CREST FACTOR COMPENSATED DRIVER

Abstract: A telephone line feed circuit for use with a telecommunication line has dual drivers with differing ratio transformer taps (72) and a sensing circuit so that under normal drive conditions a first driver supplies substantially all power through a first transformer of a first turn ratio (Tsp1) under normal in put signal conditions and a second driver supplies substantially all power through a second transformer of second turns ratio (Tsp2) under high crest factor input signal conditions.

Description: CREST FACTOR COMPENSATED DRIVER BACKGROUND OF THE INVENTION This invention relates to a method and system for reducing power consumption in a line driver used in telecommunication systems. Efforts have been made by manufacturers of Digital Subscriber Line (DSL) hardware to reduce overall power dissipation of DSL line cards. Although overall power reduction improvements have been made, significant power improvements in the field of line drivers have evidently not yet been made. Known relatively minor improvements have been due to crest factor reductions that in turn have precipitated a slight driver voltage rail reduction at the expense of processing speed, or so-called DSP MIPS. For all these applications the driver power has not significantly improved. To make DSL technology attractive, the overall power dissipation must be reduced beyond the presently offered solutions. This power dissipation manifests itself in the form of increased operation temperature of the equipment. A number of designs constraints are introduced in order to maintain circuits at a reasonable operating temperature, including the inclusion of additional fans, air conditioning, heat sinks and space for thermal ventilation. These constraints significantly increase the material, labor and maintenance cost associated with the system. Furthermore, excessive heat may restrict the density of equipment, thereby increasing the size of the facility hosting the system and/or limiting the number of customers that can be served by a fixed size facility. Thus, reducing the power consumption in communications systems can be a key aspect of any system design. The line driver is a component that consumes a significant amount of power. Typically, the line driver includes an amplifier for receiving an analog signal from a preceding circuit, such as a digital to analog (DA) converter, to drive this signal through a source resistance and a line transformer onto a twisted pair telephone line or loop. The DSL line drivers commonly in use include a source resistance equal to the reference impedance of the loop, usually 100 ohms implemented as a series resistance. Typical line drivers use two amplifiers working differentially. A factor that exacerbates driver design is the presence of statistically infrequent high crest-factor occurrences in the received signal to be driven onto the loop. One solution is to power the amplifiers from a single bipolar high voltage rail, typically +15V and -15V The voltage being chosen so that the signal being driven is not clipped at the high crest factor occurrences In this cases a large portion of the power is dissipated in the real source resistance used to match the impedance on the line Another solution uses a differential voltage rail, which requires the addition of a second voltage rail thereby increasing the required rails from two to four The rails could be +12V(or +15V), -12V(or -15V), +5V and -5V The high voltage rails handle the high crest factor occurrences, whereas the low voltage rails handle the normal signal condition For example Texas Instruments THS6032 is an ultra low-power, differential line driver designed for Central Office (CO) ADSL applications The THS6032 has Class-G architecture that enables the device to be powered from both low voltage and high voltage power supplies The voltage rails are switched when low probability peak power events occui In this way the low voltage rails are used most of the time thereby keeping power low The high voltage rail is switched to gracefully when large signal levels are transmitted In this way the high power dissipation is only encountered when actually required The application of such an amplifier m xDSL modems is disclosed in US Patent 5,898,342 The above solution is costly both from a direct cost and from a board space point of view Accordingly there is a need for a low po er drive stage that is capable of handling high crest factors while mitigating at least some of the above disadvantages SUMMARY OF THE INVENTION According to the invention, a telephone line feed cncuit is piovided for use with a telecommunication line which has dual drivers with transformer taps and a sensing circuit so that under normal drive conditions a first driver supplies substantially all powei through a first transformer of a first turns ratio under normal input signal conditions and a second driver supplies substantially all power through a second transformer of a second turns ratio under high crest factor input signal conditions More specifically, a first driver circuit has an output connected via a first transformer having a first turns ratio to a tip terminal and a ring terminal of a telecommunication