AN1304: ADSL CO Driver Distortion and Backmatch Resistor Trade-Offs

ADSL CO Driver Distortion and
Backmatch Resistor Trade-Offs
®
Application Note
July 11, 2007
AN1304.0
By Tamara Ahrens and Mike Wong
Digital Subscriber Lines have enjoyed almost instant
success as they expand among Internet access, fiber optics,
and cable uses. One stumbling block in the integration of a
complete digital system is the power required from the CO
(central office) driver. Many current solutions burn excess
power and add unnecessary distortion. A trade-off with the
backmatch resistor will allow a well-designed amplifier to
operate more efficiently and effectively. Optimizing the
performance in conjunction with the power dissipation will
allow greater numbers of lines into each central card, while
providing the capability to service more customers.
If N is selected as 1.41 (a typical value), the peak to peak
differential voltage at the driver side of the transformer is:
33.52V ⁄ 1.41 = 23.8V
Given the dual driver configuration, the swing from each side
is 11.88V. The output swing required of each amplifier is
greater, though, to account for the loss across the back
termination resistance.
Why is there a resistor in series with our output? The answer
lies not in driving the line, but in handling any power
reflected back to the driver by imperfect terminations and
connections on the line encountered by the signal. The
termination resistor serves to match the impedance of the
line and to dissipate any of this power reflected back to the
source.
An ADSL CO driver are must supply peak line power of
20dBm on POTS line impedance of 100Ω, with a crest factor
of 14.5dB.
We can calculate average output line voltage current, from
output line power and line impedance:
2
P OUT = ( V OUT ( rms ) ) ⁄ R LINE
For 100% termination, the resistor value is 25Ω (half the line
impedance divided by the square of the turns ratio of the
transformer). The output swing required from the driver
amplifier is 23.1V. For 60% termination, the resistor value
reduces to 15Ω. Likewise, the output swing decreases to
18.45V. While this relaxes the requirements for the output of
the driver, it also allows a portion of any reflected power to
reach the driver amp. The output circuitry of this amplifier
must be designed to handle such reflections.
V OUT ( rms ) = 3.16V
(EQ. 1)
The crest factor determines the ratio of peak to average
signal on the line. Given the recent calculations of average
signal, the maximum allowed swing on the line can then be
calculated.
CF ( crest factor ) = 20 log ( PAR ) = 14.5dB
(EQ. 2)
PAR ( peak to average ratio ) = 5.3
(EQ. 3)
The distortion at the two output swing levels are included in
Figure 2. These linearity curves are from the EL1503, a DSL
driver. With positive and negative 12V supplies, the output is
quite linear for any output voltage range less than 20V.
Specifically, the distortion of the 2nd and 3rd order
harmonics remain below -68dB. However, when the device
is configured with 100% termination, it must provide an
output swing of 23.1V. The distortion graph displays a sharp
increase for swings larger than 20V, a 10dB increase for 2nd
harmonic results as well as a 25dB increase for 3rd
Differential V OUT ( P-P ) = V OUT ( rms ) × PAR × 2 = 33.52V
Figure 1 shows a typical differential driver circuit
configuration. As calculated previously, the peak to peak
output voltage is 33.52V. The required driver output to
produce this line voltage is determined by the transformer
turns ratio, N, and back termination resistor value, RTERM.
+
12V
VCC
50/N2
U1
4
V+
V-
RTERM
167.6mA*N
TX1
-
T1
VEE
33.52V/N
32.52
RLINE
100
+
12V
U2 V+
2
4
17
1
N
RTERM
B
V-
FIGURE 1. DRIVER CIRCUITRY
1
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Application Note 1304
harmonic results. This distortion will degrade the channel
capacity of the system.
If the driver cannot handle any reflected power at its output
and 100% termination is required, then the only option is to
increase the power supply from ±12V to ±15V, or some
nonstandard value between them. Of course, if the power
supply is increased, the power dissipation and amount of
heat generated will also increase. Not all available drivers
operate with 15V supplies. The best choice for power
dissipation, performance, and temperature considerations is
a driver with partial termination.
Partial termination relaxes the output swing requirements on
the driver. The goal is to use as small percentage of
termination as possible that doesn't overwhelm the driver
with reflected power. Empirical testing has lead to the
conventional use of 60% termination. If a 60% termination,
or 15Ω resistor, is chosen, the driver's output swing
becomes 18.5V. This output swing is within the driver's linear
range and thus significantly improves the driver
performance.
-40
FREQUENCY = 100kHz
RLOAD = 80Ω
-45
RSET = 1.5k
-50
3rd HD
DISTORTION
-55
-60
2nd HD
-65
-70
-75
-80
-85
0
5
10
15
20
25
VOUT(P-P) (V)
FIGURE 2. DISTORTION OF EL1503 DRIVER vs OUTPUT SWING AT FULL POWER
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verify that the Application Note or Technical Brief is current before proceeding.
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2
AN1304.0
July 11, 2007
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