RECONFIGURABLE NODE FOR A COMMUNICATIONS NETWORK

Abstract: A node for a communications system includes a first circuit board including a first amplifier for amplifying a first signal and a second circuit board removably mounted to the first circuit board as a plug-in module. The second circuit board including at least one of an optical transmitter and an optical receiver.

Description: RECONFTGURABLE NODE FOR A COMMUNICATIONS NETWORK 1. Technical Field The present invention generally relates to a node for a communications network and, more particularly, to a reconfigurable node for a communications network which is adapted to be reconfigured using a plug-in module. 2. Description of Related Art In the past, broadband coaxial community area or cable television (CATV) systems have been designed with a system architecture known as "trunk and feeder." The function of a trunk coaxial cable is to deliver broadband television signals from a reception center, or headend, over the shortest distance with the least amount of amplification to a plurality of distribution points. The distribution points are connected to feeder coaxial cables which emanate from the trunk coaxial cable and contain subscriber tap off devices. Over long stretches of coaxial distribution runs, the broadband television signals are amplified at spaced locations by distribution amplifiers. The broadband television signals are transmitted over the distribution network from the headend in a forward direction to a plurality of subscribers, which is generally termed a one-way system. However, more complex systems include a reverse signal path. The reverse channel is used for system control, messaging, pay-per-view events, monitoring, and the like. In recent years, there has been a great deal of interest in the transmission of various types of information including, for example, broadband television signals, via optical fibers. Optical fibers intrinsically have more information carrying capacity than do the coaxial cables which are used in present CATV systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals. Consequently optical fibers are capable of spanning longer distances between signal regenerators or amplifiers than are coaxial cables. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by electrical shorting or radio frequency pick-up. Finally, optical fiber is immune to ambient electromagnetic interference ("EMI") and generates no EMI of its own. In view of these advantages of optical fibers, attempts are being made to push optical fiber deeper and deeper into the distribution system, i.e., closer to the homes (subscribers) in the system. However, such attempts require that current distribution equipment be changed in order to accommodate the optical fiber. Such changes can result in significant higher infrastructure costs. In some instances, the distribution equipment can be upgraded to accommodate fiber. For example, a system distribution amplifier model 6920 (available from Scientific-Atlanta, Inc. of Norαoss, Georgia) is capable of being upgraded to include a fiber receiver. However, in order to implement such an upgrade, all of the electronics must be removed from the housing and new electronics must then be installed. In addition, a different amplifier housing cover must be provided. Thus, such upgrades result in considerable equipment and labor costs. Accordingly, it would be desirable to provide a simple method and apparatus which permits a simple and inexpensive upgrade of subscriber television systems and, in particular, which permits a simple and inexpensive migration of current coaxial systems to fiber optic systems. SUMMARY OF THE INVENTION In accordance with one aspect of the invention, a node for a communications network includes a first circuit board including a first amplifier for amplifying a first signal and a second circuit board removably mounted to the first circuit board as a plug-in module. The second circuit board includes a second amplifier for amplifying a second signal and at least one diplex filter connected to the second amplifier. The diplex filter is further connected to the first amplifier when the second circuit board is mounted to the first circuit board. In accordance with another aspect of the invention, a node for a communications network includes a first circuit board including a first amplifier for amplifying a first signal and a second circuit board removably mounted to the first circuit board as a plug-in module. The second circuit board includes at least one of an optical transmitter and an optical receiver. As set forth herein, the functionality of a node for a communications network may be changed or upgraded using a plug-in module. Such changes and upgrades in functionality may be accomplished without a significant change to currently installed components and may be accomplished using the same node housing. These and other features and aspects of the invention will be more clearly understood and better described if the following detailed description is read in conjunction with the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a node for a communications network in accordance