line, a second driver circuit has an output connected via a second transformer with a second turns latio to the tip terminal and the ring terminal of said telecommunication line, and a switching circuit is provided sense and switch the input signal to the driver circuits so that substantially all of the drive power is provided by the first driver under noπΗal input signal conditions and substantially all of the dπve power is provided by the second driver under high crest factor input signal conditions, the turns ratios of the two transformers having been selected such that the signal driven onto the line is not clipped In an embodiment the driver circuits includes an active impedance synthesis circuit for generating the line impedance for DSL signals In a further embodiment of the invention the line feed circuit is coupled to an integrated POTS/xDSL line card These and other features of the embodiments of the invention will become more apparent m the following detailed description in which reference is made to the appended drawings BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a line feed circuit with a series source resistance according to the prior art Figure 2 is a schematic diagram of a dnect-coupled line feed circuit Figure 3 is a schematic diagram of a dual powei supply line feed circuit Figure 4 is a schematic diagram of a dual transformer driv e line feed circuit Figure 5 is a schematic diagram showing an implementation of the circuit of Figure 4 Figures 6(a)-6(d) are voltage and cuπent waveforms for the circuit of Fiεure 5 Figure 7 is schematic diagram of a further implementation of the cncuit of Figure 4 DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to Figure 1, a line feed circuit 10 according to the pπor art is shown generally by numeral 10 The line feed circuit 10 includes driver stage 12, feed resistors 16 and a line transformer 18 The driver stage 12 normally comprises two dnvei amplifier circuits 14 and 16 (normally fixed-gain amplifiers) for supplying a subscriber line comprising tip T and ring R lines, via lespective series feed resistors Rj. 18 A load resistance R \ normally terminates the line Foi xDSL signals the line is normally terminated by a load resistance R of typically 100 ohms, thus the series feed resistors Rj are chosen to equal the lefeicnce impedance of the loop, I e 50 ohms each Other values of the feed resistors are chosen for different signals such as POTS signals. Furthermore as illustrated in Figure 1, the drive amplifiers 14 and 16 are each powered by (+)V supp i y voltage rails referenced to ground. Alternatively the drive amplifiers may be powered by (-)V supp i y /2 and (+)V supp ι y /2 voltage rails. For convenience the following definitions of specific terms that are used in the following description are provided: V h - total voltage headroom required on side of the voltage rail for allowing proper biasing of the drive amplifiers, typically 3V/2; R s = total source resistance; R tr = tip and ring load resistance, 100 ohms for xDSL; V tI = tip and ring voltage, 2V rms; I tr = tip and ring current, 20 mA rms (or 16.2 dBm into 100 ohms); σ = CF = crest factor, which is the ratio of the peak signal to the rms signal ( typically 5.3 for the g.Lite standard); R = load impedance; Rj.- = feed impedance; n = transformer turns ratio; Vp d = peak output voltage at one driver; P t = total power for two driver amplifiers; V p _ p = total peak-to-peak voltage at output of drivers; I h = quiescent bias current per drive amplifier; s upph = total driver amplifier supply voltage across supply rails; and V2/π = (average DSL current )/(rms DSL current). Referring back to the circuit of Figure 1 , the peak voltage at one driver may be derived as follows: V p , d = (R F + R L )/(n*Rι )*V t ,/2*σ; the supply voltage rails required, given the desired voltage headroom above is then given by: V SUpp , y = (V h + V p/d )*2. The total differential driver power consumption for xDSL signals can be defined in terms of the driver supply voltage and the required current to be driven onto the line, as follows: P t = V supplv *(V2/π*n*V lr /R L + 2 *I b ) P dm e = 2*(V h + ((R s +R tr )/(n*R tl )*V tl Λ2 * σ)*(V2/π*nV tr /R L + 2*I b ) (1) The supply voltage V supp ] Y is a function of the total resistance, the current and the ciest factoi (signal excursion) requirement Power consumption can be calculated for the circuit m Figure 1 assuming C F = 5 3, V h = 1 5V, R F = 100 Ω, and R L = 100 Ω Then using equation (1 ) the driver power P dr ,ve = 2*(1 5V + (2 * 2V rms /2 * 5 3)) * V2/π* 2/100 +2*7 mA - 557 mW One solution is to reduce the nominal DSL dnver power consumption (ideally by 50%) by driving the DSL signal directly onto the line and not through a source resistance A feedback circuit actively generates the souice lesistance Accordingly, referring to Figure 2, a low power drive stage is shown geneially by numeial 20 The drive stage 20 includes two drive amplifiers shown schematically by block 22 the outputs of which are each directly coupled to the secondary winding 25 of a 1 1 line transformer 28, the primary 29 of which is coupled to tip and ring lines The amplifier 22 includes an impedance generation circuit for maintaining