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AD8022 Scheda tecnica(PDF) 11 Page - Analog Devices |
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AD8022 Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 15 page AD8022 –11– REV. 0 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. 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. BITS/TONE 60.00 0 50.00 40.00 30.00 20.00 10.00 0.00 510 15 Figure 36. ADSL DMT SNR vs. Bits/Tone 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 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. 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. |
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