Wave transmission lines and networks – Automatically controlled systems – Impedance matching
Reexamination Certificate
2001-05-29
2003-05-27
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Automatically controlled systems
Impedance matching
C333S150000, C455S121000, C455S123000
Reexamination Certificate
active
06570462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to the field of antennas used in wireless communication devices and particularly to adaptive tuning of such antennas.
2. Description of the Prior Art
There has been a proliferation in recent years in the field of wireless telecommunications. Items such as cordless and cellular telephones, pagers, wireless modems, wireless email devices, personal digital assistants (PDAs) with communication functions, and other mobile communication devices are becoming commonplace, particularly among individuals who need to be quickly contacted from remote locations. With such devices, it is very important to maintain a clear, strong signal that preserves the integrity of the transmission.
The antennas used with previous wireless communication devices are prone to many significant problems. Some devices, such as pagers and cellular telephones, are usually worn on the person of the user. However, the human body has certain inherent dielectric properties that create an electromagnetic boundary. The boundary conditions of the body of the user change the surrounding impedance, affecting the antenna current distribution and the signal radiation pattern, thus lowering the gain of the antenna by about 4 dB. In this way, the antenna is “detuned”. Antenna detuning may also be caused by the presence of certain objects such as metallic bodies and also various ground plane conditions. This effect results in a shorter operating radius and poor in-building performance for some wireless communications devices.
Such boundary effects can be particularly pronounced in modern mobile communication device designs, in which embedded antennas are common. A signal transmitted from or received by an antenna which is integrated into a communication device may encounter several boundaries, including for example a printed circuit board, a battery, a display screen, a device housing, a device carrying case and any of a multitude of other elements or components associated with the device, in addition to a user's body. All such boundaries affect signal propagation and the surrounding impedance seen by the antenna.
Previous devices also suffer from performance problems related to the polarization characteristics of transmission and reception signals. Electromagnetic radiation propagates in any plane and can thus be regarded as having vertical and horizontal polarizations. In order to receive a strong signal, an antenna must be property aligned with the polarization plane of the incoming signal. However, when a mobile device is in operation, it may be turned in all different directions and may not be optimally aligned to receive an incoming signal. In a two-way device, transmissions from the device can be affected by a similar problem. Known device antennas incorporate a loop design, which is nominally effective at implementing the two polarizations but suffers from low gain and low bandwidth. Boundary sources also affect the reception of a polarized signal.
At least a portion of the signal power losses associated with antennas in wireless devices is due to signal reflection. Ideally, all of the signal power of a signal input to an antenna should be converted into an output signal. In reference to
FIG. 1
, all of the power in a signal generated by a communication unit
12
in a wireless communication device
10
and input to an antenna
16
through a line
14
would ideally be radiated out into the air by antenna
16
. A communication signal received over the air by antenna
16
would similarly be converted into a received signal and input to the communication unit
12
. However, in reality the characteristic impedance of the communication unit
12
and line
14
interacts with the characteristic impedance of the antenna
16
. Unless these impedances are equal, some signal reflection will occur at the interface between line
14
and antenna
16
.
One known method of addressing the above-noted problems is to provide an impedance matching circuit between communication circuitry and an antenna, as shown in FIG.
2
. Communication device
20
is substantially the same as device
10
, but includes impedance matching circuit
24
in line
14
. As will be apparent to those skilled in the art to which the invention pertains, impedance matching circuit
24
will normally be an LC circuit with inductance and capacitance elements connected in any one of a number of standard matching circuit topologies. On the communication unit side of line
14
, characteristic impedance is relatively easily determined in accordance with known techniques. For example, line
14
may be a coaxial cable having a standard characteristic impedance of 50 &OHgr;, in which case the impedance matching circuit
24
would be designed and implemented such that the overall impedance of the matching circuit
24
, in conjunction with the characteristic impedance of the antenna
16
, is also 50 &OHgr;.
A major problem with impedance matching in mobile or other wireless communication systems is that impedance matching circuits are normally calibrated during device manufacture and do not normally provide for adjustments in the field, whereas surrounding impedance affecting antennas is rarely constant. In the example receiver
20
, over the air signals transmitted and received by antenna
16
may encounter such dielectric boundaries as the housing of device
20
, printed circuit boards on which the communication unit
12
is built, electronic components in the communication unit
12
, batteries for powering the device
20
, display
18
, input device
22
and the body of a device user, all of which will affect the impedance seen by the antenna
16
. Such impedances can be estimated, but are dependent upon the orientation of the device with respect to its surroundings. Thus, even the best estimates of impedance matching requirements cannot possibly remain accurate for all device operating conditions.
Another known technique intended to compensate for signal reflection effects is shown in FIG.
3
. The device
30
is similar to devices
10
and
20
, but includes signal power measurement and amplifier control arrangements in addition to the impedance matching circuit
24
. The arrangement shown in
FIG. 3
is normally used only in a transmit signal path, as indicated by the illustrated unidirectional connections between components. In
FIG. 3
, a signal generated in communication unit
12
for transmission from device
30
is amplified by power amplifier
26
and then fed to directional power coupler
28
. The transmission signal is split between the impedance matching circuit
24
and a termination
34
. A reflected signal induced by the combination of antenna
16
and matching circuit
24
is then fed back to a signal power measurement unit
32
, which develops an amplifier control signal. Such conventional arrangements measure only signal magnitude and are not designed or intended to determine signal phase. Since effective antenna and surrounding impedances are dependent upon orientation of the antenna, phase information can be important for accurate impedance matching. Furthermore, instead of correcting the underlying problem causing signal power losses, these conventional systems merely boost signal power so that the signal losses can be tolerated.
Therefore, there remains a need for an improved impedance matching arrangement and technique. According to the invention, surface acoustic wave (SAW) technology is used to determine the magnitude and phase of reflected signals and thereby impedance magnitude and phase in a wireless communication device. The determined magnitude and phase are then used to develop control signals which are advantageously used to adjust components in an impedance matching circuit to thereby provide adaptive tuning in a communication device.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved impedance matching method and system which provide for an increased operating radius for a wireless communication device.
A related ob
Campbell Colin K.
Edmonson Peter J.
Jones Day
Meyer, Esq. Charles B.
Pathiyal, Esq. Krishna K.
Research In Motion Limited
Summons Barbara
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