Abstract:
In a novel apparatus for and method of discrete spurious frequency leakage cancellation a radio frequency RF switch is used to couple the upstream path signal to the CATV cable only during transmission bursts. In between transmission bursts, the upstream signal is disconnected from the CATV cable. In a circuit for canceling frequency spurs from a victim signal, a radio frequency (RF) switch is operative to connect and disconnect the victim signal to/from the output in accordance with a switch control signal which is generated by a switch control module. The victim signal is coupled to said RF switch output during transmission bursts only.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Divisional of and claims priority to U.S. Nonprovisional patent application No. 12/426,424 filed Apr. 20, 2009, hereby incorporated by reference herein in its entirety. 
     
    
     FIELD 
       [0002]    The present invention relates to the field of data communications and more particularly relates to a discrete spurious frequency leakage cancellation method and apparatus for use in a cable modem such as a Data Over Cable Service Interface Specification (DOCSIS) compliant cable modem. 
       BACKGROUND 
       [0003]    Currently there are more than 50 million high-speed Internet access customers in North America. Recently, the cable modem has become the broadband connection of choice for many Internet users, being preferred over the nearest rival broadband technology, Digital Subscriber Line (DSL), by a significant margin. 
         [0004]    Cable modems are well known in the art. A cable modem is a type of modem that provides access to a data signal sent over the cable television (CATV) infrastructure. Cable modems are primarily used to deliver broadband Internet access, taking advantage of unused bandwidth on a cable television network. In 2005 there were over 22.5 million cable modem users in the United States alone. 
         [0005]    A cable modem is a network appliance that enables high speed data connections to the internet via data services provided by the local cable company. Data from the home is sent upstream on a carrier that operates on the 5 MHz to 42 MHz band of the cable spectrum. Downstream data is carried on a 88 MHz to 860 MHz band. The cable modem system can have additional networking features such as Voice over IP (VoIP), wireless connectivity or network switch or hub functionality. 
         [0006]    The term cable Internet access refers to the delivery of Internet service over the cable television infrastructure. The proliferation of cable modems, along with DSL technology, has enabled broadband Internet access in many countries. The bandwidth of cable modem service typically ranges from 3 Mbps up to 40 Mbps or more. The upstream bandwidth on residential cable modem service usually ranges from 384 kbps to 30 Mbps or more. In comparison, DSL tends to offer less speed and more variance between service packages and prices. Service quality is also far more dependent on the client&#39;s location in relation to the telephone company&#39;s nearest central office or Remote Terminal. 
         [0007]    Users in a neighborhood share the available bandwidth provided by a single coaxial cable line. Therefore, connection speed varies depending on how many people are using the service at the same time. In most areas this has been eliminated due to redundancy and fiber networks. 
         [0008]    With the advent of Voice over IP telephony, cable modems are also being used to provide telephone service. Many people who have cable modems have opted to eliminate their Plain Old Telephone Service (POTS). An alternative to cable modems is the Embedded Multimedia Terminal Adapter (EMTA). An EMTA allows multiple service operators (MSOs) to offer both High Speed Internet and VoIP through a single piece of customer premise equipment. A multiple system operator is an operator of multiple cable television systems. 
         [0009]    Many cable companies have launched Voice over Internet Protocol (VoIP) phone service, or digital phone service, providing consumers a true alternative to standard telephone service. Digital phone service takes the analog audio signals and converts them to digital data that can be transmitted over the fiber optic network of the cable company. Cable digital phone service is currently available to the majority of U.S. homes with a large number of homes are now subscribing. The number of homes subscribing is currently growing by hundreds of thousands each quarter. One significant benefit of digital phone service is the substantial consumer savings, with one recent study saying residential cable telephone consumers could save an average of $135 or more each year. 
         [0010]    The Data Over Cable Service Interface Specification (DOCSIS) compliant cable modems have been fueling the transition of cable television operators from a traditional core business of entertainment programming to a position as full-service providers of video, voice, and data telecommunications services. 
         [0011]    Cable systems transmit digital data signals over radio frequency (RF) carrier signals. To provide two-way communication, one carrier signal carries data in the downstream direction from the cable network to the customer and another carrier signal carries data in the upstream direction from the customer to the cable network. Cable modems are devices located at the subscriber premises that functions to convert digital information into a modulated RF signal in the upstream direction, and to convert the RF signals to digital information in the downstream direction. A cable modem termination system (CMTS) performs the opposite operation for multiple subscribers at the cable operator&#39;s head-end. 
         [0012]    Typically, several hundreds of users share a 6 MHz downstream channel and one or more upstream channels. The downstream channel occupies the space of a single television transmission channel in the cable operator&#39;s channel lineup. It is compatible with digital set top MPEG transport stream modulation (64 or 256 QAM), and provides up to 40 Mbps. A media access control (MAC) layer coordinates shared access to the upstream bandwidth. 
         [0013]    The DOCSIS 2.0 specification provides for both more efficient modulation techniques and increased RF channel bandwidth in the return path under two different allowed multi-access protocols: a time division multi-access (TDMA) protocol and a synchronous code division multi-access (S-CDMA) protocol. Under the DOCSIS 2.0 TDMA protocol, the maximum allowed RF channel bandwidth is increased from 3.2 to 6.4 MHz and three new higher-order modulation techniques are specified: 8 QAM, 32 QAM, and 64 QAM. As a result, the maximum raw data rate is increased from 10.24 Mbps in the case of DOCSIS 1.0/1.1 (16 QAM in 3.2 MHz) to 30.72 Mbps (64 QAM in 6.4 MHz). 
         [0014]    Under TDMA, individual channel users are assigned a distinct time slot during which they transmit a QAM burst that encodes multiple information bits. Under CDMA, the in-phase and quadrature (I and Q) components of each QAM symbol are first encoded into a stream of sub-bits, or ‘chips’. Each user is assigned one or more distinct code chip sequences that are recognized by a matched correlator at the receiver that rejects all other users&#39; code sequences. In this manner, multiple users are able to transmit simultaneously in the same time slot. The DOCSIS S-CDMA protocol is actually a time division multiplexed CDMA that employs 128-chip spreading codes and mini-time slots spanning multiple CDMA symbols. 
         [0015]    A potential problem in the design of cable modems is spurious emissions from signal leakage from the PHY circuitry into the upstream path. Out of band spurious emissions can be filtered out relatively easily. In-band spurious emissions, however, are more difficult to eliminate. 
         [0016]    In accordance with the DOCSIS 2.0 specification, the spurious emissions specifications are separated into two regions based on the transmit power. Region 1 is defined to have a power range of +14 dBmV to (Pmax−3), i.e. the central region. Region 2 is defined from +8 dBmV to +14 dBmV and (Pmax−3) to Pmax, i.e. the low and high ends of the transmit power. 
         [0017]    For S-CDMA mode, when a modem is transmitting fewer than four spreading codes, the region 2 specifications are used for all transmit power levels. Otherwise, for all other numbers of spreading codes (e.g., 4 to 128) or for TDMA mode, the spurious emissions specifications are used according to the power ranges defined for regions 1 and 2 above. 
         [0018]    The noise and spurious power cannot exceed the levels given in Table 1 below. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 DOCSIS 2.0 Spurious Emissions 
               
