Patent Publication Number: US-5255267-A

Title: Apparatus for combining broadband and baseband signal transmissions

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to locate area network RF transmission systems, more particularly the invention relates to the combining of broadband and baseband transmissions on a baseband network. 
     A popular form of local area network is based on the IEEE 802.3 10 base 2 standard for the thin Ethernet network which uses a 50 ohm coaxial cable as the transmission medium. Transmitted signals are digital data with a carrier frequency below 25 MHz using a Manchester encoded baseband signal. Other signals do not normally coexist with the Manchester encoded signals on the same cable. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to extending the bandwidth of a thin Ethernet network, for example, to support broadband transmission (i.e. 50-350 MHz). The RF broadband signal must be DC isolated, equalized, and converted from 75 ohm broadband coaxial cable impedance to 50 ohms. Further, the RF broadband signal must be split into a plurality of segments supported by a baseband repeater and suitably filtered to prevent interference with the baseband signals in each segment and provide a high impedance to the baseband signals. 
     Each of the baseband segments is low pass filtered to remove high frequency signals which can interfere with the RF broadband signals. The filtered baseband and broadband signals are then combined in each segment. 
     In accordance with one feature of the invention, these functions are accomplished with a bandwidth bridge. 
     Taps are provided on each segment at user nodes to access combined baseband and broadband signals. Each tap provides low pass filtering of signals from an MAU or computer interface to prevent signals from interfering with the broadband signals. Baseband signals at each tap are transmitted equally to the repeater side and the network side of the segment, while a broadband signal at each tap is transmitted with a directional coupler towards the bridge. 
     The invention can be implemented in a shielded twisted pair cable distribution system as well as a coaxial cable distribution system. 
     The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a conventional Ethernet thin net base network. 
     FIG. 2 is a schematic of one embodiment of a combined FR broadband and baseband network using the Ethernet network of FIG. 1. 
     FIG. 3 is a schematic of a bridge for combining a broadband signal and a baseband signal in a baseband distribution network in accordance with one embodiment of the invention. 
     FIG. 4 is a schematic of a tap for interfacing broadband and baseband signals between users and the network. 
     FIG. 5 is a schematic of a tap for a most remote broadband node of the network, or for a star wired system where there is only one user per network. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now the drawing, FIG. 1 is a schematic of a conventional thin Ethernet baseband network. Typically a repeater 10 supports multiple segments 12 with each segment comprising a 50 ohm coaxial cable. Alternatively, each segment can comprise a shielded twisted pair cable instead of a coaxial cable. A plurality of nodes (BNC&#34;T&#34;s) 14 are provided in each segment for interfacing a MAU or personal computer 16 to the segment. 
     Typically, the entire frequency spectrum of the medium is used to form the baseband signal which comprises digital data typically transmitted at 10 MBPS. Transmission is bidirectional with a signal inserted at any tap on the medium propagating in both directions to the ends. The Ethernet signal is a Manchester encoded baseband signal that does not normally allow other signals to coexist on the same cable. 
     FIG. 2 is a schematic of one embodiment of a combined RF broadband and baseband network in accordance with the invention using the Ethernet baseband network of FIG. 1. The transmission of the broadband RF transmissions (50-350 MHz) allows the network to support video devices 18 and RF telephonic devices 20 as well as personal computers 16. Special tap devices 22 are provided at each node for access to the thin Ethernet baseband and RF broadband signals transmitted in each segment. As will be described further hereinbelow, each tap provides filtering and load impedance conversion for optimal coupling of signals to and from the network cable. Higher harmonics (e.g. above 25 MHz) of the Ethernet baseband signal are filtered out at each node to prevent interference with the broadband signal, and the broadband signal is filtered to remove signals below 35 MHz so that the broadband signal does not interfere with the thin baseband signal. 
     A bridge 24 is connected between the Ethernet repeater 10 and the Ethernet segments 12. The bridge contains a broadband power splitter for connection to a plurality (e.g. 32) thin Ethernet segments and provides filter networks for the broadband and Ethernet signals. The broadband signal is tapped from the broadband trunk cable and is amplified by a bi-directional RF line amplifier. The amplifier provides the gain and frequency equalization needed to drive the bridge. 
     The bridge 24 is illustrated in more detail in the schematic of FIG. 3 in which a broadband trunk is interconnected with the Ethernet system. The broadband trunk is connected to a 75/50 ohm transformer 34 in the bridge 24. The output of transformer 34 is connected to a 32 way signal splitter 36 for coupling of the broadband signal to up to 32 segments. Each segment is connected through a 35 MHz high pass filter 38 to keep noise below 35 MHz from getting into the thin Ethernet network and provide a high impedance to the thin Ethernet signals. Each of the thin Ethernet signals from the repeater is passed through a 25 MHz low pass filter 40 to remove high frequency signals which could interfere with the RF broadband signals. The filtered thin Ethernet and RF broadband signals are then combined and connected to each segment to single or multiple users over a 100 meter max thin Ethernet segment cable. 
     FIG. 4 is a schematic of a tap device in accordance with the invention for providing access to the thin Ethernet baseband and broadband signals at each Ethernet node. The repeater (rep) is connected to terminal 42 of the tap, and the segment (NET) is connected to terminal 44 of the tap. The RF broadband signal is connected through a directional coupler 46 to the tap terminal 48, and a personal computer is connected to the MAU terminal 50. The MAU is connected through 25 MHz low pass filters 52 positioned between the MAU and the repeater and segment, respectively. The RF broadband signals bypass the low pass filters 52 through a 35 MHz high pass filter 54 connected between terminals 42,44. Thus signals from the MAU above 25 MHz are prevented from getting onto the network with the filters 5 providing low loss and low distortion to the network from high impedance and low impedance loads at the MAU port. The MAU presents a high impedance load when it is receiving signals and a low impedance load when it transmits signals onto the coaxial cable. The filters 52 couple the signal equally to the repeater side and the network side of the tap. Directional coupler 46 couples the broadband signal only toward the repeater side of the tap since all of the broadband signals go toward the bridge. 
     Thus, the tap filter is designed to low pass filter (LPF) the MAU signal before it gets onto the network cable and to pass the broadband signal (50-350 MHz) from the repeater to the network port. The MAU signal must be coupled equally to the repeater and to the network ports. The filter consists of a third order 35 MHz LPF in parallel with a third order 35 MHz HPF connected between the repeater and network ports. This filter combination passes the broadband and the baseband data signals. The MAU port is coupled to the center element of the LPF through a series resonator tuned to 54 MHz. With this configuration the MAU port is coupled to the repeater and to the network ports, with the baseband signal filtered to prevent interference with the broadband signal. 
     FIG. 5 is a schematic of another tap in accordance with the invention which is normally located at the most remote RF broadband node and separates the broadband and Ethernet signals. The broadband tap is connected through a 50/75 ohm transformer 60 and a 35 MHz high pass filter 62 to the repeater (hub) terminal 64. An MAU terminal 66 is connected through a 25 Mhz low pass filter 68 to terminal 64. Thus the tap can be used to terminate the end of a long cable for RF broadband and Ethernet signals or terminate the broadband signal and allow the Ethernet signal to continue on as a normal thin net cable. At the end of a network segment, the net terminal should be terminated with 50 ohms. 
     Following are system, bridge, and tap specifications for one embodiment of the invention: 
     