with a first embodiment of the present invention. Figure 2 is a general block diagram of a diplex filter. Figure 3 is a block diagram of a node for a communications network in accordance with a second embodiment of the present invention. Figures 4-8 are block diagrams of plug-in modules usable in the nodes of Figures 1 and 3. Figures 9A-9C illustrate a housing for containing a motherboard and a plug-in module. DETAILED DESCRIPTION The node of the present invention will be described below in terms of a distribution amplifier usable in a distribution system of a broadband communications network. However, the present invention is not limited in this respect and the principles set forth herein are generally applicable to any node in a communication network including by way of example, but not by way of limitation, system amplifiers, line extenders, taps, and nodes functioning as electrical repeaters and signal regenerators. In Figure 1, there is shown a node 100 having a plug-in module 200 for use in a two-way broadband communications network. As configured in Figure 1, node 100 constitutes a distribution amplifier with a reverse amplifier. In accordance with the present invention, the components of plug-in module 200 are provided on a first circuit board and the remaining components are provided on a motherboard or some other circuit boards. Plug-in module 200 is adapted to be removably attached to at least one of the motherboard or the other circuit boards. Node 100 further includes an environmental housing for protecting the circuit boards. Node 100 includes a first stage forward path signal amplifier 102, a second stage forward path signal amplifier 104, and third stage forward path signal amplifiers 106 and 108. These amplifiers function to amplify forward path signals supplied to a port 101. Plug-in module 200 of node 100 includes a reverse path signal amplifier 202 for amplifying reverse path signals supplied to the node as described below. Plug-in module 200 further includes diplex filters 204, 206, and 208. Specifically, to allow the forward path signals and the reverse path signals to use the same coaxial cable without interference, a diplex filter 204 is provided on the input side of the forward path signal amplifiers and the output side of the reverse path signal amplifier and diplex filters 206 and 208 are provided on the output side of the forward path signal amplifiers and the input side of the reverse path signal amplifier. Each diplex filter 204, 206, and 208 is a three-port -5- passive device having a high-pass filter and a low pass filter joined at a common point. For example, Figure 2 generally illustrates diplex filter 204 which includes a high pass filter 40 and a low pass filter 45 connected at a common point 38. A terminal 44 of high pass filter 40 is coupled to the forward path signal amplifiers and a terminal 50 of the low pass filter 45 is coupled to the reverse path signal amplifier. Additional details of diplex filters are set forth, for example, in U.S. Patent No. 5,390,337, which is incorporated herein by reference. In the configuration of Figure 1, the diplex filters separate the forward path high frequency broadband signals (e.g., television channels) and the reverse path low frequency signals (e.g., data or monitoring channels). The output of the high pass filter 205a of diplex filter 204 is supplied to first stage amplifier 102 via a pad circuit 116 for setting the signal level of the signal supplied to first stage amplifier 102 and an equalizing circuit 118 for frequency compensation of the signal supplied to first stage amplifier 102. A coupler 120 couples a portion of the output of equalizing circuit 118 to a test point TP for testing purposes. The output of first stage amplifier 102 is supplied to second stage amplifier 104 via a trim circuit 122. The output of second stage amplifier 104 is provided to third stage amplifiers 106 and 108 via an optional system trim circuit 124, an on-board trim circuit 126, an optional equalizer circuit 128, a coupler 130, and pad circuits 132 and 134. System trim 124 is an optional plug-in device which compensates for the frequency response variations of the system, where the system includes the amplifiers and the coaxial distribution network. On-board trim circuit 126 includes tunable devices (variable capacitors and inductors) which are tuned during production to yield the flattest overall frequency response. Equalizing circuit 128 (if provided) frequency compensates the signals supplied to third stage amplifiers 106 and 108 while pad circuits 132 and 134 set the levels of the signals supplied to third stage amplifiers 106 and 108, respectively. Node 100 optionally may also include a thermal compensation circuit 136 and an automatic gain control circuit 138. A suitable thermal compensation circuit for use in node 100 is described in a commonly assigned application entitled "Thermal Compensation Circuit" (Attorney Docket No. 1263.56288), filed on an even date herewith and incorporated herein by reference. A suitable automatic gain control circuit suitable for use in node 100 is described in a commonly assigned application entitled "Method and Apparatus for Dynamic Automatic Gain Control When Pilot Signal is Lost " (Attorney Docket No. 1263.56285), filed on May 31, 1996, and incorporated herein by reference. The output of third stage amplifier 106 is supplied to high pass filter 209a of diplex filter 208 via a coupler 140 and is output to the distribution system at a port. The output of third stage amplifier 108 is supplied to high pass filter 207a of diplex filter 206 via a coupler 148 and is output to the distribution system. An optional coupler 160 may couple the output of high pass filter 207a to the distribution system at two ports. A portion of the output of third stage amplifier 106 is coupled to a test pad TP by couplers 140 and 142; to automatic gain control circuit 138 (if provided) via couplers 140, 142, and 144; and to an optional status monitor circuit 146 via couplers 140, 142, and 144. A suitable status monitor circuit for use in node 100 is described in a provisional application entitled "Apparatus for Automatic Sensing of Product Configuration for Status Monitoring" (Attorney Docket No. 1263.56372), filed on an even date herewith and incorporated by herein by reference. A portion of the output of third stage amplifier 108 is coupled to a test pad TP by couplers 148 and 150 and to status monitor circuit 146 via couplers 148 and 150. AC circuits 152, 154, 156, and 158 are used to decouple AC from the RF frequencies. The AC circuits filter off the AC signal which is then fed to the amplifier power supply (not shown), where a DC supply voltage for the amplifier is generated from the incoming AC waveform. The AC can be input from any port and can be passed to any port. Reverse path signals from the distribution system are supplied to reverse path signal amplifier 202 via low pass filter 207b of diplex filter 206, low pass filter 209b of diplex filter 208, and couplers 210 and 212. Coupler 212 couples a portion of the reverse path signals to a test point TP; couples any reverse path signals locally injected at injection point 214 to reverse path amplifier 202 via pad circuit 216 for adjusting the signal level; and couples reverse path signals from status monitor circuit 146 to reverse path amplifier 202. The output of reverse path signal amplifier 202 is supplied to low pass filter 205b of diplex filter 204 via a coupler 218, an adjustable pad circuit 220 and an adjustable slope circuit 222. A portion of the output of reverse path signal amplifier 202 is coupled to a test point TP by coupler 218. Since plug-in module 200 is adapted to be removably attached to at least one of the other circuit boards, the functionality of node 100 is easily reconfigured. In addition, when a plug-in module is replaced, the environmental housing of the node may be left in its original location, thereby saving costs which would be associated with replacement and or physical relocation. In addition, the plug-in modules may be designed so as to fit within the space of existing environmental housings so as not require replacement of the housing or components thereof (e.g., housing covers). Figure 3 illustrates a node 300 having a plug-in module 400 in accordance with another embodiment of the present invention. Node 300 includes a first stage forward path signal amplifier 302, a second stage forward path signal amplifier 304, and third stage forward path signal amplifiers 306, 308, and 310. Plug-in module 400 of node 300 includes an optical receiver 482 for converting received forward path optical signals into electrical signals and supplying the electrical signals to the forward path signal amplifiers. Plug-in module 400 further includes an optical transmitter 484 for converting reverse path electrical signals into optical signals. The connection of optical fibers to the optical receiver and transmitter is described in a commonly assigned application entitled "Fiber Routing and Retention Assembly with Modular Fiber Connector Support" (Attorney Docket No. 1263.56284), filed on an even date herewith and incorporated herein by reference. A reverse path signal amplifier 486 amplifies the reverse path electrical signals and supplies the amplified signals to optical transmitter 484. Plug-in module 400 further includes diplex filters 406, 407, and 408. The output of optical receiver 482 is supplied to first stage amplifier 302 via a coupler 320, a pad circuit 316 for setting the signal level of the signal supplied to first stage amplifier 302, and an equalizing circuit 318 for frequency compensation of the signal supplied to first stage amplifier 302. Coupler 320 couples a portion of the output of optical receiver 482 to a test point TP for testing purposes. The output of first stage amplifier 302 is supplied to second stage amplifier 304 via a trim circuit 322. The output of second stage amplifier 304 is provided to third stage amplifiers 306, 308, and 310 via an optional system trim circuit 324, an optional equalizer circuit 328, couplers 330 and 331, and pad circuits 332, 333, and 334. Node 300 optionally may also include a thermal compensation circuit 336 and an automatic gain control circuit 338 as described above. The output of third stage