TA0313 STMICROELECTRONICS | Alldatasheet
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This paper describes STMicroelectronics ADSL analog line interface solutions. After a short overview of the ADSL application environment, this article focuses on the implementation of line drivers. Magnetic circuits such as hybrid or line transformer circuits will not be described in detail. The ADSL concept Asymmetric Digital Subscriber Line (ADSL) is a modem technology, which converts existing twisted-pair telephone lines into access paths for multimedia and high speed data communications. An ADSL modem is connected to a twisted-pair telephone line, creating three information channels: a high-speed downstream channel (up to 1.1MHz and 2.2MHz for ADSL2+) depending on the implementation of the ADSL architecture, a medium-speed upstream channel (up to 135kHz or 230kHz) and a POTS (Plain Old Telephone Service), split off from the modem by filters. ADSL allows the wide-band access necessary to transmit media such as movies, television, remote CD- ROMs via LANs and the Internet into individual workplaces and homes. Figure 1: Typical spectral representation of a DMT ADSL signal (subscriber side) 135kHz30kHz p.s.d. Power Spectral Density POTS 1.1MHz Upstream Downstream DMT sub-channel (Discrete Multi-Tone) 160kHz4.5kHz STMicroelectronics Solutions for ADSL Line Interfaces TA0313 Technical Article
STMicroelectronics Solutions for ADSL Line Interfaces TA0313 The resistances R1, R2 and R3 allow us to calculate the gain of the structure as follows: The damping factor can be derived from these resistances and the capacitances C1, C2, C3 and C4: with: The higher the gain, the more sensitive the damping factor is. When the gain is higher than 1 it is preferable to use very stable resistance and capacitance values. The value of the gain does not affect the cut-off frequency, fc, which is derived as follows: Moreover this expression shows that it is possible to shift the cut-off frequency by simply changing the values of the resistances R4, R5 or R6, R7 — with neither a change of capacitance nor of the damping factor. The following table shows a calculations of components for a cut-off frequency around 130kHz for the ADSL over POTS and 270kHz for the ADSL over ISDN. The final, accurate settings are made by compromising between the attenuation of the highest frequencies of the upstream signal and the impact of the distortion on the downstream signal. This is best done directly in the application. Nevertheless we can start with the following initial values: Transformation ratio In differential mode, the TS613 and TS612 are able to deliver a typical amplitude signal of 18V peak to peak. The dynamic line impedance is 100 Ω. The typical value of the amplitude signal required on the line is up to 12.4V peak to peak. By using a 1:2 transformer ratio the reflected impedance back to the primary will be a quarter (25 Ω) and therefore the amplitude of the signal required with this impedance will be halved (6.2 V peak to peak). Assuming a 25 Ω series resistance (12.5 Ω for both outputs) is necessary for impedance matching, the output signal amplitude required is 12.4V peak to peak. This value is acceptable for both TS613 and TS612. In this case, the load impedance is 25 Ω for each driver in single ended. (Ω) (Ω) (Ω) (Ω) (nF) (nF) Gain (dB) fc (kHz) ζ 180 536 261 261 10 2.2 16.8 130 0.73 180 536 127 127 10 2.2 16.8 270 0.73 Gain 1 2R2 ζ 2C1 αC2–
2 C1C2
2 C3C4
α 2R2 fc 1 2π R4R5C1C2 2π R6R7C3C4
STMicroelectronics Solutions for ADSL Line Interfaces TA0313 As Vo° equals Vo without loading, the gain in this case becomes: The gain for the loaded system will be: Equation 1 As shown in Figure 9, this system is an ideal generator, with a synthesized impedance equal to the internal impedance of the system. Therefore, the output voltage becomes: Equation 2 with Ro the synthesized impedance and Iout the output current. On the other hand Vo can be expressed as: Equation 3 By identifying of both Equation 2 and Equation 3, the synthesized impedance is, with Rs1=Rs2=Rs: Equation 4 Unlike the level of Vo° required for passive impedance, Vo° will be smaller than 2Vo in this case. Let us write Vo°=kVo with k the matching factor varying between 1 and 2. Assuming that the current through R3 is negligible, the resistance divider becomes: After choosing the k factor, Rs will equal to 1/2RL(k-1). Figure 9: Equivalent schematic, where Ro is the synthesized impedance G Vo noload() 1 2R2 1 R2 GL Vo withload() 2--- 1 2R2 1 R2 Vo ViG() RoIout()–= Vo Vi 1 2R2 1 R2 1 R2 Ro Vi.Gi Iout 1/2RL Ro Rs 1 R2 Ro kVoRL
TA0313 STMicroelectronics Solutions for ADSL Line Interfaces A good impedance matching assumes: Equation 5 From Equation 3 and Equation 5 we derive: Equation 6 By fixing an arbitrary value for R2 in Equation 6, we arrive at: Finally, the values of R2 and R3 allow us to extract R1 from Equation 1 so that: Equation 7 with GL the required gain. Capabilities The table below shows the calculated components for different values of k for a differential load of 25 Ω. In all cases, R2=1000 Ω and the gain=16dB. The last column displays the maximum amplitude level on the line regarding the TS613 maximum output capabilities (18Vp-p diff.) and a 1:2 line transformer ratio. Measurement of power consumption in application Conditions: /c108 Passive impedance matching /c108 Transformer turns ratio: 2 /c108 Maximum level required on the line: 12.4Vpp /c108 Maximum output level of the driver: 12.4Vpp GL (gain for the loaded system) GL is fixed for the application requirements GL=Vo/Vi=0.5(1+2R2/R1+R2/R3)/(1-R2/R3) R1 2R2/[2(1-R2/R3)GL-1-R2/R3] R2 (=R4) Arbitrarily fixed R3 (=R5) R2/(1-Rs/0.5RL) Rs 0.5RL(k-1) Active matching k R1 (Ω) (Ω) Rs (Ω) TS613 Output Level to get 12.4Vpp on the line with a turn ratio of 2. (Vp-p diff) Maximum Line Level (Vp-p diff) 1.4 590 1620 5 (10//10) 8.68 25.7 1.7 261 3300 8.71 (10//68) 10.52 22.3 Passive matching 12.4 18 Ro 1 2---RL= R3 R2 1 2Rs R1 2R2 21 R2 GL 1– R2
STMicroelectronics Solutions for ADSL Line Interfaces TA0313 /c108 Crest factor: 5.3 (Vp/Vrms) /c108 Power Supply: 12V Power consumption of the driver during emission on 900 and 4550 meter twisted pair telephone lines: /c108 TS613: 360mW /c108 TS612: 450mW Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publicati on are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectro nics. The ST logo is a registered trademark of STMicroelectronics All other names are the property of their respective owners © 2004 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Ital y - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America