Direct digitally tunable microwave oscillators and filters

#This invention relates generally to a communications device and in particular to direct digitally tunable microwave oscillators and filters that enable a low-cost high-speed digital transceiver in the millimeter wave frequency range.

#Telecommunication deregulation and the growth of the Internet are causing a large demand for broadband communications network access to homes and offices at increasing bit rates. For Internet access, a Digital Subscriber Loop (DSL) service and similar techniques are becoming popular. However, the speed of the DSL service is limited by the fact that the data travels over long telephone twisted copper wire pairs. Cable modems are also becoming prevalent methods of Internet and telephony access. However, the return channel for a cable modem is quite limited in speed by the nature of cable distribution technology. Fiber optics offers higher access speeds than the other systems and services, but fiber does not currently reach most homes and offices and installation of new fiber is very expensive.

#To overcome these limitations with current systems, service providers and users are considering using digital wireless communications as a possible alternative access system. Various radio frequency bands are in fact currently allocated to applications that include broadband wireless access, however, most of the available bandwidth is in the millimeter wave range, ranging roughly from 15 to 66 GHz. Among these bands, the USA Local Multipoint Distributed Services (LMDS) bands in the 28 to 30 GHz range are a good example of a licensed band intended for telecom access applications. Since LMDS is a licensed band, the license owner has the exclusive rights to use the band for such services, thus ensuring interference-free operation.

#FIG. 1 is a block diagram of a typical radio access terminal including a radio transceiver module portion;

#FIG. 2 is a block diagram of a typical radio access terminal divided into indoor and outdoor units with a digital interface between them;

#FIG. 3 is a block diagram illustrating an example of a typical radio transceiver module;

#The invention is particularly applicable to digitally controlled oscillators and filters for use in a microwave transceiver or microwave access terminal and it is in this context that the invention will be described. It will be appreciated, however, that the device and method in accordance with the invention has greater utility, such as to other communications systems that require a tuned oscillator or filter. Before describing the invention, a typical microwave subscriber terminal that may include a digitally controlled filter or oscillator in accordance with the invention will be described.

#A typical digital microwave transceiver 100 for a subscriber terminal in a fixed wireless network is shown as a block diagram in FIG. 1. In particular, FIG. 1 shows an integral outdoor transceiver 100 while FIG. 2 shows a split transceiver 200, including an indoor unit 210, an outdoor unit 220 and a connecting cable 201 between them. The integral unit of FIG. 1 includes a user interface 101 for interfacing between the communications system and the microwave transmission and reception system and converting user traffic to a serial bitstream for transmission over the air, an air-interface media access control layer (MAC) 102 if applicable for the wireless network in use, a system controller 103, for configuration, protocol processing and network management tasks and a antenna 110 for receiving/transmitting the microwave signals. The transceiver-related functions more directly related to the present invention are depicted inside a block 111 having a gray background. These functions are included in a modem 104 for converting the serial bit stream into modulated signals and vice versa, an up/down frequency converter 105 for converting between different frequencies, a synthesizer 106 for setting the proper radio frequency for communication and a millimeter wave front-end 107 that performs the higher frequency transceiver functions. The control signals between these units and the system controller are shown although not described since these are well known. The functions depicted in FIG. 1 are well known and typical for a microwave transceiver and will not be described here in any more detail. The separation of the transmit-receive functions, the number of conversion stages, the DC power supply and other radio-related features are omitted from the figure since these are also well known in the art.

#FIG. 2 illustrates a split transceiver 200 including an indoor unit 210 and an outdoor unit 220. The split transceiver of FIG. 2 includes the same blocks as the transceiver shown in FIG. 1 and further includes an indoor to outdoor connection medium 201 that may be a coaxial cable, a fiber optic cable, a twisted pair cable or combinations of such media. An integral transceiver is likely to be more cost-effective, however various constraints in the field, such as distance from the user to the outdoor unit, may necessitate a split implementation. In a split transceiver, one typical partition between the indoor unit and the outdoor unit is to keep the modem indoors and use intermediate frequency (IF) signals for indoor to outdoor transmission. A lower cost typical approach is shown in FIG. 2 in which the modem 104 is outdoors and the media 201 transmits multiplexed digital information to the modem using a digital multiplexer 203 and a media physical layer driver (Media PHY) 204. The multiplexed signal contains the data, timing reference and control information. For example, a transmission may consist of a start-stop protocol as used in a serial port of a UART. The start bit indicates when a transmission in the air should start. A media PHY driver 207 and multiplexer 205 in the outdoor unit 220 can interpret the stop bit and control the modem 104 via a transmit control bus 206. The media PHY driver can be an off the shelf transceiver for the appropriate medium, such as 100BaseT driver for twisted pair cabling. Now, more details of the portion of the transceiver in the gray background will be described.