Patent Publication Number: US-7218187-B2

Title: Bow tie coupler

Description:
This is a continuation of U.S. patent application Ser. No. 10/797,492, filed Mar. 10, 2004. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to radio frequency test equipment, and more particularly, is directed to a coupler for use in a test enclosure and for coupling to test equipment to enable wireless communication with a device under test. 
   Wireless communication equipment are subject to various standards relating to wireless transmission, including but not limited to power emissions standards and interference standards. The four main cellular frequency bands cover 824 to 960 MHz and 1710 to 1990 MHz. Bluetooth, Wireless LAN (WLAN) and/or global positioning system (GPS) functionality is being added to many wireless products; the center frequencies of these systems are 2450 MHz and 1575 MHz respectively. Such wireless devices, e.g., cellphones, personal digital assistants (PDAs) and smart phones, must be tested prior to sale, to ensure they comply with appropriate standards, and in general, function properly. 
     FIG. 1A  shows a typical radio frequency (RF) testing enclosure. A device under test is placed in an enclosure that contains a coupler for wirelessly coupling between test equipment and the device under test. 
   Conventional couplers are designed to operate over specific frequency bands. Accordingly, when testing a device designed to operate at several frequency bands, the testing procedure must include switching the coupler for each of the frequency bands being tested. The need to switch between different couplers to test the same device decreases the reliability and repeatability of tests, increases the cost of testing, increases the difficulty of calibrating the tests, and increases the test time. 
   Thus, there is a need for a wide bandwidth RF coupler, operating in several frequency bands. 
   SUMMARY OF THE INVENTION 
   In accordance with an aspect of this invention, there is provided a coupler comprising a first element having a rectangular portion and a tapered portion with a nose, a second element having a rectangular portion and a tapered portion with a nose, a third element disposed between the nose of the first element and the nose of the second element, a matching network for electrically connecting the first, second and third elements. 
   In accordance with another aspect of this invention, there is provided a bow tie coupler comprising a first element having a tapered nose portion for connecting to a first portion of a signal feed structure, a second element having a tapered nose portion, a third element for connecting to a second portion of the signal feed structure, the third element located between the tapered nose portions of the first and second elements, and a matching network for electrically connecting the first, second and third elements. 
   In accordance with a further aspect of this invention, there is provided a coupler for use in a radio frequency test chamber, comprising a first element having a tapered nose portion for connecting to a first portion of a signal feed structure, a second element having a tapered nose portion, a third element for connecting to a second portion of the signal feed structure, and a matching network for electrically connecting the first, second and third elements. 
   It is not intended that the invention be summarized here in its entirety. Rather, further features, aspects and advantages of the invention are set forth in or are apparent from the following description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a typical RF enclosure; 
       FIG. 1B  is a block diagram showing hand-held mobile communication device  1 ; 
       FIG. 1C  shows a conventional bow tie antenna; 
       FIGS. 2A–2B  show views of a bow tie coupler according to an embodiment of the present invention; 
       FIGS. 3A–3F  show measured radiation patterns of the coupler of  FIG. 2A ; 
       FIG. 4  is a graph showing the Voltage Standing Wave Ratio (VSWR) for the coupler of  FIG. 2A ; 
       FIG. 5  is a graph showing the VSWRs for several couplers; 
       FIG. 6  is a graph showing the antenna gain for several couplers; and 
       FIG. 7  is a diagram of another bow tie coupler according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1B  shows hand-held mobile communications device  1 , which is an example of a device that may be tested in the enclosure of  FIG. 1A . 
     FIG. 1B  shows the conventional operating environment of device  1 . Hand-held mobile communication device  1  includes a housing, a keyboard  14  and an output device  16 . The output device shown is a display  16 , which is preferably a full graphic LCD. Other types of output devices may alternatively be utilized. A processing device  18 , which is shown schematically in  FIG. 1B , is contained within the housing and is coupled between the keyboard  14  and the display  16 . The processing device  18  controls the operation of the display  16 , as well as the overall operation of the mobile device  1 , in response to actuation of keys on the keyboard  14  by the user. 
   The housing may be elongated vertically, or may take on other sizes and shapes (including clamshell housing structures). The keyboard may include a mode selection key, or other hardware or software for switching between text entry and telephony entry. 
