Wave transmission lines and networks – Coupling networks – Delay lines including long line elements
Reexamination Certificate
1999-08-26
2002-03-12
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Delay lines including long line elements
C333S164000, C342S368000
Reexamination Certificate
active
06356166
ABSTRACT:
BACKGROUND
It is often desirable to provide a means to phase shift a signal, such as a radio frequency (RF) carrier signal. For example, relative phase shifts between simulcast signals may be used to provide radiation pattern shaping and beam forming from an antenna. Moreover, it may be desirable to provide for adjusting or selecting such relative phase shifts in order to provide steerable antenna beams.
In a beam-forming architecture where the antenna is mechanically fixed, phase shift control allows radiation patterns to be created which can handle multiple simultaneous cellular phone calls, or improvements to radar systems which require spatially agile antennas. In order to make an antenna pattern electronically changeable, a phase shifter is, for example, often used. Such phase shifters are used to change the relative phase between the individual radiating elements of an antenna, while keeping frequency and amplitude fixed. The electronics that are suitable for phase shift control can fall into many different categories, depending on design parameters, signal requirements, and the like. Accordingly, adjustable phase shifter designs are realized through various technologies, each with its own performance advantages and disadvantages. For example, a ferrite core phase shifter requires a lot of space, is very heavy, and experiences hysteresis (a previous command must be erased before another command is written to the device). They also consume large amounts of DC power, but can handle high signal power.
A quadrature modulator is another effective way to change the phase of a signal, but such devices are very limited in terms of their input power-handling capability. Therefore, it exacerbates the design of the network around it, in order to control the input signal level, while accommodating a large variation after the modulator. Using a quadrature modulator to phase shift a signal requires applying DC voltages to its intermediate frequency (IF) inputs, but to do so requires learning this behavior, and then providing a lookup table of phase versus IF voltage in order to use the device. However, quadrature modulators are typically compact (often realized as integrated chip sets) and provide substantially unlimited phase resolution.
Another type of phase shifter, “loaded line,” uses selectable transmission lines to reactively load, i.e., with capacitance or inductance, to change the insertion phase of the device. These loaded line phase shifters can have high insertion loss—a significant portion of the input signal power is lost. They can also have low linearity, measured as a figure of merit called intercept performance, and therefore distort a signal even at low power levels. Accordingly, although loaded line phase shifters are generally compact, they are lossy, have poor dynamic range, are typically expensive, and employ high-risk designs.
Finally, there are switched-line phase shifters, which allow selection of one signal path or another, made of various lengths of transmission lines, which effectively changes the electrical length, and therefore the phase, of the composite path. Switched line phase shifters are generally limited to relatively narrow bandwidths because they utilize transmission line lengths designed for one particular frequency. As other frequencies are utilized therewith, the phase shifter will appear electrically longer or shorter. Dynamic range is limited by the linearity of the switching devices, and only quantized states are available. Also, they typically occupy large areas, and can have poor isolation. However, switched line phase shifters employ low risk designs that are inexpensive, have low loss, and provide adequate RF power handling.
From the above, it can be seen that there is a trade off between several designs, and operational factors, such as insertion loss, intercept performance (which controls linearity of the signal), bandwidth, and the like. Cost is also a factor, as is the number of separate phase shifters required and available space. For example, in a system which provides multiple narrow antenna beams in order to provide complete 360° coverage about a cellular base transceiver station, not only are multiple phase shifters required to form a single beam, but multiple sets are required to form multiple beams. Thus, factors such as cost and size can become critical,
Accordingly, there is a need in the art for providing phase shifters which provide a desired degree of phase resolution, with low insertion loss, high linearity, good dynamic range, low power consumption, small size, low risk, and low cost.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a system and method employing a phase shifter design which uses a switched line structure on a printed circuit board, or other support structure. Systems operating over a relatively narrow bandwidth, such as 869 to 894 MHz associated with the cellular transmit band for example, are very acceptable for the application of this form of phase shifter.
In order to provide fine resolution, i.e., an ability to provide for relatively small incremental changes in phase shift, over a broad range, a preferred embodiment of the present invention defines multiple switched line sections with increasingly smaller lengths. Preferably, the lengths are designed to be cardinal states of a digital control word. Accordingly the line lengths are successively divided in half, i.e., if the greatest line length in the switched line phase shifter is 180° (&lgr;/2, where &lgr; is the wavelength in the dielectric media of the design frequency), the next is 90° (&lgr;/4), the next is 45° (&lgr;/8), and the next is 22.5° (&lgr;/16), until the finest resolution desired is reached. Accordingly, if a specific phase shift is desired, a control system may be operated to activate, or command, certain sections of the phase shifter, corresponding to a specific digital control word, in order to achieve a net phase shift.
It should be appreciated that the above described phase shifter could require a substantial amount of space, such as a large surface area on a circuit board, in order to provide the line lengths necessary to achieve both the desired range of phase shift and a desired resolution. Moreover, there is the likelihood that placing a number of such switched lines, either from a single phase shifter of multiple segments or multiple such phase shifters, in near proximity, such as on a circuit board, may result in adjacent sections being electrically coupled to each other. In other words, poor signal isolation can be a result if steps are not taken to control this phenomenon.
The preferred embodiment phase shifter is small in order to take up less space on a circuit board, or other supporting structure, contrary to generally accepted practice. Additionally, the preferred embodiment phase shifter provides high linearity in order for it to handle higher power signals. Likewise, the phase shifter of the present invention preferably exhibits relatively low loss. Preferably, the phase shifter of the present invention provides phase shifting with low DC power consumption, which becomes very important in circumstances where a relatively large number of phase shifters are utilized, such as when many simultaneous antenna beams are generated.
In achieving the above described attributes, the phase shifter of a preferred embodiment of the present invention utilizes microstrip and/or stripline transmission lines of selected lengths to provide switched line lengths which are small in size, low loss, inexpensive, and provide good isolation of signals. A stripline is an RF transmission line disposed, or “sandwiched,” between two ground planes, most often within a dielectric media. Accordingly, striplines may be buried within the strata of a circuit board and, thus, can utilize the same area in one dimension (e.g., the x-y plane), while being separated in another dimension (e.g. the z-axis), to occupy a small amount of space per phase shifter. Of course, other structure for providing phas
Goldsmith Gary S.
Harvey Donn
Meredith Sheldon K.
Shafer Thomas
Fulbright & Jaworski L.L.P.
Lee Benny
Metawave Communications Corporation
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