an optimum power transfer to the line by actively generating the equivalent source resistance 31 As illustrated the impedance generation circuit includes a feedback of the voltage or the cun ent sensed, which is combined with the signal to be transmitted and fed through either a current amplifier oi a v oltage amphfiei driving the line The feedback is lllustiated as being taken from the primary side of the transformer, however, it ma equally well be taken from the secondary side of the transformer For further details on directly coupling the driver stages to the line with active impedance generation, reference may be made to copendmg United States Patent Application, entitled "REDUCED POWER LINE DRIVER", which is in the name of the present inventors and which is assigned to the assignee of the subject application, and fully incorporated herein Power consumption for the circuit 20 of Fig e 2 is calculated as follows, assuming R r = 0— because the driver circuit 22 is coupled directly to the transformer 29 — and R| = 100 Ω, then from equation 1 P d t = 2 + (l 5V+ (1 * 2Vrms/2 + 5 3 2)) *V2/π* 2/100 + 2*7 mA = 313 mW Referring to Figure 3, a direct coupled line feed circuit 35 as illustrated in Figure 2 is used; however in this embodiment of circuit 35 a dual power supply is provided to the amplifiers, similar to a class G arrangement. Referring now to Figure 4, a generalized embodiment of a dual transformer driver circuit according to an embodiment of the present invention is shown generally by numeral 40. The circuit 40 comprises first and second dual amplifiers 42 and 44, the outputs of which are coupled to respective first 48 and second secondary 50 windings of a line transformer 46, and attenuation circuit 52 for attenuating an input signal T x received from an xDSL circuit (not shown) which is applied to the input of the second amplifier 44. The circuit 40 also includes a switching circuit 54 for monitoring the output signal from the xDSL circuit and applying the signal to the first amplifier 42 upon the occurrence of a statistically low crest factor signal and applying the output signal to the attenuator 52 coupled to the second amplifier 44 upon the occurrence of a statistically high crest factor signal. The ratios of the first secondary winding to its corresponding primary winding is chosen to be 1 : 1 (nl) while the ratio of the second secondary winding to its primary winding is chosen to be in the ratio of 1 :3 (n2). It may be noted that the switching circuit may be placed at the output of the drivers or the drivers may be designed so that the second set of drivers exhibit a high output impedance when the first set of drivers are driving the line. The circuit configuration 40 is shown with the amplifiers connected to their respective secondary windings without the use of typically serially connected feed resistors RF. In this implementation, it is assumed that the amplifiers may incorporate an impedance generation circuit as described with reference to Figure 3 above. Furthermore, the implementation shown in Figure 4 is used to facilitate an understanding of the operation of the circuit. The aπangements of this implementation are discussed hereinafter. Referring back to Figure 4, the general operation of the circuit may be explained as follows. In operation, the switch circuit 54 allows normal low level signals from the xDSL equipment to pass to the primary drive source (the first driver) 42 through the transformer 46 using a turns ratio nl in the order of 1 : 1. This is a relatively low power circuit and need be supplied only from a single low voltage power supply. Upon the occurrence of high crest factor signals, the switching circuit automatically diverts the signals to the second driver 44 which, because it is driving an attenuated version of these high crest factor signals, need only be supplied from the same low voltage supply However, because of the higher turns ratio n2 in the order of 1 3, the voltage output on the tip and πng line is amplified by the appropriate amount The signal diversion is invoked by causing the first driver to go to a high impedance state from its normal low impedance state when the second dπver is providing the power to the loop, effectively opening the one circuit when the other circuit is closed Similarly under low signal excursion the second driver goes to its high impedance state when the first driver is providing the power Thus, the second driver amplifier 44 dissipates only quiescent power when the primary drivers 42 are in control of the loop The secondary power driver dissipation is only high when the primary drivers are not driving the loop The second set of drivers are transparent to the primary set of drivers when the primary set of dπvers are driving the loop Turning now to Figure 5, a schematic diagram of a dual drive circuit, implemented accoiding to the embodiment of Figure 4, is shown generally by numeral 70 An output signal T x is coupled through an input resistor R, n and a buffer amplifier 71 into a fust pair of drive amplifiers D p) , the outputs of which are coupled through a respective feed resistance R d ι across the ends T ι of a secondary w inding of the line transform ei 72 The secondary w inding