   In addition to the processing device  18 , other parts of the mobile device  1  are shown schematically in  FIG. 41 . These include a communications subsystem  100 ; a short-range communications subsystem; the keyboard  14  and the display  16 , along with other input/output devices  106 ,  108 ,  11  and  112 ; as well as memory devices  116 ,  118  and various other device subsystems  120 . The mobile device  1  is preferably a two-way RF communication device having voice and data communication capabilities. In addition, the mobile device  1  preferably has the capability to communicate with other computer systems via the Internet. 
   Operating system software executed by the processing device  18  is preferably stored in a persistent store, such as a flash memory  116 , but may be stored in other types of memory devices, such as a read only memory (ROM) or similar storage element. In addition, system software, specific device applications, or parts thereof, may be temporarily loaded into a volatile store, such as a random access memory (RAM)  118 . Communication signals received by the mobile device may also be stored to the RAM  118 . 
   The processing device  18 , in addition to its operating system functions, enables execution of software applications  130 A– 130 N on the device  1 . A predetermined set of applications that control basic device operations, such as data and voice communications  130 A and  130 B, may be installed on the device  1  during manufacture. In addition, a personal information manager (PIM) application may be installed during manufacture. The PIM is preferably capable of organizing and managing data items, such as e-mail, calendar events, voice mails, appointments, and task items. The PIM application is also preferably capable of sending and receiving data items via a wireless network  140 . Preferably, the PIM data items are seamlessly integrated, synchronized and updated via the wireless network  140  with the device user&#39;s corresponding data items stored or associated with a host computer system. Communication functions, including data and voice communications, are performed through the communication subsystem  100 , and possibly through the short-range communications subsystem. The communication subsystem  100  includes a receiver  150 , a transmitter  152 , and one or more antennas  154  and  156 . In addition, the communication subsystem  100  also includes a processing module, such as a digital signal processor (DSP)  158 , and local oscillators (LOs)  160 . The specific design and implementation of the communication subsystem  100  is dependent upon the communication network in which the mobile device  1  is intended to operate. For example, a mobile device  1  may include a communication subsystem  100  designed to operate with the Mobitex™, Data TAC™ or General Packet Radio Service (GPRS) mobile data communication networks and also designed to operate with any of a variety of voice communication networks, such as AMPS, TDMA, CDMA, PCS, GSM, etc. Other types of data and voice networks, both separate and integrated, may also be utilized with the mobile device  1 . 
   Network access requirements vary depending upon the type of communication system. For example, in the Mobitex and DataTAC networks, mobile devices are registered on the network using a unique personal identification number or PIN associated with each device. In GPRS networks, however, network access is associated with a subscriber or user of a device. A GPRS device therefore requires a subscriber identity module, commonly referred to as a SIM card, in order to operate on a GPRS network. 
   When required network registration or activation procedures have been completed, the mobile device  1  may send and receive communication signals over the communication network  140 . Signals received from the communication network  140  by the antenna  154  are routed to the receiver  150 , which provides for signal amplification, frequency down conversion, filtering, channel selection, etc., and may also provide analog to digital conversion. Analog-to-digital conversion of the received signal allows the DSP  158  to perform more complex communication functions, such as demodulation and decoding. In a similar manner, signals to be transmitted to the network  140  are processed (e.g. modulated and encoded) by the DSP  158  and are then provided to the transmitter  152  for digital to analog conversion, frequency up conversion, filtering, amplification and transmission to the communication network  140  (or networks) via the antenna  156 . 
   In addition to processing communication signals, the DSP  158  provides for control of the receiver  150  and the transmitter  152 . For example, gains applied to communication signals in the receiver  150  and transmitter  152  may be adaptively controlled through automatic gain control algorithms implemented in the DSP  158 . 
   In a data communication mode, a received signal, such as a text message or web page download, is processed by the communication subsystem  100  and is input to the processing device  18 . The received signal is then further processed by the processing device  18  for an output to the display  16 , or alternatively to some other auxiliary I/O device  106 . A device user may also compose data items, such as e-mail messages, using the keyboard  14  and/or some other auxiliary I/O device  106 , such as a touchpad, a rocker switch, a thumb-wheel, or some other type of input device. The composed data items may then be transmitted over the communication network  140  via the communication subsystem  100 . 
