Patent Grant
7321276
The demand for smaller and lower cost components for consumer electronics has increasingly led to efforts to reduce the sizes of various microwave components. An example of such a component is a microwave directional coupler utilized in wireless terminals for monitoring transmitted power. In such applications, size and weight may be critical parameters.
A conventional microwave directional coupler utilizes two 50 ohm transmission lines, each having an electrical length of one quarter wavelength at the operating frequency. The spacing between the transmission lines is selected to provide the desired electromagnetic coupling. At an operating frequency of 1.95 GHz, the length of a conventional microstrip directional coupler is 19 millimeters (mm). This dimension is large in proportion to the overall package size of typical wireless terminals.
A directional coupler is a passive device which couples part of the transmission power by a known amount out through another port, often by using two transmission lines set close enough together such that energy passing through one is coupled to the other. The term “main line” refers to the main transmission line. On some directional couplers, the main line is designed for high power operation (large connectors), while the coupled port may use a small SMA (SubMiniature version A) connector. Usually the isolated port is terminated with an internal or external matched load (typically 50 ohms).
Physical considerations such as an internal load on the isolated port will limit port operation. The coupled output from the directional coupler can be used to obtain the information (i.e., frequency and power level) on the signal without interrupting the main power flow in the system. It should be recognized that the coupled response is periodic with frequency. For example, a ¼ coupled line coupler will have responses at n/4 where n is an odd integer.
Common properties desired for all directional couplers are wide operational bandwidth, high directivity, and a good impedance match at all ports when the other ports are terminated in matched loads.
Microstrip directional couplers having a capacitor or other reactive element connected between the two transmission lines are disclosed in U.S. Pat. Nos. 4,216,446 and 5,159,298. The capacitor or other reactive element is stated to improve the directivity of the directional coupler.
A directional coupler having a capacitor connected between transmission lines and shunt capacitors connected between each transmission line and ground is disclosed in U.S. Pat. No. 5,243,305. The capacitors are connected at the center of the transmission lines and are stated to increase the directivity of the directional coupler.
A capacitively compensated microstrip directional coupler is disclosed in U.S. Pat. No. 4,999,593. Reactive coupling networks are coupled between the transmission lines of the directional coupler at each end. Each reactive coupling network includes a first capacitor coupled between a common node and the first transmission line, a second capacitor coupled between the common node and the second transmission line, and a third capacitor coupled between the common node and ground. This interconnection however eliminates the independence between the transmission lines.
All known prior art microwave directional couplers have had one or more drawbacks, including but not limited to unacceptable physical size and a large number of compensation components. Accordingly, there is a need for improved microwave directional couplers.
Directional couplers as disclosed above are a well known element for radio frequency equipment. The directional coupler (a.k.a. a power sampler) allows a sample of a radio frequency signal, which is input at an input terminal and output at an output terminal, to be extracted from the input signal. Properly designed, the directional coupler can distinguish between a signal input at the input terminal and a signal input at the output terminal. This characteristic is of particular use in a radio frequency transmitter in which both the input signal and a signal which is reflected from a mismatched antenna can be independently monitored. One or the other or both of these signals can be utilized in a power control circuit to control the output power of the transmitter.
Another element well known in the output circuit of a transmitter is a harmonic filter, which is employed to reduce the energy coupled to an antenna at harmonic frequencies of the desired output signal. In a system which consists of a transmitter coupled to an antenna, the harmonic filter can be a relatively simple low pass filter. In a system where the transmitter must share the same antenna with other equipment, e.g., a companion receiver, the harmonic filter may take on a somewhat more complex configuration. For example, a bandpass filter which passes only a relatively narrow band of frequencies at which the transmitter is designed to operate while rejecting all other frequencies has been used in critical applications such as cellular radiotelephones. In order to achieve the lowest insertion loss within the smallest practical size, frequency resonant structures such as helical or coaxial resonators have been the choice of radio equipment designers. Unfortunately, resonant structures experience a reduction in their attenuation characteristics at frequencies which are approximately odd order harmonics of the passband frequency. Such a response is known as flyback. In order to overcome the flyback response, equipment designers have placed additional filtering in series with the resonant structure bandpass filter. One example of this additional filtering may be found in U.S. Pat. No. 5,023,866.