of the nans former is partitioned by taps T SP I and T p 2 , such that each of the partitions having a secondary primary turns ratio of 1 3 (n2) The primary winding of the transformer is coupled to the respective tip T and ring R lines of the loop The first pan of drive amphfieis D p ι thus see an effectiv e turns ratio of 3 3 (nl ) A second pan of drivers D p have then inputs connected by a resistive di idei netwoi k 72 hich performs the attenuation function m the input signal T The outputs Irom the second pan of drivers D p -> aie each connected to a lespective resistor R p2 which is in turn connected to ones of the taps T P I or T SP2 on the secondary winding Thus the second pair of drivers see an effective turns ratio n2 between the secondary and pπmaiy of 1 3 An a c signal feedback path is implemented by a resistor R f and a capacitor C t coupled m series from the primary of the transformer to the input of the buffer amplifier 71 The opeiation of the circuit may be moie clearly understood by referring to the voltage and current waveforms shown in Figures 6(a) - 6(d) at various points as indicated in the circuit of Figure 5 Specifically Figures 6(a) and 6(b) show respective v oltage and current waveforms under overdrive conditions and Figure 6(c) and 6(d) show respective voltage and current waveforms at points in the circuit under normal operating conditions Under normal operating conditions, the first pair of driver amplifiers D pϊ provide substantially all the drive across the secondary windings of the transformer 72 The second pair of drivers D p2 under these conditions receive a scaled version of the input signal due to the resistive divider network R sl and R s2 Thus, the first set of drivers D p] prov ides a normal current to the secondary winding of the transformer and the D p2 drivers ideally appear invisible m the circuit That is ideally no current flows through R p] and R p2 when the D pl drivers are in contiol Under overdrive conditions, 1 e , at high crest factor occurrences, the drivers D p ι tend to saturate as shown in the voltage plot V DPI of Figure 6(a) However, the drivers D p2 which receive a scaled version of the input signal T do not saturate and prov ide an undipped output V pπ2 as shown m Figuie 6(a) This voltage is then stepped- up by the transformer due to the 1 3 turns ratio The voltage output V out on the T and R line under overdπv e conditions is shown bv the plot 88 in Figure 6(a) Thus, it may be seen that when statistically low occurrence peaks occur, the D p] amplifiers run out of drive capability and the scaled D p2 dnvers take over the sourcmg of current Effective turns ratio for D p2 dnvers to the primary of the transformer is 1 3 In this way, a relativ ely low voltage supply is able to power the dπv ers Furthermoie, when the D p) dnv ers are sourcing current, the entne secondaiy side of the winding is used to couple the signal to the primary side Referring to Figure 7 a variation of the abov e circuit is shown generallv by numeral 80 In this circuit 80 a separate set of secondarv w indings rathei than a tapped secondary is used for the secondaiy dπveis Once again, as with the circuit shown in Tiguie 5, the effective turns ratio nl of the first transformer is 1 1 and the effectiv e turns ratio n2 of the secondaiy transfonrier is 1 3 The operation of the circuit is otherwise the same as above In the circuits of Figures 5 and 7, it is assumed that the dnvers are powered by a single rail power supply Because large crest-factor signals occur statistically infrequently, the power consumed at these occurrences is negligible in comparison to normal conditions Thus simply adding the P dp ι power to the P dp 9 bias power approximates the total po ei consumed The power consumption for the circuits 70 and 80 is, as given by the following equations Pj p (2)bιas = total bias powei foi both dp2 drivers P dP ι(2) = total power for both dpi drivers in this case a=V2=l .414 Using Equation 1 Pd P ι(2) = 2* (1.5V +(l *2Vιms/2* 1.414))* V2/IT 2/100+2.7mA) = 5.83V *(9mA + 14mA) = 134 mW Vsupply = 5.83 V P dp2 (2)bias = V supp ι y *2I b , as = 5.83V* 14mA = 82mW Ptotai = Pd P ι(2) + P dp2 (2)bias = 134 mW + 82 W = 216 mW Power comparison : 216/313 = 67% The number of voltage rails required is one, in comparison to other solutions which require two or four voltage rails. Rail reduction 2/1 or 4/1 = 2 times or 4 times. A comparison between the power consumption of traditional class G amplifiers and a circuit according to the present invention is shown as follows: Class G approach biasing current effect: V C c h , c h = bias high voltage and current V C ci, JL I = bias low voltage and current Pb.a = (Lch * V LCh ( total) + L , * V CLl (total)r # of amplifiers = (4 ma * 30V + 5 ma * 10V) * 2 (120 mW + 50mW)*2 340 mW Dual drive, according to an embodiment of the present invention, with transformer taps approach' I tl. , V L = supply bias current and voltage Pb, ai = L c * V cc (total) * # of amplifiers 9ma * 5V * 4 180 ma The bias power savings: 180/340 = 53% The circuits illustrated herein are merely exemplary and may be implemented in other forms as will be evident to those skilled in the art. Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.