             
          
           
               
                 Parameter 
                 Transmitting Burst 
                 Between Bursts 
               
               
                   
               
               
                 Inband 
                 −40 dBc 
                 The greater of −72 dBc 
               
               
                   
                   
                 or −59 dBmV 
               
               
                 Adjacent Band 
                 See Table 6-10 
                 The greater of −72 dBc 
               
               
                   
                   
                 or −59 dBmV 
               
               
                 3 or Fewer Carrier- 
                 Region 1: −50 dBc 
                 The greater of −72 dBc 
               
               
                 Related Frequency 
                 for transmitted 
                 or −59 dBmV 
               
               
                 Bands (such as second 
                 modulation 
               
               
                 harmonic, if &lt; 42 MHz) 
                 rate = 320 ksps and 
               
               
                   
                 above; −47 dBc for 
               
               
                   
                 transmitted 
               
               
                   
                 modulation 
               
               
                   
                 rate = 160 ksps 
               
               
                   
                 Region 2: −47 dBc 
               
               
                 Bands within 5 to 42 
                 See Table 6-11 
                 The greater of −72 dBc 
               
               
                 MHz (excluding 
                   
                 or −59 dBmV 
               
               
                 assigned channel, 
               
               
                 adjacent channels, and 
               
               
                 carrier-related channels) 
               
               
                 CM Integrated Spurious 
               
               
                 Emissions Limits 
               
               
                 (all in 4 MHz, 
               
               
                 includes discretes) 1   
               
               
                   42 to 54 MHz 
                 max (−40 dBc, 
                 −26 dBmV 
               
               
                   
                 −26 dBmV) 
               
               
                   54 to 60 MHz 
                 −35 dBmV 
                 −40 dBmV 
               
               
                   60 to 88 MHz 
                 −40 dBmV 
                 −40 dBmV 
               
               
                 88- to 860 MHz 
                 −45 dBmV 
                 max (−45 dBmV, 
               
               
                   
                   
                 −40 dB ref d/s 2 ) 
               
               
                 CM Discrete Spurious 
               
               
                 Emissions Limits 1   
               
               
                   42 to 54 MHz 
                 −max (−50 dBc, 
                 −36 dBmV 
               
               
                   
                 −36 dBmV) 
               
               
                   54 to 88 MHz 
                 −50 dBmV 
                 −50 dBmV 
               
               
                  88 to 860 MHz 
                 −50 dBmV 
                 −50 dBmV 
               
               
                   
               
             
          
         
       
     
         [0019]    In Table 1 above, in-band spurious emissions may include noise, carrier leakage, clock signal lines, synthesizer spurious products and other undesired transmitter products. The measurement bandwidth for in-band spurious is equal to the modulation rate (e.g., 160 to 5120 kHz). All requirements expressed in dBc are relative to the actual transmit power that the cable modem emits. 
         [0020]    The measurement bandwidth for the three (or fewer) Carrier-Related Frequency Bands (below 42 MHz) is 160 kHz, with up to three 160 kHz bands, each with no more than the value given in Table 1, allowed to be excluded from the “Bands within 5 to 42 MHz Transmitting Burst” specifications of Table 2 below. Carrier-related spurious emissions include all products whose frequency is a function of the carrier frequency of the upstream transmission, such as but not limited to carrier harmonics. The measurement bandwidth is also 160 kHz for the Between Bursts specifications of Table 1 below 42 MHz. 
         [0021]    The Transmitting Burst specifications apply during the mini-slots granted to the cable modem (when the cable modem uses all or a portion of the grant), and for 32 modulation intervals before and after the granted mini-slots. The Between Bursts specifications apply except during a used grant of mini-slots, and the 32 modulation intervals before and after the used grant. 
         [0022]    In TDMA mode, a mini-slot may be as short as 32 modulation intervals, or 6.25 microseconds at the 5.12 Msymbol/sec rate, or as short as 200 microseconds at the 160 ksym/sec rate. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Spurious Emissions in 5 to 42 MHz Relative to the Transmitted Burst 
               
               
                 Power Level 
               
             
          
           
               
                 Possible 
                 Specification 
                 Specification 
                 Initial measurement 
               
               
                 modulation rate 
                 in the interval, 
                 in the interval, 
                 interval and distance 
               
               
                 in this interval 
                 Region 1 
                 Region 2 
                 from carrier edge 
               
               
                   
               
               
                  160 kHz 
                 −54 dBc 
                 −53 dBc 
                 220 kHz to 380 kHz 
               
               
                  320 kHz 
                 −52 dBc 
                 −50 dBc 
                 240 kHz to 560 kHz 
               
               
                  640 kHz 
                 −50 dBc 
                 −47 dBc 
                 280 kHz to 920 kHz 
               
               
                 1280 kHz 
                 −48 dBc 
                 −44 dBc 
                 360 kHz to 1640 kHz 
               
               
                 2560 kHz 
                 −46 dBc 
                 −41 dBc 
                 520 kHz to 3080 kHz 
               
               
                 5120 kHz 
                 −44 dBc 
                 −38 dBc 
                 840 kHz to 5960 kHz 
               
               
                   
               
             
          
         
       
     
