the antenna  20  (which is not necessarily the same amount that is radiated) may be used to approximate the radiated power to simplify the monitoring. For example, the power amplifier  26  may provide a signal that is proportional to delivered power. The controller  34  may then compare this to the desired tuning frequency for the antenna  20  to determine a drift from the desired center frequency. The controller  34  may then adjust the capacitive load via the variable capacitive module  30  or  32 . The appropriate capacitance of the variable capacitive module  30  or  32  to produce the desired tuning of the antenna  20  may be calculated by the controller  34  or accomplished by, for example, a look-up table  42  within the controller  34 .  
         [0018]     Thus, as the antenna  20  is constantly detuned due to external factors such as repositioning of the host wireless device with respect to the surrounding environment, embodiments of the invention may continuously compensate in real time to keep the antenna  20  tuned to a desired center frequency.  
         [0019]     Still referring to  FIG. 4 , various variable capacitor schemes may be used. In one embodiment, the variable capacitor module  30  comprises a bank of high-Q capacitors,  50 ,  52 ,  54 , and  58  connected in parallel, each of which may have a different fixed capacitive value. Each of the capacitors  50 ,  52 ,  54 , and  58  may be switched on or off by a MEMS switch  60 ,  62 ,  64 ,  66 , or  68 , respectively. A MEMS switch may be preferred to a solid state-switch since solid state switches are generally non-linear devices which create undesirable frequency sidebands which can interfere with other wireless devices.  
         [0020]     As shown, the variable capacitor module  30  comprises a bank of five fixed capacitors  50 - 58  and associated MEMS switches  60 - 68 . The capacitors  50 - 58  may for example have values of 1 pF (picofarad) to 5 pF, respectively. By selecting one of more of the MEMS switches to close, a wide range of variable capacitance values may be realized to keep the antenna  20  tuned to a desired center frequency. This is of course by way of example only as more or less than five capacitors may be used and the capacitive value of each may comprise different values than those offered.  
         [0021]     In another embodiment, the variable capacitive module  32  may comprise a variable MEMS parallel plate capacitor  70  where one plate is made to move to change the capacitance value. Variations of suitable variable MEMS capacitors may be found with reference to U.S. Pat. Nos. 6,355,534 to Ma et al. and  6 , 593 , 672  to Cheng et al. As shown the variable capacitor  70  may comprises a fixed charge plate  72 , a movable charge plate  74  disposed above the fixed charge plate  72  by spacers  75 . A stiffener  76  may be affixed to the movable charge plate  74 . In operation, when an actuation voltage is applied to the variable MEMS capacitor, such as by the controller  34 , the moveable charge plate  72  is caused to flex in a downward direction, illustrated by dashed lines as movable charge plate  74 ′. In this manner the MEMS capacitor may produce a continuous range of variable capacitance values the proper value of which may be selected to tune the antenna  20  to the desired center frequency.  
         [0022]     According to embodiments of the invention, the antenna  20  may be switched to multiple desired center frequencies and thereafter continuously monitored and tuned to maintain the desired frequency to facilitate higher antenna efficiency. Power may be efficiently radiated under changing environmental conditions as opposed to being dissipated promoting longer battery life and improved range.  
         [0023]     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
         [0024]     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.