   In a voice communication mode, overall operation of the device is substantially similar to the data communication mode, except that received signals are output to a speaker  110 , and signals for transmission are generated by a microphone  112 . Alternative voice or audio I/O subsystems, such as a voice message recording subsystem, may also be implemented on the device  1 . In addition, the display  16  may also be utilized in voice communication mode, for example to display the identity of a calling party, the duration of a voice call, or other voice call related information. 
   The short-range communications subsystem enables communication between the mobile device  1  and other proximate systems or devices, which need not necessarily be similar devices. For example, the short-range communications subsystem may include an infrared device and associated circuits and components, or a Bluetooth™ communication module to provide for communication with similarly-enabled systems and devices. 
   The frequency bands of interest for cellular and smart phones are: 850 MHz GSM (824–894 MHz), 900 MHz GSM (880–960 MHz), GPS (1575.42 MHz), DCS (1710–1880 MHz), PCS (1850–1990 MHz), and WLAN (2400–2484 MHz). 
   A wide bandwidth coupler has two outer elements around a center element. The outer elements are rectangular at their outside portions and each have a tapered nose portion next to the center element. A matching network electrically connects the two outer elements and the center element. 
   The coupler exhibits better than 2:1 Voltage Standing Wave Ratio (VSWR), stable antenna gain characteristics and a dipole-like radiation pattern over a wide frequency range. In one embodiment of the present invention, the coupler exhibits the above characteristics over a frequency range of 824 to 2484 MHz, that is, all of the frequency bands for cellular and smart phones. Over each frequency band the coupler has very stable antenna gain. These characteristics minimize system error and thus maximize device failure detection during testing. The coupler can be etched easily on printed circuit board material. The wide bandwidth coupler is useful in an RF testing enclosure. 
   The wide bandwidth coupler eliminates the test time needed to switch the coupler of an RF test chamber, and reduces calibration time. Additionally, the wide bandwidth coupler enables simultaneous testing of multiple bandwidths, and improves the reliability and repeatability of test measurements. 
   Since couplers wear out sooner if they are switched frequently, the present wide bandwidth coupler should last longer as it will need to be switched less often. 
     FIG. 1C  shows an example of a conventional bow tie antenna having two triangular portions and a signal feed structure connected to the inner vertices of the triangular portions. With the inner vertices having 60° angles, the conventional bow tie antenna could provide a voltage standing wave ratio (VSWR)&lt;2 over a bandwidth of 30% to 40% of the center frequency, when its length L=0.8λ at the center frequency, where λ is the wavelength of a signal being transmitted or received. 
     FIG. 2A  shows bow tie coupler  10  according to an embodiment of the present invention. Small element  50  is disposed between medium element  20  and large element  30 . Matching network  40  electrically connects small element  50 , medium element  20  and large element  30 . 
   In one embodiment, bow tie coupler  10  is located on a printed circuit board (PCB) RF substrate, such as a FR4 substrate, with no ground plane opposing the coupler. The elements of bow tie coupler  10  are created on the PCB using a board milling machine or by an etching method. Other methods of manufacturing bow tie coupler  10  will be apparent to those of ordinary skill in the art. 
   Small element  50  is coupled to the center pin (not shown) of a signal feed structure, such as a coaxial cable or microstrip line, connected to test equipment. Other suitable signal feed structures will be apparent to those of ordinary skill in the art. Small element  50  has a square shape. 
   Medium element  20  is coupled to the outer sleeve (not shown) of the coaxial cable connected to the test equipment, that is, the signal ground. Medium element  20  has length len 20 . Medium element  20  has an outer rectangular portion and an inner tapered portion. Sides  23  and  24  taper to edge  22 , forming a tapered nose portion. 
   Bow tie coupler  10  wirelessly receives and transmits with the device under test (not shown), that is, acts as an antenna for converting electromagnetic energy to electrical energy and vice versa. Large element  30  has length len 30 . Generally, len 30  is greater than or equal to len 20 , with the specific length values chosen in view of the signal frequency range and/or center frequency. However, len 30  and len 20  may be the same in some embodiments. In one embodiment, len 20  is about 20 mm and len 30  is about 40 mm. Large element  30  has an outer rectangular portion and an inner tapered portion. Sides  33  and  34  taper to edge  32 , forming a tapered nose portion. 