         [0023]    In the worst case, the maximum spurious level relative to the transmission level permitted during transmission is −54 dBc. Spurious emissions, other than those in an adjacent channel or carrier related emissions listed above, may occur in intervals (frequency bands) that could be occupied by other carriers of the same or different modulation rates. To accommodate these different modulation rates and associated bandwidths, the spurious emissions are measured in an interval equal to the bandwidth corresponding to the modulation rate of the carrier that could be transmitted in that interval. This interval is independent of the current transmitted modulation rate. 
         [0024]    Table 2 above lists the possible modulation rates that could be transmitted in an interval, the required spurious level in that interval, and the initial measurement interval at which to start measuring the spurious emissions. Typically, the modulation is set by the CMTS utilizing the downstream link. 
         [0025]    For example, consider a 35 MHz clock used to drive a cable modem PHY that leaks to the output of a PGA circuit output at a sufficiently high magnitude to cause a violation of the DOCSIS in-band spurious level specifications. The magnitude of the leakage will typically vary by the particular PCB payout used and the configuration of the 1.5 V decoupling capacitors. 
         [0026]    In the worst case, for DOCSIS 2.0 for bands within 5 to 42 MHz, the maximum allowed spurious emissions between transmission bursts is -59 dBmV which translates to approximately −107.75 dBm or 1.122 μV on 75 ohm. A spur emitted at 35 MHz (PHY clock driver) cannot be filtered because is falls within the upstream frequency range of 5 to 42 MHz. 
         [0027]    One approach to solving this problem is to modify the design of the PHY circuit which may be a complex, FPGA or ASIC. A disadvantage of this approach is complicated and very expensive process (in terms of human resources) of analyzing and investigating the circuit to find the leakage path. Therefore, in the case of in-band spurious emissions (e.g., noise, carrier leakage, clock signal lines, synthesizer spurious products, etc.), a mechanism is needed to substantially minimize or cancel the spurious emissions. The mechanism should meet the requirements of the DOCSIS cable modem specification and operate efficiently, be of low complexity, exhibit high performance, consume minimal board and chip area and be able to be manufactured at low cost. 
       SUMMARY 
       [0028]    The present invention is a novel apparatus for and method of discrete spurious frequency leakage cancellation for use in a cable modem. The spurious leakage cancellation mechanism is particularly suitable for use in cable modem systems adapted to implement the DOCSIS 2.0 specification which specifies both downstream and upstream channels. 
         [0029]    In one embodiment, the spurious emission cancellation mechanism cancels the spurious emissions by first creating a replica of the aggressor clock signal having the same amplitude but 180 degree phase shift as the spurious signal. The phase shifted spurious replica is added to the original spurious signal thus cancelling the spurious signal. 
         [0030]    In another embodiment, an RF switch is used to couple the upstream path signal to the CATV cable only during transmission bursts. In between transmission bursts, the upstream signal is disconnected from the CATV cable. This embodiment takes advantage of the less stringent spurious requirements in the DOCSIS 2.0 specification for transmission bursts. In between transmission bursts, when stricter spurious requirements apply, the upstream signal is disconnected from the CATV cable. 
         [0031]    To aid in understanding the principles of the present invention, the description is provided in the context of a DOCSIS 2.0 capable cable system comprising a cable modem adapted to receive an DOCSIS compatible RF signal feed from a cable head-end (i.e. CMTS) and to distribute video, Internet and telephony to a subscriber premises. It is appreciated, however, that the invention is not limited to use with any particular communication device or standard and may be used in optical, wired and wireless applications. Further, the invention is not limited to use with a specific technology but is applicable to any transmission circuit wherein it is desirable to cancel or substantially eliminate in-band spurious emissions. 
         [0032]    Several advantages of the discrete spurious leakage cancellation mechanism of the present invention include (1) relatively low cost of manufacturing; (2) stable circuit operation over temperature and voltage fluctuations; (3) simple and clear implementation to satisfy users and customers; (4) relatively to implement; and (5) can be removed from the cable modem circuit design without requiring changes to the PCB layout. 
         [0033]    Note that many aspects of the invention described herein may be constructed as software objects that are executed in embedded devices as firmware, software objects that are executed as part of a software application on either an embedded or non-embedded computer system running a real-time operating system such as WinCE, Symbian, OSE, Embedded LINUX, etc. or non-real time operating system such as Windows, UNIX, LINUX, etc., or as soft core realized HDL circuits embodied in an Application Specific Integrated Circuit (ASIC) or Field Programmable Gate Array (FPGA), or as functionally equivalent discrete hardware components. 
         [0034]    There is thus provided in accordance with the present invention, a circuit for canceling frequency spurs from a victim signal, the frequency spurs originating from an aggressor clock source comprising a canceling clock source for generating a canceling clock signal, a conditioning circuit operative to generate an amplitude and phase adjusted cancellation signal from the canceling clock source and combining means for applying the cancellation signal to the victim signal thereby substantially canceling the frequency spurs. 
         [0035]    There is also provided in accordance with the present invention, a method of canceling frequency spurs from a victim signal, the frequency spurs originating from an aggressor clock source, the method comprising the steps of providing a canceling clock source for generating a canceling clock signal, conditioning the canceling clock signal to generate an amplitude and phase adjusted cancellation signal therefrom and combining the cancellation signal with the victim signal to generate an output signal having substantially reduced frequency spur energy. 
         [0036]    There is further provided in accordance with the present invention, a cable modem connected to a Community Antenna Television (CATV) infrastructure comprising a memory, one or more interface ports, a downstream path including a tuner, an upstream path for generating an upstream signal to be transmitted over the CATV infrastructure, the upstream path comprising a canceling clock source for generating a canceling clock signal, a conditioning circuit operative to generate an amplitude and phase adjusted cancellation signal from the canceling clock source, combining means for applying the cancellation signal to the upstream signal thereby substantially canceling the frequency spurs, a PHY circuit coupled to the downstream path and the upstream path and a processor coupled to the memory, the one or more interface ports and the PHY circuit, the processor operative to implement a media access control (MAC) layer operative to generate an output video stream. 
         [0037]    There is also provided in accordance with the present invention, a circuit for canceling frequency spurs from a victim signal, the frequency spurs derived from an aggressor clock source comprising a radio frequency (RF) switch having an output and operative to connect and disconnect the victim signal to the output in accordance with a switch control signal and a switch control module operative to generate the switch control signal, wherein the victim signal is coupled to the RF switch output during transmission bursts only. 
         [0038]    There is further provided in accordance with the present invention, a method of canceling frequency spurs from a victim signal, the frequency spurs originating from an aggressor clock source, the method comprising the steps of providing a radio frequency (RF) switch having an output and operative to connect and disconnect the victim signal to the output in accordance with a switch control signal and generating the switch control signal whereby the victim signal is coupled to the RF switch output during transmission bursts only. 
         [0039]    There is also provided in accordance with the present invention, a cable modem connected to a Community Antenna Television (CATV) infrastructure comprising a memory, one or more interface ports, a downstream path including a tuner, an upstream path for generating an upstream signal to be transmitted over the CATV infrastructure, the upstream path including a radio frequency (RF) switch having an output and operative to connect and disconnect the upstream signal to the output in accordance with a switch control signal, a PHY circuit coupled to the tuner and the RF switch, the PHY circuit comprising a switch control module operative to generate the switch control signal, wherein the upstream signal is coupled to the RF switch output during transmission bursts only and a processor coupled to the memory, the one or more interface ports and the PHY circuit, the processor operative to implement a media access control (MAC) layer operative to generate an output video stream. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0041]      FIG. 1  is a block diagram illustrating an example cable modem system incorporating the upstream system of the present invention; 
           [0042]      FIG. 2  is a block diagram illustrating an example cable modem including an upstream system incorporating the spur reduction mechanism of the present invention; 
           [0043]      FIG. 3  is a simplified block diagram illustrating the processor of the cable modem of  FIG. 2  including an upstream system incorporating a first embodiment of the spur reduction mechanism of the present invention; 
           [0044]      FIG. 4  is a schematic diagram illustrating the spur cancellation circuit of  FIG. 3  in more detail; 
           [0045]      FIG. 5  is a block diagram illustrating a model of a portion of the upstream circuit without the spur cancellation mechanism of the present invention; 
           [0046]      FIG. 6  is a block diagram illustrating a model of a portion of the upstream circuit incorporating the spur cancellation mechanism of the present invention; 
           [0047]      FIG. 7  is a spectrum plot illustrating frequency response of the spur cancellation circuits of  FIGS. 5 and 6 ; 
           [0048]      FIG. 8  is a time domain plot of the spur without the spur cancellation mechanism of the present invention; 
           [0049]      FIG. 9  is a frequency spectrum of the spur of  FIG. 8 ; 
           [0050]      FIG. 10  is a time domain plot of the spur with the spur cancellation mechanism of the present invention; 
           [0051]      FIG. 11  is a frequency spectrum of the spur of  FIG. 10 ; and 
           [0052]      FIG. 12  is a simplified block diagram illustrating the processor of the cable modem of  FIG. 2  including an upstream system incorporating a second embodiment of the spur reduction mechanism of the present invention; 
       
