Reduction of aggregate emi emissions of multiple transmitters

Abstract

A power efficient and reduced electromagnetic interference (EMI) emissions multi-transmitter system for unshielded twisted pair (UTP) data communication applications. For each transmitter, digital transmit data is converted to a current-mode differential signal analog waveform by a digital-to-analog converter (DAC). The output current from each DAC is used to generate the required transmit voltage on the respective UTP line. Timing circuitry staggers the time base of each transmitter to reduce the aggregate EMI emissions of the multi-transmitter system.

1 . A data communication device comprising:
 a plurality of transmitters for driving a plurality of transmission channels; and   a plurality of I/O circuits electrically coupled to the plurality of transmitters wherein, the I/O circuits define a plurality of time staggered output signals.   
     
     
         2 . The data communication device of  claim 1 , wherein the I/O circuits drive the output of each of the plurality of transmitters by a respective staggered output signal in such a way as to reduce transmission signal harmonics causing electromagnetic emission. 
     
     
         3 . The data communication device of  claim 2 , wherein the plurality of transmitters include M transmitters with a data sampling frequency of Fi, and the plurality of output signals are staggered in time by 1/(Fi*M). 
     
     
         4 . A low aggregate electromagnetic emission transmission system comprising:
 a plurality of transmitters;   means for generating a plurality of timing references wherein each of the plurality of timing references is staggered in time by a time duration which is a function of a transmission data rate and the number of the transmitters in the system; and   means for driving the output of each of the plurality of transmitters by one of the staggered timing references, respectively.   
     
     
         5 . The system of  claim 4 , wherein the means for generating a plurality of timing references includes a phase-locked loop driven by a master clock. 
     
     
         6 . The system of  claim 4 , wherein the means for generating a plurality of timing references includes a plurality of delay circuits. 
     
     
         7 . The system of  claim 4 , wherein signal harmonics generating a portion of the emission are centered around integer multiples of a data sampling frequency Fi and the plurality of transmitters comprises M transmitters, and wherein the staggered time duration is 1/(M*Fi). 
     
     
         8 . The system of  claim 4 , wherein the data rate is 125 MHZ. 
     
     
         9 . The system of  claim 4 , wherein the transmission system has four transmitters and the data rate is 20 MHZ. 
     
     
         10 . The system of  claim 4 , wherein the transmission system has four transmitters and the data rate is 25 MHZ. 
     
     
         11 . A low aggregate electromagnetic emission transmission system comprising:
 a plurality of transmitters; and   means for driving the output of each of the plurality of transmitters to produce a plurality of transmitted analog signals, each of the analog signals having high frequency signal components, and each high frequency signal component being phase shifted by 360°/M relative to high frequency signal component of another analog signal, where M is the number of transmitters.   
     
     
         12 . A low aggregate electromagnetic emission transmission system comprising:
 a plurality of analog transmitters;   means for switching each of the transmitters on, to generate an analog transmit signal, at a different time from the other transmitters, the interval between switching transmitters being inversely proportional to the number of transmitters.   
     
     
         13 . The system of  claim 12 , wherein the interval between switching transmitters is also inversely proportional to a transmission data rate. 
     
     
         14 . The system of  claim 13 , wherein the interval between switching transmitters is proportional to 1/M*Fi, where M is the number of transmitters and Fi is the data sampling frequency. 
     
     
         15 . A method for reducing aggregate electromagnetic emission caused by signal images centered around integer multiples of a data sampling frequency Fi in a transmission system having M transmitters, said method comprising the steps of:
 generating M timing references staggered in time by 1/(Fi*M); and   driving the output of each of the M transmitters by one of the M staggered timing references, respectively.   
     
     
         16 . A method for reducing aggregate electromagnetic emission caused by signal harmonics of a plurality of transmitters in a transmission system comprising the steps of:
 generating a plurality of timing references staggered in time; and   driving the output of each of the plurality of transmitters by one of the respective staggered timing references in such a way as to reduce the signal harmonics causing the emission.   
     
     
         17 . The method of  claim 16 , wherein the transmission system includes M transmitters, the data sampling frequency is Fi, and the plurality of timing references are staggered in time by 1/(Fi*M).