[Design Application]
Optimizing Performance In An xDSL Line Driver
A Simple Analysis Supplies An Output Amplifier's Voltage And Current Needs As A Function Of The Line Transformer's Turns Ratio.
In the rush to deliver broadband services through the installed copper infrastructure, it becomes critical to accurately assess the true trade-offs in the line-driver design. Starting from the required line power and crest factor, a simple analysis will give the required output amplifier voltage and current requirements as a function of the transformer turns ratio going out onto the line. Optimizing the design then requires an assessment of the internal driver power dissipation and harmonic distortion.
This optimization is done as the driver operating point is adjusted to meet the varying peak load voltage and current requirements versus the turns ratio. Once the basic driver design is set, other features, like an active filter, can be added to the driver.
In any line-driver design, the most fundamental starting point is the voltage that must be delivered onto the line. In the alphabet soup of DSL, the specific modulations, target BER, and frequency allocations for each version are hotly debated issues that strongly influence the line-voltage requirements. For a general-purpose line-driver discussion, we'll assume that the targets are given by the application. The importance of these assumptions, however, will be clearly illustrated in the driver internal power calculations. The minimal specifications required to start the line-driver design are:
Average power level required on the line (PL)
Crest factor for the modulation chosen (CF)
Line impedance assumed for the average power specification
Transmission frequency band
Target harmonic distortion specifications
The first three of these may be used to compute the initial requirements for any line driver, which at a minimum, must deliver both the required voltage and current output swing at the peaks. Once this most basic requirement is met, the designer may consider the additional signal performance issues (principally associated with harmonic distortion and power efficiency).
The maximum required line voltage, VPP, may be computed by stepping through the following equations:
Here, PL is the specified average line power developed from the power spectral density mask being considered; RL is the assumed line impedance; and CF = VP/VRMS.
The first equation is solved for VRMS. Then, CF is used to calculate the maximum desired VPP on the line. PL is typically given in dBm, but it may be converted to watts using PAVG = (0.001)*10(PL/10). The line power in dBm will be referred to as PL, while PAVG will represent average line power in watts.
The example calculation shown is a typical upstream transmission requirement for full-power, DMT-based ADSL where the frequency band is in the 30- to 138-kHz region (see the table). Because most xDSL applications use asymmetric data rates, the upstream frequency band for FDM systems usually goes below the downstream band. This way, it experiences less attenuation, permitting a lower-power transmitter on the customer side. The principles described here apply to either side, but the examples will focus on the lower-power upstream (or CPE or ATU-R) side.
The CF strongly influences the driver's power dissipation. Called the peak-to-average ratio (PAR), this is set by the modulation scheme chosen and limited by the DSP. PAR is related to CF by CF = 10(PAR/20). The Issue 1 and 2 T1.413 ADSL standard specifies a PAR of 15 dB, which translates to a CF of 5.6, very close to the more common 5.33 (see the table, again).1
A higher CF will always dissipate more power in the line driver. That's because the higher peak-output-voltage excursions require a higher supply voltage, which will require a higher quiescent power. In almost all cases, the actual power delivered out onto the line from the power supplies (through the driver) is very small compared to the total power dissipated by the line driver internally.
The maximum VPP on the line may be taken as a fixed design goal (referred to as VLPP). For a given VLPP and impedance, the amplifier's output voltage and current can be traded off with the line transformer's turns ratio. As a result, one can obtain the total VPP across two amplifier outputs and their peak output currents with the transformer turns ratio (n) as a variable (Fig. 1).
The two amplifiers form a push/pull output stage through impedance matching resistors into a transformer. The resistors ensure a line match on the output side of the transformer and limit fault currents, but they will dissipate half the total power delivered at the output. The desired line power accounts for the other half.
Once VLPP is known, the designer can use the transformer turns ratio to trade off the required voltage and current out of the amplifier. This allows different types of amplifiers over a range of supply voltages to be considered for a particular requirement. The transformer turns ratio forms a primary design variable that will determine the required amplifier output voltage and current. Increasing the turns ratio will decrease the required voltage swing, but at the expense of a higher current output. Moreover, increasing the turns ratio will allow a lower power-supply voltage and, in turn, use of low-voltage components.
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