    
    
     DETAILED DESCRIPTION 
     Notation Used Throughout 
       [0053]    The following notation is used throughout this document. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Term 
                 Definition 
               
               
                   
                   
               
             
             
               
                   
                 AC 
                 Alternating Current 
               
               
                   
                 ADC 
                 Analog to Digital Converter 
               
               
                   
                 ASIC 
                 Application Specific Integrated Circuit 
               
               
                   
                 ATM 
                 Asynchronous Transfer Mode 
               
               
                   
                 CATV 
                 Community Antenna Television or Cable TV 
               
               
                   
                 CDMA 
                 Code Division Multiple Access 
               
               
                   
                 CM 
                 Cable Modem 
               
               
                   
                 CMTS 
                 Cable Modem Termination System 
               
               
                   
                 CO 
                 Central Office 
               
               
                   
                 CPU 
                 Central Processing Unit 
               
               
                   
                 DAC 
                 Digital to Analog Converter 
               
               
                   
                 DCAS 
                 Downloadable Conditional Access Systems 
               
               
                   
                 DECT 
                 Digital Enhanced Cordless Telecommunications 
               
               
                   
                 DHCP 
                 Dynamic Host Control Protocol 
               
               
                   
                 DOCSIS 
                 Data Over Cable Service Interface Specification 
               
               
                   
                 DS 
                 Downstream 
               
               
                   
                 DSL 
                 Digital Subscriber Line 
               
               
                   
                 DSP 
                 Digital Signal Processor 
               
               
                   
                 DVR 
                 Digital Video Recorder 
               
               
                   
                 EEROM 
                 Electrically Erasable Read Only Memory 
               
               
                   
                 EMTA 
                 Embedded Multimedia Terminal Adapter 
               
               
                   
                 FPGA 
                 Field Programmable Gate Array 
               
               
                   
                 GPIO 
                 General Purpose I/O 
               
               
                   
                 HDL 
                 Hardware Description Language 
               
               
                   
                 I/F 
                 Interface 
               
               
                   
                 I/O 
                 Input/Output 
               
               
                   
                 IC 
                 Integrated Circuit 
               
               
                   
                 IP 
                 Internet Protocol 
               
               
                   
                 LAN 
                 Local Area Network 
               
               
                   
                 LED 
                 Light Emitting Diode 
               
               
                   
                 MAC 
                 Media Access Control 
               
               
                   
                 MPEG 
                 Moving Picture Experts Group 
               
               
                   
                 MSO 
                 Multiple Service Operator 
               
               
                   
                 NB 
                 Narrowband 
               
               
                   
                 PC 
                 Personal Computer 
               
               
                   
                 PC 
                 Personal Computer 
               
               
                   
                 PCB 
                 Printed Circuit Board 
               
               
                   
                 PCC 
                 Passive Cancellation Circuit 
               
               
                   
                 PDA 
                 Personal Digital Assistant 
               
               
                   
                 PGA 
                 Programmable Gain Amplifier 
               
               
                   
                 PLL 
                 Phase Locked Loop 
               
               
                   
                 POTS 
                 Plain Old Telephone Service 
               
               
                   
                 PSTN 
                 Public Switched Telephone Network 
               
               
                   
                 QAM 
                 Quadrature Amplitude Modulation 
               
               
                   
                 RAM 
                 Random Access Memory 
               
               
                   
                 RF 
                 Radio Frequency 
               
               
                   
                 ROM 
                 Read Only Memory 
               
               
                   
                 SLIC 
                 Subscriber Line Interface Card 
               
               
                   
                 SONET 
                 Synchronous Optical Network 
               
               
                   
                 SPDT 
                 Single Pole Double Throw 
               
               
                   
                 TB 
                 Tuning Band 
               
               
                   
                 TDMA 
                 Time Division Multiple Access 
               
               
                   
                 US 
                 Upstream 
               
               
                   
                 USB 
                 Universal Serial Bus 
               
               
                   
                 VCO 
                 Voltage Controlled Oscillator 
               
               
                   
                 VGA 
                 Variable Gain Amplifier 
               
               
                   
                 VoIP 
                 Voice over IP 
               
               
                   
                 WAN 
                 Wide Area Network 
               
               
                   
                 WB 
                 Wideband 
               
               
                   
                 WLAN 
                 Wireless Local Area Network 
               
               
                   