amplifier 306 is supplied to the high pass filter of diplex filter 408 via a coupler 340 and is output to the distribution system at a port. The output of third stage amplifier 308 is supplied to the high pass filter of diplex filter 406 via a coupler 348 and is output to the distribution system. The output of third stage amplifier 310 is supplied to the high pass filter of diplex filter 407 via a coupler 360 and is output to the distribution system. A portion of the output of third stage amplifier 306 is coupled to a test pad TP by couplers 340 and 342; to automatic gain control circuit 338 (if provided) via couplers 340, 342, and 344; and to an optional status monitor circuit 346 via couplers 340, 342, and 344. A portion of the output of third stage amplifier 308 is coupled to a test pad TP by couplers 348 and 350 and to status monitor circuit 346 via couplers 348 and 350. A portion of the output of third stage amplifier 310 is coupled to a test pad TP by couplers 360 and 362 and to status monitor circuit 346 by couplers 360 and 362. Reverse path signals from the distribution system are supplied to reverse path signal amplifier 486 via the low pass filter of diplex filter 406, the low pass filter of diplex filter 407, the low pass filter of diplex filter 408, and combiner 410. Optional pad circuits 430, 431, and 432 may be provided to set the level of the reverse path signals supplied to reverse amplifier 486. The output of reverse path signal amplifier 486 is supplied to optical transmitter 484 which converts the electrical signal to an optical signal and outputs the optical signal to an optical fiber. A coupler 488 supplies status monitor signals to reverse amplifier 486. It can be seen that using plug-in module 400, it is possible to easily upgrade a node in a communications system to include a optical receiver and/or transmitter. The use of a plug-in module to accomplish this upgrade avoids the expense associated with removing all of the electrical components in current node to accomplish an upgrade. This ability is very beneficial as fiber is pushed deeper and deeper in the distribution plant of communications systems. In addition, an upgraded fiber node may be provided which has good frequency response flatness performance. In general, broadband amplifiers have certain performances over the operating range of frequencies. This is termed the frequency response of the amplifier. The frequency response is a measure of how each portion of the amplifier bandwidth is amplified in the node. In many cases, it is optimal to have a flat frequency response, in which case all the frequencies (or, in the case of CATV systems, the transmitted channels) exhibit constant gain. The end result is that the levels of the various channels when delivered to the end user are at a constant and consistent level. In accordance with the invention, a fiber receiver design may employ frequency matching components which are stable over temperature and component tolerances. In addition, the fiber receiver can be tested at an automatic test station where its frequency response can be tuned to be the same as other fiber modules. Since the fiber receiver's frequency response will not vary from unit to unit, the receiver's response can be tailored to the frequency response of the amplifier and thus greater overall flatness can be achieved. Additional details of flatness response are described in a commonly assigned application entitled "Flatness Compensation of Diplex Filter Roll-Off Using Active Amplifier Peaking Circuit" (Attorney Docket No. 1263.56289), filed on an even date herewith and incorporated by reference. Figures 4-8 show additional modules which may be used in place of either or both of the modules 200 and 400 in Figures 1 and 3. Figure 4 is an example of a module which provides a reverse optical transmitter 501 and an optical receiver 503. It is preferable that the power consumption of the optical transmitter and receiver be minimized since it is difficult to heat-sink a plug-in module, and otherwise the temperature of the module due to power dissipation may be too high. The module provides for two reverse inputs from the coaxial distribution plant which are supplied via the low pass filters of diplex filters 505 and 507 to a reverse signal amplifier 509. The module of Figure 4 may, for example, be used to replace module 200 of Figure 1 in which case the high pass filters of diplex filters 505 and 507 are supplied with the output of the forward signal amplifiers 106 and 108 and the output of optical receiver 503 is supplied to pad circuit 116. The reverse path signals are supplied from the low pass filters to the reverse signal amplifier via pad circuits 511 and 513, couplers 515 and 517, and an adjustable pad circuit 519. Status monitor information and local reverse signal injection are supplied to reverse path amplifier 509 via couplers 517 and 523. Coupler 517 also serves to couple off a portion of the signal to a test point TP. The output of reverse path amplifier 509 is supplied to optical transmitter 501 via a coupler 527. Coupler 527 couples off a portion of the signal to a test point TP. Jumpers 528 