AD8018
20
4
3
2
0
–20
–40
–60
1
0
–1
–2
–3
–80
–0.25
–0.05
0
1.5
–0.2 –1.5 –1.0
0.05
1.0
0.2
50
FREQUENCY – kHz
0
100
150
TIME – ms
Figure 9. DMT Waveform in the Frequency Domain
Figure 10. DMT Signal in the Time Domain
Generating DMT Signals
MTPR versus transformer turns ratio is depicted in TPC 21 and
covers a variety of line power ranging from +12 dBm to +14 dBm.
As the turns ratio increases, the driver hybrid can deliver more
undistorted power due to higher output current capability.
Significant degradation of MTPR will occur if the output of the
driver swings to the rails, causing clipping at the DMT voltage
peaks. Driving DMT signals to such extremes not only compro-
mises “in-band” MTPR, but will also produce spurs that exist
outside of the frequency spectrum containing the desired DMT
power. “Out-of-band” spurious free dynamic range (SFDR) can
be defined as the relative difference in amplitude between these
spurs and a tone in one of the upstream bins. Compromising
out-of-band SFDR is equivalent to increasing near end cross-
talk (NEXT). Regardless of terminology, maintaining out-of-band
SFDR while reducing NEXT will improve the overall performance
of the modems connected at either end of the twisted pair.
At this time, DMT-modulated waveforms are not typically
menu-selectable items contained within AWGs. Even using
AWG software to generate DMT signals, AWGs that are available
today may not deliver DMT signals sufficient in performance
with regard to MTPR due to limitations in the D/A converters
and output drivers used by AWG manufacturers. Similar to
evaluating single-tone distortion performance of an amplifier,
MTPR evaluation requires a DMT signal generator capable of
delivering MTPR performance better than that of the driver
under evaluation. Generating DMT signals can be accom-
plished using a Tektronics AWG 2021 equipped with Option
4, (12-/24-bit, TTL Digital Data Out), digitally coupled to
Analog Devices’ AD9754, a 14-bit TxDAC®, buffered by an
AD8002 amplifier configured as a differential driver. Note that
the DMT waveforms (available on the Analog Devices website,
http://www.analog.com), or similar .WFM files are needed to
produce the digital data required to drive the TxDAC from the
optional TTL Digital Data output of the TEK AWG2021.
TPC 21 shows how SFDR varies versus transformer turns ratio
for line power ranging from +12 dBm to +14 dBm. As line
power increases, or turns ratio decreases, SFDR degrades. The
power contained in the spurs can be measured relative to the
power contained in a typical upstream carrier and is expressed
in dBc as SFDR, similar to MTPR.
The supply voltage of the driver can also affect SFDR. As the
supply voltage is increased, voltage swing is increased as well,
resulting in the ability to deliver more power to the line with-
out sacrificing performance. This can be seen in TPC 22. Less
undistorted power is available when lower turns ratio transform-
ers are used due to voltage clipping of the signal.
TxDAC is a trademark of Analog Devices, Inc.
–12–
REV. 0
ADI [ ADI ]