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION 
       [0054]    Embodiments of the invention are a novel apparatus for and method of discrete spurious frequency leakage cancellation for use in a cable modem. The spurious leakage cancellation mechanism is particularly suitable for use in cable modem systems adapted to implement the DOCSIS 2.0 specification which specifies both downstream and upstream channels. 
         [0055]    To aid in understanding the principles of the present invention, the description is provided in the context of a DOCSIS 2.0 capable cable system comprising a cable modem adapted to receive an DOCSIS compatible RF signal feed from a cable head-end (i.e. CMTS) and to distribute video, Internet and telephony to a subscriber premises. It is appreciated, however, that the invention is not limited to use with any particular communication device or standard and may be used in optical, wired and wireless applications. Further, the invention is not limited to use with a specific technology but is applicable to any transmission circuit wherein it is desirable to cancel or substantially eliminate in-band spurious emissions. 
         [0056]    It is noted that the spur reduction mechanism of the present invention can be used in cable modems designed for use not only in North America, but also for use with the Euro DOCSIS standard using a similar configuration. 
         [0057]    Note that throughout this document, the term communications device is defined as any apparatus or mechanism adapted to transmit, or transmit and receive data through a medium. The communications device may be adapted to communicate over any suitable medium such as RF, wireless, infrared, optical, wired, microwave, etc. In the case of wireless communications, the communications device may comprise an RF transmitter, RF receiver, RF transceiver or any combination thereof. 
         [0058]    The term cable modem is defined as a modem that provides access to a data signal sent over the cable television infrastructure. The term voice cable modem is defined as a cable modem that incorporates VoIP capabilities to provide telephone services to subscribers 
       Cable System Incorporating Spur Reduction Mechanism 
       [0059]    A block diagram illustrating a cable modem system incorporating the upstream system of the present invention is shown in  FIG. 1 . The system, generally referenced  10 , comprises an operator portion  11  connected to the public switched telephone network (PSTN)  12  and the Internet  14  or other wide area network (WAN), a link portion  13  comprising the RF cable  28  and a subscriber portion  15  comprising the subscriber premises  34 . 
         [0060]    The operator portion  11  comprises the cable head-end  17  which is adapted to receive a number of content feeds such as satellite  16 , local antenna  18  and terrestrial feeds  26 , all of which are input to the combiner  24 . The cable head-end also comprises the voice over IP (VoIP) gateway  20  and Cable Modem Termination System (CMTS)  22 . The combiner merges the TV programming feeds with the RF data from the CMTS. 
         [0061]    The Cable Modem Termination System (CMTS) is a computerized device that enables cable modems to send and receive packets over the Internet. The IP packets are typically sent over Layer 2 and may comprise, for example, Ethernet or SONET frames or ATM cell. It inserts IP packets from the Internet into MPEG frames and transmits them to the cable modems in subscriber premises via an RF signal. It does the reverse process coming from the cable modems. A DOCSIS-compliant CMTS enables customer PCs to dynamically obtain IP addresses by acting as a proxy and forwarding DHCP requests to DHCP servers. A CMTS may provide filtering to protect against theft of service and denial of service attacks or against hackers trying to break into the cable operator&#39;s system. It may also provide traffic shaping to guarantee a specified quality of service (QoS) to selected customers. A CMTS may also provide bridging or routing capabilities. 
         [0062]    The subscriber premises  34  comprises a splitter  38 , cable appliances  36  such as televisions, DVRs, etc., cable modem  40 , router  48 , PCs or other networked computing devices  47  and telephone devices  51 . Cable service is provided by the local cable provider wherein the cable signal originates at the cable head end facility  17  and is transmitted over RF cable  28  to the subscriber premises  34  where it enters splitter  38 . One output of the splitter goes to the televisions, set top boxes, and other cable appliances via internal cable wiring  37 . 
         [0063]    The other output of the splitter comprises the data portion of the signal which is input to the cable modem  40 . The cable modem is adapted to provide both Ethernet and USB ports. Typically, a router  48  is connected to the Ethernet port via Ethernet cable  54 . One or more network capable computing devices  47 , e.g., laptops, PDAs, desktops, etc. are connected to the router  48  via internal Ethernet network wiring  46 . In addition, the router may comprise or be connected to a wireless access point that provides a wireless network (e.g., 802.11b/g/a) throughout the subscriber premises. 
         [0064]    The cable modem also comprises a subscriber line interface card (SLIC)  42  which provides the call signaling and functions of a conventional local loop to the plurality of installed telephone devices  51  via internal 2-wire telephone wiring  52 . In particular, it generates call progress tones including dial tone, ring tone, busy signals, etc. that are normally provided by the local loop from the CO. Since the telephone deices  51  are not connected to the CO, the SLIC in the cable modem must provide these signals in order that the telephone devices operate correctly. 
         [0065]    The cable modem also comprises a downstream system (not shown) and an upstream system  44  which incorporates the spur reduction mechanism of the present invention. A digital video output signal is displayed to the user (i.e. cable subscribers) via televison set  53  (i.e. video display device or other cable appliance). 
       DOCSIS 2.0 Channel Cable Modem 
       [0066]    A block diagram illustrating an example cable modem including an upstream system incorporating the spur reduction mechanism of the present invention is shown in  FIG. 2 . The cable modem, generally referenced  70 , comprises a duplexer  74 , CATV RF tuner circuit  76  incorporating, DOCSIS PHY (analog/digital)  78 , DOCSIS compatible processor  80 , DOCSIS MAC  82 , VoIP processor  108 , voice codec  110 , subscriber line interface card (SLIC)  112 , phone port  114 , wireless local area network (WLAN)  122  and associated antenna  120 , DECT  126  and associated antenna  124 , Bluetooth  130  and associated antenna  128 , Ethernet interface  96 , Ethernet LAN ports  98 , general purpose (I/O) (GPIO) interface  100 , LEDs  102 , universal serial bus (USB) interface  104 , USB port  106 , cable card/Downloadable Conditional Access Systems (DCAS)  92 , video interface (I/F)  94 , video processor  90 , upstream system  116  including spur reduction circuit  118 , AC adapter  134  coupled to mains utility power via plug  132 , power management circuit  136 , battery  138 , RAM  84 , ROM  86  and FLASH memory  88 . 
         [0067]    Note that in the example embodiment presented herein, the cable modem and DOCSIS enabled processor are adapted to implement the DOCSIS 2.0 standard. Although the invention is applicable to cable modems designed to implement this standard, the invention is not limited to use therein. It can be applied to other standards and systems, and should not be limited to use in the example cable modem application presented herein. 
         [0068]    In operation, the cable modem processor is the core chip set which in the example presented herein comprises a central single integrated circuit (IC) with peripheral functions added. The voice over IP (VoIP) processor  108  implements a mechanism to provide phone service outside the standard telco channel. Chipset DSPs and codecs  96  add the functionality of POTS service for low rate voice data. 
         [0069]    The cable modem also comprises a subscriber line interface card (SLIC)  112  which functions to provide the signals and functions of a conventional local loop to a plurality of telephone devices connected via the phone port  114  using internal 2-wire telephone wiring. In particular, it generates call progress tones including dial tone, ring tone, busy signals, etc. that are normally provided by the local loop from the CO. Since the telephone deices are not connected to the CO, the SLIC in the cable modem must provide these signals in order that the telephone devices operate correctly. 
         [0070]    In a traditional analog telephone system, each telephone or other communication device (i.e. subscriber unit) is typically interconnected by a pair of wires (commonly referred to as tip and ring or together as subscriber lines, subscriber loop or phone lines) through equipment to a switch at a local telephone company office (central office or CO). At the CO, the tip and ring lines are interconnected to a SLIC which provides required functionality to the subscriber unit. The switches at the central offices are interconnected to provide a network of switches thereby providing communications between a local subscriber and a remote subscriber. 
         [0071]    The SLIC is an essential part of the network interface provided to individual analog subscriber units. The functions provided by the SLIC include providing talk battery (between 5 VDC for on-hook and 48 VDC for off-hook), ring voltage (between 70-90 VAC at a frequency of 17-20 Hz), ring trip, off-hook detection, and call progress signals such as ringback, busy, and dial tone. 
         [0072]    A SLIC passes call progress tones such as dial tone, busy tone, and ringback tone to the subscriber unit. For the convenience of the subscriber who is initiating the call, these tones normally provided by the central office give an indication of call status. When the calling subscriber lifts the handset or when the subscriber unit otherwise generates an off hook condition, the central office generates a dial tone and supplies it to the calling subscriber unit to indicate the availability of phone service. After the calling subscriber has dialed a phone number of the remote (i.e. answering) subscriber unit, the SLIC passes a ring back sound directed to the calling subscriber to indicate that the network is taking action to signal the remote subscriber, i.e. that the remote subscriber is being rung. Alternatively, if the network determines that the remote subscriber unit is engaged in another call (or is already off-hook), the network generates a busy tone directed to the calling subscriber unit. 