A radio equipment designer wishing to design high performance radio equipment may elect to employ a directional coupler, a resonant structure bandpass filter and an odd order harmonic flyback filter, but heretofore, has been constrained to use conventionally realized individual circuit elements. Such a configuration, with individual circuit elements, can experience potentially higher failure rates and dramatically increased size and cost of equipment.
Other prior art solutions lack the versatility desired by radio equipment designers and operators.
As illustrated in
As illustrated in
In view of the deficiencies of the prior art, it is an object of the present subject matter to obviate these deficiencies by presenting an independently tunable, combined coupler and filter.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.
The present subject matter is advantageously used in modules in which functions are incorporated sequentially; that is, in a line, one after another. The sequential arrangement of these functions consumes much physical room in the module. These microwave modules tend to be long and narrow, and there is often not enough room for multiple functions in a linear row. This present subject matter combines two functions-filtering and coupling (power sampling) which are usually done sequentially, into one structure along with the ability to independently tune the coupler and the filter. Therefore, less room is required, especially along the length of the module.
Additionally, the filter and coupler of the present subject matter are tunable based on frequency, insertion loss, coupling factor and/or return loss, not just frequency.
Microwave power amplifiers often have a harmonic filter followed by a coupler for power sampling on the output. In modules, multiple functions are generally realized sequentially. The present subject matter by combining two functions into one, minimizes the physical room required, and the module can be made shorter with the same functionality. Also the combined functionality provides for improved specifications over either function alone; i.e., it can be considered a sampling power detector which also reduces harmonic content or a harmonic filter which also provides for power sampling.
The present subject matter works by realizing a low pass harmonic stub-type filter. Power sampling couplers are usually at least 10 dB down from the sampled signal, and sampling this level of power from the filter structure interacts with the filter minimally. The coupled line section is placed on the side of the filter opposite to that of the stubs, such that it does not load or otherwise interfere with the filter substantially.
The directional coupling or power sampling portion 311 of the combined directional coupler and filter has an output end and a terminal end. The power sampling portion 311 is substantially parallel to and laterally spaced with the filter portion 302. The coupling portion 301 is in electromagnetic connection with the filter portion 302 as necessary to extract a portion of the signal. The sampling portion 311 is located opposite the stubs of the filter, as shown in
Additionally, a variable adjustable reactance circuit 441 may be placed on the terminal end of the sampling portion 301 and connected to ground 450. The adjustable reactance circuit 441 may include resistors, varactors and other components that allow for changing the terminal impedance of the coupler. This adjustment is independent of the adjustments made to the filter portion 302.
In addition, the combined filter and power sampler can be tuned based upon harmonic rejection, insertion loss, return loss and coupling factor instead of only frequency as shown in the prior art.
In block 603, a plurality of varactors are connected between the low pass filter and ground, and in block 605 a variable reactance circuit between the power sampler and ground which serves to terminate one end of the coupler. The varactors associated with the low pass filter base are then adjusted based on harmonic rejection, insertion loss and return loss and the variable reactance circuit of the power sampler is independently adjusted based on desired characteristics such as coupling factor and insertion loss as shown in block 607.
Harmonic rejection of a representative embodiment of the present subject matter is graphically illustrated in
Insertion loss is the loss in signal due to the filter and or coupler existing in the circuit, whereas return loss is the attenuation of a reflected signal in proportion to the forward signal. The insertion loss of an embodiment of the adjustable combined directional coupler and filter are shown in
While preferred embodiments of the-present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.
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| Number | Date | Country | |
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