Patent Application
20070139135
This invention relate to outdoor units used in microwave communications systems, such as millimeter wave (MMW) applications, and more particularly, to diplexers used in outdoor units.
The increased demand for high-speed, high data rate communications has created an immediate need for broadband access to the related network infrastructure. New applications include computer-to computer communications, gaming, and video-based services. Wireless solutions offer benefits in ease of deployment without the requirement of destroying streets to lay fiber. Wireless solutions also offer increased flexibility because new communication links can be added to the network as customers are added. Wireless solutions are also less expensive compared to optical fiber and hardwired solutions.
The use of millimeter wave (MMW) frequency bands allows wireless links to produce up to about an estimated one thousand times the data capacity of digital subscriber loop (DSL) or cable modem systems, and offer a higher bandwidth than available at lower operating frequencies. Currently, many terrestrial wireless systems are built using point-to-point, point-to-multipoint, Local Multipoint Distribution Services (LMDS), and mesh architectures. Each link end contains an indoor unit (IDU) and an outdoor unit (ODU). The indoor unit usually has a modem and a power supply. The outdoor unit, which represents about 60% of the cost of the link, typically contains a number of subassemblies, such as a millimeter wave transmitter and receiver or an integrated transceiver, a frequency synthesizer circuit, a power supply, a controller, and monitoring circuits.
One of the most challenging aspects of these types of wireless communication systems is to maintain the ability to operate in very specific frequency bands without interfering with adjacent bands. Although wide frequency bands (1 to 2 GHz) are available at millimeter wave frequencies, these bands typically are segmented and allocated in small channels of only a few megahertz (MHz). Regulatory rules, however, strictly dictate low interference levels from adjacent channels, and compliance with these high isolation requirements usually mandates the use of waveguide filters and a diplexer in any wireless system.
Waveguide filters and diplexers have been used extensively in communication systems to pass desired signals with low insertion loss, while rejecting unwanted signals. Because of the high level of segmentation of the frequency bands and the small channel width, dozens of different diplexers are typically required to cover each band. These waveguide filters and diplexers typically are long lead items and cost over $200 (two hundred dollars) in present day value, even when purchased in large quantity. It is not unusual for wireless radio manufactures to carry hundreds of waveguide filters/diplexers in inventory to cover an entire frequency band, thus allowing the manufacturer to react to changes in customer requirements in a reasonable time acceptable to a customer.
A diplexer includes a diplexer housing having transmit and receive waveguide channels formed therein. A cover is received on the diplexer housing over the transmit and receive waveguide channels. A septum is inserted between the diplexer housing and cover and configured to provide isolation between any transmitter and receiver signals and a desired frequency band of operation.
In yet another aspect of the present invention, the transmit and receive waveguide channels can each be serpentine configured. The diplexer housing can include opposing sides and the respective transmit and the receive waveguide channels are formed on the opposing sides. An end cover can be received over each opposing side. Each end cover can have a respective waveguide channel engaging the respective waveguide channel within the housing. A common waveguide port can interconnect the transmit and receive waveguide channels.
In yet another aspect of the present invention, the septum can be formed by forming resonators within the septum for imparting a desired frequency band of operation and isolation between transmitter and receiver signals. In yet another aspect, the waveguide channels can be formed as multiple segments, each representing “n” number of poles. The diplexer housing can be formed as a base plate and the cover can include transmit and receive waveguide ports formed therein and operative with the transmit and receive waveguide channels.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the invention which follows, when considered in light of the accompanying drawings in which:
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
In accordance with one non-limiting example of the present invention, a low cost diplexer covers very wide frequency bands and can be set to specific narrow segments by changing an insert inside a waveguide. Radio manufacturers can reduce their inventory and reduce the cost of a diplexer by at least a factor of four (4) in present day economic terms.
In addition, waveguide filters and diplexers in accordance with non-limiting examples are reduced in size by implementing a serpentine septum filter, without sacrificing functionality, performance or reliability. The reduction in size has many benefits, including easier integration into communication systems.