         [0073]    The SLIC also acts to identify the status to, or interpret signals generated by, the analog subscriber unit. For example, the SLIC provides −48 volts on the ring line, and 0 volts on the tip line, to the subscriber unit. The analog subscriber unit provides an open circuit when in the on-hook state. In a loop start circuit, the analog subscriber unit goes off-hook by closing, or looping the tip and ring to form a complete electrical circuit. This off-hook condition is detected by the SLIC (whereupon a dial tone is provided to the subscriber). Most residential circuits are configured as loop start circuits. 
         [0074]    Connectivity is provided by a standard 10/100/1000 Mbps Ethernet interface  96  and Ethernet LAN port  98 , USB interface  104  and USB port  106  or with additional chip sets, such as wireless 802.11a/b/g via WLAN interface  122  coupled to antenna  120 . In addition, a GPIO interface  100  provides an interface for LEDs  102 , etc. The network connectivity functions may also include a router or Ethernet switch core. Note that the DOCSIS MAC  82  and PHY  78  may be integrated into the cable modem processor  80  or may be separate. 
         [0075]    In the example embodiment presented herein, the tuner  76  is coupled to the CATV signal from the CMTS via port  72  and is operative to convert the RF signal received over the RF cable to an IF frequency in accordance with the tune command signals received from the processor. 
         [0076]    The cable modem  70  comprises a processor  80  which may comprise a digital signal processor (DSP), central processing unit (CPU), microcontroller, microprocessor, microcomputer, ASIC, FPGA core or any other suitable processing means. The cable modem also comprises static read only memory (ROM)  86 , dynamic main memory  84  and FLASH memory  88  all in communication with the processor via a bus (not shown). 
         [0077]    The magnetic or semiconductor based storage device  84  (i.e. RAM) is used for storing application programs and data. The cable modem comprises computer readable storage medium that may include any suitable memory means, including but not limited to, magnetic storage, optical storage, semiconductor volatile or non-volatile memory, biological memory devices, or any other memory storage device. 
         [0078]    Any software required to implement the spur reduction mechanism of the present invention is adapted to reside on a computer readable medium, such as a magnetic disk within a disk drive unit. Alternatively, the computer readable medium may comprise a floppy disk, removable hard disk, Flash memory, EEROM based memory, bubble memory storage, ROM storage, distribution media, intermediate storage media, execution memory of a computer, and any other medium or device capable of storing for later reading by a computer a computer program implementing the system and methods of this invention. The software adapted to implement the spur reduction mechanism of the present invention may also reside, in whole or in part, in the static or dynamic main memories or in firmware within the processor of the computer system (i.e. within microcontroller, microprocessor or microcomputer internal memory). 
       Spur Reduction Mechanism 
       [0079]    In accordance with the invention, two solutions are presented to reduce or eliminate the spurious emission leakage problem whereby the clock (e.g., 35 MHz clock) that drives the PHY circuit leaks to the output of the PGA circuit causing the cable modem to violate DOCSIS limits on spurious emission levels. 
         [0080]    The first embodiment for reducing the spur power level comprises a passive cancellation circuit (PCC). This circuit uses the 35 MHz PHY clock, available from a dedicated pin on the PHY integrated circuit (IC), and creates from it a modified amplitude and phase cancellation signal. This cancellation signal is applied to the single ended output of the balun before the diplexer, thereby cancelling the spur power present at that point. 
         [0081]    The second embodiment for reducing the spur power level comprises an RF switch whereby an RF switch is inserted into the US path between the PGA&#39;s balun and diplexer. The RF switch provides wideband isolation between US transmission bursts, reducing the 35 MHz spur power, in addition to any additional power that may reside in the bandwidth of the transmitted signal. 
         [0082]    The first embodiment is preferred due its lower cost and is the more robust solution that meets DOCSIS specifications. The second embodiment has application in cases where there is concern for additional noise injection into the RF output from the PCB assembly. Each of the embodiments will now be described in more detail. 
       First Embodiment: Spur Cancellation Circuit 
       [0083]    A simplified block diagram illustrating the processor of the cable modem of  FIG. 2  including an upstream system incorporating a first embodiment of the spur reduction mechanism of the present invention is shown in  FIG. 3 . The example cable modem, generally referenced  150 , comprises diplexer  154  coupled to a CATV input  152 , RF tuner circuit  156 , processor  158  and upstream path circuit  116 . 
         [0084]    The upstream circuit  116  comprises image reject filter  172 , PGA  174 , balun  176  and spur cancellation circuit  177 . The processor  158  comprises an analog to digital converter (ADC)  160 , PHY circuit  162 , digital to analog converter (DAC)  170 , PGA control circuit  178 , power supply control  180  and MAC  168 . Power is supplied by an external power source  182  e.g., utility power, etc. or a battery  184 . 
         [0085]    In operation, in the downstream (i.e. receive) direction, the receive signal from the diplexer is input to the CATV RF tuner circuit  156 . The tuner output signal is input to the ADC to provide I and Q input signals to the PHY circuit. The PHY circuit provides a tuner control signal  157  that controls the tuning of the tuner circuit. After MAC processing, one or more MPEG video streams  169  are output of the cable modem. 
         [0086]    In the upstream (US) (i.e. transmit) direction, a digital TX output signal provided by the PHY circuit is converted to analog by the DAC. The analog signal is then filtered via the image reject filter  172  before being amplified by the PGA whose gain is controlled by a PGA control signal  173  generated by the PGA control circuit  178 . 
         [0087]    The output of the PGA circuit is input to one side of the balun  176 . The other side of the balun is input to the diplexer  154  which couples the US signal to the CATV cable  152 . In accordance with the invention, spur cancellation circuit  177  functions to substantially cancel the in-band spurious emissions from the US signal before input to the diplexer. 
         [0088]    The spur cancellation circuit is operative to adjust the amplitude and phase of the 35 MHz MPEG clock  179  such that when combined with the US signal, the spur signals are cancelled or substantially cancelled. Note that the spur cancellation circuit operates both during US transmission bursts and in between bursts. The 35 MHz MPEG clock is used to generate the cancellation signal assuming that the source of the spur is the PHY clock, which is based on the 35 MHz MPEG clock. It is appreciated that the source signal used to generate the cancellation signal is not limited to the clock shown in the example circuit presented herein but can be any clock or other signal source depending on the particular implementation of the invention. 
         [0089]    The spur cancellation circuit  177  is essentially a passive cancellation circuit (PCC). It is based on the assumption that the interfering frequency spur is narrow band and has predictable characteristics of frequency, phase and amplitude. The 35 MHz spur that is coupled to the output path of the PGA is dependent on the particular ground separation regime implemented and the 1.5 V digital power supply decoupling capacitor arrangement. It is preferable that there be a single solid ground (including the PGA ground) and to use decoupling capacitors of 1 nF or less on the digital 1.5 V power supply network. Experiments by the inventors have shown that this configuration results in a spur level of less then −55 dBmV. Use of the spur cancellation circuit of the present invention reduces this level further. 
         [0090]    The 35 MHz clock  179  (internal or external) is highly correlative with the DOCSIS PHY clock (which is the source of the spur). The spur cancellation circuit  177  functions to condition the amplitude and phase of the MPEG clock. After signal conditioning, the clock signal is applied to the output of the balun  176 . This reduces the level of the spur to a worst case of −61 dBmV and a typical level of −7 dBmV over temperature and sample variation and +/−5% voltage changes, which translates to a 6 dB mnimum/12 dB typical improvement. 
         [0091]    The MPEG clock output  179  is a digital 3.3 V peak-to-peak clock signal which translates to 64 dBmV. The amplitude of the cancellation signal  181  preferably should be equal to the amplitude of the spur, i.e. −55 dBmV. Thus, a relatively high attenuation of approximately 120 dB is required, The exact attenuation can be determined empirically for maximum cancellation. 
         [0092]    The phase of the cancellation signal  181  (relative to the MPEG clock) is set empirically for maximum cancellation. Measurements have shown that a good starting point is −95 degrees. 
         [0093]    A schematic diagram illustrating the spur cancellation circuit of  FIG. 3  in more detail in shown in  FIG. 4 . The circuit shown herein represents a preferred amplitude and phase conditioning scheme. It is appreciated that other schemes using other circuit topologies may be used to achieve similar results without departing from the scope of the invention. 
         [0094]    The circuit, generally referenced  177 , comprises resistors R 1 , R 2 , R 3 , R 4 , R 5  and capacitors C 1 , C 2 . Example values of the resistor and capacitor component values for circuit  177  are given below in Table 3. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Example component values 
               
