AD8022
process DMT and other heavily modulated waveforms. A typical
xDSL downstream DMT signal may contain as many as 256
carriers (subbands or tones) of QAM signals. MTPR is the rela-
tive difference between the measured power in a typical subband
(at one tone or carrier) versus the power at another subband
specifically selected to contain no QAM data. In other words, a
selected subband (or tone) remains open or void of intentional
power (without a QAM signal) yielding an empty frequency
bin. MTPR, sometimes referred to as the “empty bin test,” is
typically expressed in dBc, similar to expressing the relative dif-
ference between single tone fundamentals and second or third
harmonic distortion components. Measurements of MTPR are
typically made at the output of the receiver directly across the
differential load. Other components aside, the receiver function
of an ADSL transceiver hybrid will be affected by the turns ratio
of the selected transformers within the hybrid design. Since a
transformer reflects the secondary voltage back to the primary
side by the inverse of the turns ratio 1/N, increasing the turns
ratio on the secondary side reduces the voltage across the pri-
mary side inputs of the differential receiver. Increasing the turns
ratio of the transformers may inadvertently cause a reduction
of the SNR by reducing the received signal strength.
Generating DMT
At this time, DMT modulated waveforms are not typically menu
selectable items contained within arbitrary waveform generators.
AWGs that are available today may not deliver DMT signals
sufficient in performance with regard to MTPR due to limita-
tions in the D/A converters and output amplifiers used by AWG
manufacturers. Similar to evaluating single tone distortion perfor-
mance 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 accomplished using a Tektronics AWG 2021 equipped with
Opt 4, (12-bit/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. See Figure
37 for schematics of a circuit used to generate DMT signals
that can achieve down to –80 dBc of MTPR performance,
sufficient for use in evaluating xDSL receivers. WFM files are
needed to produce the necessary digital data required to drive
the TxDAC from the optional TTL Digital Data output of
the TEK AWG2021. Copies of .WFM files for upstream and
downstream DMT waveforms with a peak-to-average ratio (crest
factor) of ~5.3 can be obtained through the Analog Devices
web site. http://products.analog.com/products/info.asp?
product=AD8022
Channel Capacity and SNR
The efficiency of an ADSL system in delivering the digital data
embedded in the DMT signals can be compromised when the
noise power of the transmission system increases. The graph
below shows the relationship between SNR and the relative maxi-
mum number of bits per tone or subband while maintaining a bit
error rate at 1E-7 errors per second.
Upstream data is contained in the ...24.wfm files and downstream
data in the ...128.wfm files. These DMT modulated signals are
used to evaluate xDSL products for Multitone Power Ratio or
MTPR performance. The data files are used in pairs (adslu24.wfm
and adsll24.wfm go together, etc.) and are loaded into Tektronics
AWG2021 arbitrary waveform generator. The adslu24.wfm is
loaded via the TEK AWG2021 floppy drive into Channel 1
while the adsll24.wfm is simultaneously loaded into Channel
2. The number in the file name, prefixed with ‘u,’ goes into
CH1 or upper channel and the ‘l’ goes into CH2 or the lower
channel. Twelve bits from channel CH1 are combined with two
bits from CH2 to achieve 14-bit digital data at the digital out-
puts of the TEK 2021. The resulting waveforms produced at the
AD9754-EB outputs are then buffered and amplified by the
AD8002 differential driver to achieve 14-bit performance from
this DMT signal source.
60.00
50.00
40.00
30.00
20.00
10.00
Power Supply and Decoupling
The AD8022 should be powered with a good quality (i.e., low
noise) dual supply of ±12 V for the best overall performance.
The AD8022 circuit will also function at voltages lower than
±12 V. Careful attention must be paid to decoupling the power
supply pins. A pair of 10 µF capacitors located in near proximity
to the AD8022 is required to provide good decoupling for lower
frequency signals. In addition, 0.1 µF decoupling capacitors should
be located as close to each of the power supply pins as is physi-
cally possible.
0.00
0
5
10
15
BITS/TONE
Figure 36. ADSL DMT SNR vs. Bits/Tone
REV. 0
–11–
ADI [ ADI ]