For purposes of explanation, a brief explanation will follow of waveguides and diplexers and related filters to better understand the embodiments of the invention as described. As known to those skilled in the art, a waveguide is a passive device that controls the propagation of an electromagnetic wave so that the wave is forced to follow a path defined by the physical structure of the guide. Waveguides typically take the form of rectangular hollow metal tubes.
Waveguide diplexers have been used extensively in wireless communication systems to isolate the output transmitter signal from the receiver input signal. The diplexer is a coupling device that permits two radio frequencies to share the same antenna. A typical waveguide diplexer shown in
The most commonly used waveguide filters and diplexers are of the cavity type, which requires manual tuning with screws to set them to the desired frequency. These filters require precision machining and extensive manual tuning to set the desired frequency response.
The length of this type of filter 50 is typically dictated by the amount of rejection and the number of required poles. To achieve high rejection with a sharp cut-off (>50 dB), 7 to 12 poles are required and the filter length could be several inches long. In addition to this large size, each filter requires precision machining and extensive manual tuning to set the filter frequency response. This tuning is achieved by adjusting the tuning screws 60, which are usually sealed to protect them when exposed to water or excessive humidity. The prevalent practice is to use an epoxy sealant over the screw locations after the tuning process.
The characteristics of this filter/diplexer 90 are determined by the number of cavities 94, 96 and their size. The cavities are typically precision machined. In addition to their high cost, cavity filters as illustrated provide no flexibility because each frequency segment requires unique cavity machining. Therefore, wireless radio manufacture must carry hundreds of waveguide filters/diplexers in inventory to cover wide frequency bands and be able to react to changes in customer requirements in a reasonable time. Here again, any filters/diplexers subjected to water or excessive humidity must also have their tuning screws sealed.
Another type of waveguide filter is the septum filter 120, shown in
As illustrated, this septum filter 120 includes a bottom half body member 122 and top half body member 124 and an insert 126 with resonators 128 formed in the insert. The insert 126 is positioned between top and bottom half body members 122, 124 in the final assembled filter 120 as shown in
E-plane septum filters as described have been used since the 1970's due to their low cost, low loss and suitability for mass manufacturing techniques. Standard manufacturing techniques for these filters involve machining the body members forming the waveguide halves and photo-etching the septum insert 126. It currently costs about $20 (twenty dollars) per filter in present day costs to machine the waveguide halves. These filters do not require any tuning screws, and instead use the illustrated type of inserts, which are typically made of copper, beryllium copper (BeCu), aluminum, or other similar materials to form the filter resonators. An insert, which is about two to about six mils thick, is sandwiched between the two halves of the waveguides formed by the body members. This type of low cost insert 132 replaces the expensive precision machining required for cavity filters and the extensive tuning required for iris coupled filters.
Although septum inserts have been used to create individual waveguide filters, they have not been used to create full diplexers. In one non-limiting example of the present invention, a septum is used with a low cost diplexer and provides radio manufacturers the flexibility required to cover segmented frequency bands without requiring extensive disassembly and assembly and an inventory of expensive parts. As used herein, a septum can refer a thin metal or similar material vane or similar planer material that has been perforated with an appropriate wave pattern. When inserted into a waveguide; it can act as a filter/diplexer.
The septum insert 216 includes rectangular configured openings 218 that are co-extensive with the transmit and receive waveguides 204, 206 when the septum insert is positioned on the diplexer base. A larger rectangular opening 220 is formed in the septum insert and received over the end portions of the transmit and receive waveguides 204, 206 and co-extensive with the transmit waveguide port 212 and receive waveguide port 214 formed in the diplexer cover 210. A common waveguide port opening 222 is formed in the septum insert and co-extensive with the area of the common waveguide port 208 on the diplexer base 207.