             
          
           
               
                   
                 Component 
                 Value 
               
               
                   
                   
               
               
                   
                 R1 
                  100 kOhm 
               
               
                   
                 R2 
                 1000 Ohm 
               
               
                   
                 R3 
                  100 kOhm 
               
               
                   
                 R4 
                 1000 Ohm 
               
               
                   
                 R5 
                 3320 Ohm 
               
               
                   
                 C1 
                   6 pF 
               
               
                   
                 C2 
                   6 pF 
               
               
                   
                   
               
             
          
         
       
     
         [0095]    The variance of the components may be configured such that its impact on the overall spur cancellation is negligible. For example, consider resistor accuracy of 1% and capacitor accuracy of 5%. Under these conditions, the component variance results in an amplitude variation of +/−0.6 dB and phase variation of +/−3 degrees. The variation in amplitude translates to a cancellation limitation of −23.5 dB and phase variation translates to a cancellation limitation of −25.62 dB. The total cancellation limitation (i.e. amplitude and phase) is −21.dB, i.e. −76 dBmV. 
         [0096]    Note that the impedance looking into the circuit  177  is 1 Ohm. The output impedance is 75 Ohm (i.e. the characteristic impedance of the cable modem) to match the impedance of the CATV cable. 
         [0097]    In operation, resistors R 1 /R 2  and R 3 /R 4  form voltage dividers which function to significantly attenuate the MPEG clock signal. Capacitors C 1  and C 2  function to shift the phase of the MPEG clock signal. The result is a cancellation signal having a phase opposite that of the spur. When combined to the output of the balun, the spur is reduced sufficiently to meet DOCSIS requirements. 
         [0098]    This approach to spur reduction has several advantages, including (1) very low cost (i.e. only a few resistors and capacitors are required); (2) relatively very quick implementation; and (3) no need for external control such as is required in the case of an RF switch (second embodiment). 
         [0099]    A block diagram illustrating a model of a portion of the upstream circuit without the spur cancellation mechanism of the present invention is shown in  FIG. 5 . The circuit, generally referenced  200 , comprises SRC 1  (the source of the spur, the PHY clock), R 6  (the balun impedance) having a 75 Ohm impedance and R 7  (representing the CATV load). 
         [0100]    A time domain plot of the spur without the spur cancellation mechanism of the present invention is shown in  FIG. 8 . The amplitude of the spur is 1.78 μV=20 log 10 (1.78 μV/0.001 mV)=−55 dBmV. A frequency spectrum of the spur of  FIG. 8  is shown in  FIG. 9 . The amplitude of the spur is −55 dBmV at 35 MHz. 
         [0101]    A spectrum plot illustrating frequency response of the spur cancellation circuits of  FIGS. 5 and 6  is shown in  FIG. 7 . The response of the circuit  200  of  FIG. 5  (without the spur cancellation mechanism) is measured across resistor R 7  (V_BALUN_OUT_NO_CANCEL) and is shown in trace  220 . The response is relatively flat at −55 dBmV. Dashed line  224  represents the DOCSIS 2.0 specification for spur level (−59 dBmV, see Table 1). Thus, the response of circuit  200  fails to meet the DOCSIS 2.0 specifications. 
         [0102]    A block diagram illustrating a model of a portion of the upstream circuit incorporating the spur cancellation mechanism of the present invention is shown in  FIG. 6 . The circuit, generally referenced  210 , comprises SRC 2  (MPEG clock source), coupling capacitor C 3 , spur cancellation circuit  177 , SRC 3  (spur source, PHY clock), R 13  (balun), R 14  (CATV load). The spur cancellation circuit  177  comprises resistors R 8 , R 9 , R 10 , R 11 , R 12 , and capacitors C 4 , C 5 . 
         [0103]    Example values of the resistor and capacitor component values for the circuit  177  are given below in Table 4. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Example component values 
               