Claims: WHAT IS CLAIMED IS 1 A telephone line feed circuit for use with a telecommunication line, "> comprising 3 a) a first driver circuit having an output connected via a first transformer 4 having a first turns ratio to a tip terminal and to a ring terminal of said telecommunication 5 line, 6 b) a second driver circuit having an output connected via a second 7 transformer having a second turns ratio to the tip terminal and the ring terminal of said 8 telecommunication line, and 9 c) a s itching circuit arranged to provide an input signal to said driver0 en cuits so that substantially all of the drive power is provided bv said fust driver circuit 1 under normal input signal conditions and substantially all of the dπve power is provided2 by the second driver circuit under high ciest factoi input signal conditions, the turns ratios 3 of the first transformer and of the second tiansformer being selected such that signals4 driv en onto the line via said first dπvei circuit and said second dnvei circuit aie not5 clipped 1 2 A cncuit as defined in claim 1 , said dnvei circuits being operative "> to receiv e si g nals fiom an integrated POTS and xDSL line card J 1 3 A circuit as defined m claim 1 , said dnvei circuit including an ? activ e impedance synthesis circuit foi generating line impedance foi DSL signals 1 4 A circuit as defined m claim 3, wherein said impedance synthesis 2 circuit is operative to synthesize an impedance by sensing a current m the 3 telecommunication line and driving a voltage source with a corresponding voltage 1 5 A circuit as defined in claim 4, wherein said current being sensed is 2 on the secondary side of one of said transformers 6 A circuit as defined in claim 4, wherein said current being sensed is ? on the pπmai y side of one of said tiansformers 7 A circuit as defined m claim 5, including a sense resistor coupled in series in said line 8 A circuit as defined m claim 6, including a sense resistor coupled m series m said line 9 A circuit as defined in claim 5, including a sense transformer coupled m senes in said line 10 A circuit as defined in claim 6, including a sense transformer coupled in series in said line 11 A circuit as defined m claim 3, wherein said impedance synthesis circuit is operative to synthesize an impedance by sensing a voltage on the telecommunication line and dnvmg a cunent souice w ith a coπespondmg current 12 A circuit as defined in claim 1 1 , wherein said voltage being sensed is on the secondary side of one of said first and second transformers 13 A circuit as defined in claim 1 1, wherein said voltage being sensed is on the primary side of one of said first and second transformers 14 A circuit as defined m claim 1 wherein said first driver circuit is switched to a high impedance state from a low impedance state while said second driver circuit is providing power under high crest factor signal input conditions, and wherein said second dπver circuit is switched to a high impedance state from a low impedance state while said first driver circuit is providing power low signal excursion conditions 15 In telephone line feed circuit for use with a telecommunication line, said line feed circuit including a first dπver circuit having an output connected via a first transformer having a first turns ratio to a tip terminal and to a ring terminal of said telecommunication line, a second driver circuit having an output connected via a second tiansformer having a second turns ratio to the tip terminal and the ring terminal of said telecommunication line, and a switching circuit, a method for inhibiting signal clipping comprising providing substantially all of an input signal at normal input signal conditions by said first driver circuit, and providing substantially all of said input signal under high crest factor input signal conditions by said second driver circuit, wherein the turns ratios of the first transformer and the second transformers being selected such that the signal driven onto the line is not clipped 16 The method according to claim 15 further including actively synthesizing impedance in said first and second driver circuits foi generating line impedance foi DSL signals 17 The method according to claim 16 wherein said impedance synthesizing step includes synthesizing an impedance by sensing a current in the telecommunication line and driving a voltage source with a coπesponding voltage 18 The method according to claim 16 wherein said impedance synthesizing step includes synthesizing an impedance by sensing a voltage on the telecommunication line and driving a current source with a corresponding current 19 The method according to claim 16 wherein said first dπv ei circuit is switched to a high impedance state from a low impedance state while said second driver circuit is providing power under high crest factor signal input conditions, and wherein said second driver circuit is switched to a high impedance state from a low impedance state while said first driver circuit is providing power

Inventors: BISSON ROBERT, TREMBLAY FRANCOIS, BISSON, ROBERT, TREMBLAY, FRANCOIS
Applicants: BISSON ROBERT, CATENA NETWORKS INC, TREMBLAY FRANCOIS, BISSON, ROBERT, CATENA NETWORKS, INC., TREMBLAY, FRANCOIS
Application year: 2000
Main classification: H04B3
IPC codes: H04B1, H04B3, H04L27, H04M11, H04Q3