This non-limiting example of a diplexer as illustrated uses the septum insert to provide the required isolation between the transmitter and the receiver signals. The tuning screws used in a typical cavity diplexer have been eliminated. Without having to seal any tuning screws, this diplexer 200 can be assembled using standard dip-brazing or dip-soldering techniques to create an inherently weatherproof assembly. Any metal housing section forming the overall structure can be made up of a hollow waveguide with no machined cavities. The thin metal insert as the septum can be designed using electro-magnetic simulation tools such as High-Frequency Structure Simulation (HFSS), creating the filter resonators and providing the desired band segmentation. It should be understood that the septum insert can provide a filtering and a diplexer function. HFSS is adequate as a design tool because it typically uses a three-dimensional full wave finite element method (FEM) to compute the electrical behavior of high-frequency and high-speed components, such as waveguides. Engineers can extract parasitic parameters (S, Y, Z) in visualized three-dimensional near and far-field, electro-magnetic fields to generate broad band models and optimize design. Thus, a full-wave electrical characterization for radio frequency (RF), microwave and high-speed components such as waveguides, IC packages, connectors, printed circuit boards and antennas can be provided.
The thin metal inserts as septum inserts 216 can be fabricated using traditional chemical etching or stamping methods and cost under $5 (five dollars) in present day value. This allows radio manufacturers to stock low-cost, blank, waveguide diplexer housings and inserts for the different frequency segments. The diplexer frequency band can be selected at the last minute, just prior to shipment. Last minute changes to the diplexer requirements will not require retesting of the unit.
In order to achieve more flexibility in reacting to last minute changes in customer different diplexer demand, the diplexer 200 shown in
In applications where space is highly restricted, the septum inserts can be designed such that the filters and diplexers can be folded in a serpentine or similar configuration. A very compact design can be achieved in accordance with different embodiments of the present inventions.
As illustrated in
The septum diplexer and serpentine filters/diplexers as described can also be manufactured using die-casting for any waveguide halves. The potential exists for diplexers suitable for this application to be manufactured for about $5 (five dollars) in large quantity at present day values. Reduced manufacturing tolerance is possibly a tradeoff for the low cost manufacturing techniques. The standard tolerance on a machined waveguide half for an E-Plane filter is about ±2 mils. The waveguide halves can be die-cast with a tolerance of about ±3 mils.
A major manufacturing issue with die-casting is the necessity to have a draft angle on any waveguide halves to enable them to be easily removed from a mold after the molding process. A draft angle of at least about 2° is generally desirable for ease of manufacture, and the draft angle should be accounted for in the electrical design of the waveguide. When applying any draft angle to the waveguide, the new waveguide cross-section dimensions are set so that the cross-sectional area of the waveguide with draft angles is equal to the cross-sectional area of the standard waveguide. This gives the smallest mismatch when connecting a standard rectangular waveguide to a waveguide with a draft angle. The addition of a draft angle, however, to the waveguide also results in an increase in its wavelength and a decrease in its cut-off wavelength, the amount of each depending on the draft angle. A two degree draft angle corresponds to a 1.35% increase in the waveguide wavelength, and a 2.05% decrease in the cut-off wavelength. Adjustments should desirably be made to the septum insert design to compensate for these effects.
This design can offer several advantages. The low cost, septum waveguide diplexer as used for wireless communications systems can be made up of a metal housing, and are formed as a chemically etched, or stamped, metal insert, allowing flexible band pass selection. The septum diplexer can use a universal housing for frequency bands, within the waveguide cut-off frequency, and narrow frequency segments can be selected by interchanging inserts. The implementation of an external diplexer in wireless communication systems and outdoor units provides the ability to change diplexers without disassembling or retesting the outdoor unit, allowing frequency band adaptation in the field. Segmentation of filters through a serpentine septum waveguide filter/diplexer design reduces the waveguide filter/diplexer size without sacrificing performance or flexibility. Ease of manufacturing of the diplexer allows use of die-casting and implementing a low cost septum diplexer with meander-line filters. Dip-brazed septum waveguide diplexer assemblies are inherently weatherproof and do not require the extra measures currently used to seal any tuning screw locations.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.