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SYSTEM SPECIFICATIONS                                                     
Parameter            Specification                                        
______________________________________                                    
Segments per BX Bridge                                                    
                     32                                                   
Length per segment   100 meters max.                                      
Nodes per segment    1 to 5                                               
Cable type           RG-58                                                
                     (thin Ethernet)                                      
Nominal Impedance    50 ohms                                              
Spectrum Allocation:                                                      
Baseband             0-25 MHz                                             
Broadband Upstream   50-108 MHz                                           
Broadband Downstream 160-350 MHz                                          
Signal Levels (at node)                                                   
Baseband             IEEE 802.3 (10Base2)                                 
                     compatible                                           
Broadband Upstream   IEEE 802.7                                           
(into tap)           compatible                                           
Broadband Downstream +7 dBmV per channel                                  
                     ±10 dB                                            
Bridge Input Signal Characteristics                                       
Broadband Downstream:                                                     
Input Signal Level   +40 to +60 dBmV per                                  
                     cH depending on cable                                
                     length, may need                                     
                     equalization to                                      
                     compensate for cable                                 
                     frequency response                                   
                     tilt.                                                
from broadband trunk +57 dBmV (350 MHz)                                   
                     per channel                                          
Nominal Impedance    75 ohms                                              
Bandwidth            160-350 MHz                                          
Broadband Upstream:                                                       
Input Signal Level   +30 dBmV ±5 dB                                    
from network         per channel                                          
Bandwidth            50-108 MHz                                           
Baseband:                                                                 
No. of Segments      32                                                   
Signal Levels        IEEE 802.3 (10Base2)                                 
                     compatible                                           
Bandwidth            IEEE B02.3 (10Base2)                                 
                     compatible                                           
Nominal Impedance    50 ohms                                              
Bridge Performance Specifications                                         
Broadband:                                                                
Input/Output VSWR    1.2:1 (75 ohms)                                      
Downstream Gain (Broadband Input to                                       
each NET output):                                                         
150-350 MHz          -22 dB ±2 dB                                      
Upstream Gain (NET input to                                               
Broadband output):                                                        
50-108 MHz           -22 dB ±2 dB                                      
Baseband:                                                                 
Input/Output VSWR    1.2:1 (50 ohms)                                      
Insertion Loss       .4 dB                                                
Bandwidth, 3 dB      &gt;25 MHz                                              
Attenuation at 50 MHz                                                     
                     &gt;40 dB                                               
TAP Input Signal Characteristics                                          
Broadband Downstream:                                                     
Output Signal Level  +10 dBmV ±10 dB                                   
                     per channel                                          
Nominal Impedance    75 ohms                                              
Bandwidth            160-450 MHz                                          
Broadband Upstream:                                                       
Input Signal Level at node                                                
                     +54 dBmV +2/-6 dB                                    
                     per channel                                          
Nominal Impedance    75 ohms                                              
Bandwidth            50-108 MHz                                           
Baseband:                                                                 
Signal Levels        IEEE 802.3 (10Base2)                                 
                     compatible                                           
Bandwidth            IEEE 802.3 (10Base2)                                 
                     compatible                                           
Nominal Impedance    50 ohms                                              
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     There has been described a passive bridge apparatus and taps which allow the combining of RF broadband signals with baseband signals in a baseband network. While the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.