             
          
           
               
                   
                 Component 
                 Value 
               
               
                   
                   
               
               
                   
                 R8 
                  100 kOhm 
               
               
                   
                 R9 
                 3500 Ohm 
               
               
                   
                 R10 
                  100 kOhm 
               
               
                   
                 R11 
                 3500 Ohm 
               
               
                   
                 R12 
                 3320 Ohm 
               
               
                   
                 C4 
                   3.9 pF 
               
               
                   
                 C5 
                   3.9 pF 
               
               
                   
                   
               
             
          
         
       
     
         [0104]    The value of the coupling capacitor C 3  is 100 nF. Resistor R 13  represents the impedance of the balun which is 75 Ohm while resistor R 14  represents the cable modem load which is 75 Ohm (to maximize power transfer). 
         [0105]    A time domain plot of the spur with the spur cancellation mechanism of the present invention is shown in  FIG. 10 . The amplitude of the spur is approximately 34 nV=20 log 10 (34 nV/0.001 mV)=˜−90 dBmV. A frequency spectrum of the spur of  FIG. 10  is shown in  FIG. 11 . The amplitude of the spur is −95.6 dBmV at 35 MHz, which represents an improvement of approximately 40 dB over the circuit without the spur cancellation circuit. 
         [0106]    Referring to  FIG. 7 , the response of the circuit  210  of  FIG. 6  (with the spur cancellation mechanism) is measured across resistor R 14  (V_BALUN_OUT_CANCEL) and is shown in trace  222 . The response is a notch with a minimum at −95.5 dBmV which is an improvement of over 40 dBmV compared to the response of circuit  200  ( FIG. 5 ). Thus, the response of circuit  210  meets the DOCSIS 2.0 specifications. Note that the response of  FIG. 7  is the results of a simulation. In actuality, the improvement of circuit  210  with the spur cancellation circuit over the circuit  200  without it may be only −15 to −20 dBmV. Thus, the notch minimum would be at approximately −70 dBmV, still well below the maximum level permitted by the DOCSIS 2.0 specification. 
       Second Embodiment: RF Switch Circuit 
       [0107]    A simplified block diagram illustrating the processor of the cable modem of  FIG. 2  including an upstream system incorporating a second embodiment of the spur reduction mechanism of the present invention is shown in  FIG. 12 . The example cable modem, generally referenced  150 , comprises diplexer  154  coupled to a CATV input  152 , RF tuner circuit  156 , processor  158  and upstream path circuit  116 . 
         [0108]    The upstream circuit  116  comprises image reject filter  172 , PGA  174 , balun  176  and RF switch  187 . The processor  158  comprises an analog to digital converter (ADC)  160 , PHY circuit  162 , digital to analog converter (DAC)  170 , PGA control circuit  178 , switch control circuit  183 , power supply control  180  and MAC  168 . Power is supplied by an external power source  182  e.g., utility power, etc. or a battery  184 . 
         [0109]    In operation, in the downstream (i.e. receive) direction, the receive signal from the diplexer is input to the CATV RF tuner circuit  156 . The tuner output signal is input to the ADC to provide I and Q input signals to the PHY circuit. The PHY circuit provides a tuner control signal  157  that controls the tuning of the tuner circuit. After MAC processing, one or more MPEG video streams  169  are output of the cable modem. 
         [0110]    In the upstream (US) (i.e. transmit) direction, a digital TX output signal provided by the PHY circuit is converted to analog by the DAC. The analog signal is then filtered via the image reject filter  172  before being amplified by the PGA whose gain is controlled by a PGA control signal  173  generated by the PGA control circuit  178 . 
         [0111]    The output of the PGA circuit is input to one side of the balun  176 . The other side of the balun is input to a single pole double throw (SPDT) RF switch  187 . One terminal of the switch is coupled to the output of the balun while the other terminal is connected to ground via 75 Ohm resistor  189 . The output of the RF switch is input to the diplexer  154  which couples the US signal to the CATV cable  152 . 
         [0112]    In accordance with the invention, the RF switch  187  functions to eliminate in-band spurious emissions from the US signal before input to the diplexer. The second embodiment is based on a wide band isolation RF switch which limits the 35 MHz spur and any additional spurs from leaking to the RF output of the cable modem in between transmission bursts. 
         [0113]    An example RF switch suitable for use with the present invention is the AS211-334, PHEMT GaAs IC SPDT Switch, manufactured by Skyworks, Woburn, Mass., USA. The RF parameters, including linearity, insertion loss and switching performance make this RF switch suitable for use in the example circuit presented herein. It is appreciated that other components with similar parameters may be used. 
         [0114]    In this example circuit, the RF switch is controlled by a switch control signal  185  generated by the switch control circuit  183  internal to the processor  158 . The switch control circuit is operative to couple the output of the balun to the diplexer during transmission bursts and to the resistor coupled to ground in between transmission bursts. 
         [0115]    Note that this second embodiment takes advantage of the relaxed spurious emission levels permitted by the DOCSIS 2.0 during transmission bursts as compared to between bursts (see Tables 1 and 2 presented supra). 
         [0116]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof 
         [0117]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. As numerous modifications and changes will readily occur to those skilled in the art, it is intended that the invention not be limited to the limited number of embodiments described herein. Accordingly, it will be appreciated that all suitable variations, modifications and equivalents may be resorted to, falling within the spirit and scope of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.