   Large element  30  has arm  35  which serves to extend element  30  closer to element  20 , thereby making it easier to connect matching network  40  between elements  20  and  30 . 
   Matching network  40  comprises matching components  41 ,  42  and  43 . Component  41  electrically connects medium element  20  and small element  50 . Component  42  electrically connects medium element  20  and large element  30 . Component  43  electrically connects small element  50  and large element  30 . 
   In one embodiment, components  41  and  42  are each a resistor having a resistance of about 190 ohms, and component  43  is an inductor having an inductance of about 1.2 nH. In another embodiment, components  41 – 43  are each resistors, while in a further embodiment, components  41 – 43  are each inductors. Other configurations of matching network  40  will be apparent to one of ordinary skill in the art, and may be comprised of combinations of resistors, capacitors and inductors. 
     FIG. 2B  shows a three-dimensional view of bow tie coupler  10 . 
     FIGS. 3A–3F  show the radiation patterns of an exemplary bow tie coupler  10 , in the E-plane (y-z plane of  FIG. 2B ) and the H-plane (x-y plane of  FIG. 2B ), measured in a 20 meter tapered anechoic chamber for various transmit frequencies. The radiation patterns at all of the frequency bands are seen to be dipole-like with good omni-directional H-plane characteristics. 
     FIG. 3A  is for the GSM850 system frequency of 839.6 MHz. 
     FIG. 3B  is for the GSM900 system frequency of 902.4 MHz. 
     FIG. 3C  is for the DCS system frequency of 47.8 MHz. 
     FIG. 3D  is for the PCS system frequency of 1880 MHz. 
     FIG. 3E  is for the GPS system frequency of 1575.42 MHz. 
     FIG. 3F  is for the wireless LAN system frequency of 2450 MHz. 
     FIG. 4  is a graph showing the measured VSWR for the exemplary bow tie coupler  10 , measured using an Agilent 8753E vector network analyzer. It can be seen that over the frequency range of at least 600 to 2600 MHz the coupler exhibits a substantially flat VSWR curve having a max-min variation of less than 1 and a VSWR better than 2:1. 
   It will be recalled that a VSWR of 2:1 corresponds to 90% of the input power being converted to output power, and is the RF standard for couplers. A VSWR of 1:1 corresponds to 100% of input power being converted to output power. 
   Ideally, the VSWR should be better than 2:1 over the entire frequency range of interest. 
     FIG. 5  is a graph showing the VSWRs for a conventional bow tie antenna, such as shown in  FIG. 1C  (dash-dot line), a commercially popular coupler (not shown) (dotted line), and bow tie coupler  10  according to the present invention (solid line). The commercially popular coupler has poor VSWR performance in that its VSWR varies from about 27:1 to close to 1:1 and is not flat. The conventional bow tie antenna has a VSWR varying from about 8:1 to close to 1:. By contrast, bow tie coupler  10  has a VSWR that is generally flat and is better than 2:1. 
     FIG. 6  is a graph showing the antenna gain for a conventional bow tie antenna, such as shown in  FIG. 1C  (dash-dot line), a commercially popular coupler (not shown) (dotted line), and bow tie coupler  10  according to the present invention (solid line). Ideally, the antenna gain should be flat over the entire bandwidth of interest. The commercially popular coupler has a triangular gain curve from about 1700–2400 MHz that has an antenna gain variation (max-min) of about 5 dB. The conventional bow tie antenna has a linearly sloped curve from about 900–1700 MHz with an antenna gain range of about 9 dB. By contrast, bow tie coupler  10  has a generally flat antenna gain curve from about 800–2500 MHz with an antenna gain range of only about 2.5 dB. 
   An alternate embodiment is shown in  FIG. 7 , a diagram of bow tie coupler  11 , which is generally similar to bow tie coupler  10 . For brevity, only the differences will be discussed. 
   The tapered edges of the noses of medium element  21  and large element  31  of bow tie coupler have a curved or exponential shape, instead of being straight edges as in bow tie coupler  10 . Small element  31  of bow tie coupler  11  has a circular shape. 
   Although an illustrative embodiment of the present invention, and various modifications thereof, have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to this precise embodiment and the described modifications, and that various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.