and 529 may be used for connecting/disconnecting the low pass filters of diplex filters 505 and 507 to coupler 515. The module ofFigure 5 is similar to the module ofFigure 4, except provision is made for three reverse inputs. The module ofFigure 5 may be used, for example, to replace the module 400 ofFigure 3, in which case the high pass filters of diplex filters 531, 533, and 535 are supplied with the outputs of third stage amplifiers 306, 308, and 310 and the forward output of optical receiver 537 is supplied to coupler 320. The module ofFigure 6 is similar to module ofFigure 4 and may be used, for example, to replace module 200 ofFigure 1. The module ofFigure 6 further includes a second optical receiver 539, pad circuits 541 and 543, switch 545, and switch control 547. The second optical receiver provides for redundancy in the case of misoperation or malfunction of optical receiver 503. In this case, switch 545 is switched to provide the output of receiver 539 to pad circuit 116. Switch control 547 may switch switch 545 based on a sensed optical input power to the photodiode. For example, if the optical input power of receiver 503 exceeds certain upper and/or lower limits (which may indicate malfunctioning), switch control 547 may control switch 545 to supply the output of optical receiver 539 to pad circuit 116. The module ofFigure 7 is similar to module ofFigure 4 and may be used, for example, to replace module 200 ofFigure 1. The module ofFigure 7 further includes a second optical receiver 551, pad circuits 553 and 555, and diplex filter 557. The second optical receiver permits two tiers of optical signals to be supplied to the node. The high pass and low pass filter outputs of diplex filter 557 are supplied to pad circuit 116 via an amplifier 559. Amplifier 559 boosts the signal which has been attenuated through the pads and the diplex filter. The module ofFigure 8 is similar to the module ofFigure 7, but provides for three reverse inputs and may be used to replace module 400 ofFigure 3. Figure 9 A illustrates a node housing portion 600 which includes a plug-in module 601 connected to the motherboard. Figures 9B and 9C illustrate a housing cover 602 which, together with node housing portion 600, protects the circuit boards from environmental elements such as dirt, rain, snow, etc. The details of a housing suitable for a node of the present invention are described in a commonly assigned application entitled "Housing for Electronic Devices Including Internal Fins for Volumetric Cooling" (Attorney Docket No. 1263.56282), filed on an even date herewith and incorporated by reference. While there has been shown and described the preferred embodiments of the invention, it will be evident to those skilled in the art that various modifications may be made thereto without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims: WE CLAIM: 1. A node for a communications system, comprising: a first circuit board including a first amplifier for amplifying a first signal; and a second circuit board removably mounted to said first circuit board as a plug-in module, said second circuit board including a second amplifier for amplifying a second signal and at least one diplex filter connected to said second amplifier, said diplex filter being further connected to said first amplifier when said second circuit board is mounted to said first circuit board. 2. A node for a communications system, comprising: a first circuit board including a first amplifier for amplifying a first signal; and a second circuit board removably mounted to said first circuit board as a plug-in module, said second circuit board including at least one optical transmitter. 3. The node according to claim 2, wherein said second circuit board further includes a second amplifier connected to said optical transmitter. 4. The node according to claim 2, wherein said second circuit board further includes a diplex filter connected to said optical transmitter, said diplex filter being further connected to said first amplifier when said second circuit board is mounted to said first circuit board. 5. The node according to claim 2, wherein said second circuit board further includes an optical receiver. 6. The node according to claim 5, wherein an output of said optical receiver is supplied to said first amplifier when said second circuit board is mounted to said first circuit board. 7. A node for a communications system, comprising: a first circuit board including a first amplifier for amplifying a first signal; and -14- a second circuit board removably mounted to said first circuit board as a plug-in module, said second circuit board including at least one optical receiver. 8. The node according to claim 7, wherein said optical receiver is connected to said first amplifier when said second circuit board is mounted to said first circuit board.

Inventors: JELINEK CATHERINE W, LABICHE MICHAEL J, PALAZZO MARK, POWELL ERIC G, VOGT STEPHEN W, JELINEK, CATHERINE, W., LABICHE, MICHAEL, J., PALAZZO, MARK, POWELL, ERIC, G., VOGT, STEPHEN, W.
Applicants: SCIENTIFIC ATLANTA, SCIENTIFIC-ATLANTA, INC.
Application year: 1997
Main classification: H04J
IPC codes: H05K1