Design of ADSL Low Noise Preamplifier Receiver

fighting

Design of ADSL Low Noise Preamplifier Receiver [Copy link]

Design of ADSL Low Noise Preamplifier Receiver

Anhui Provincial Hospital Information Center Wang Ping Zong Ning

1 Introduction With the rapid development of the Internet, people's demand for information has also extended from narrowband telephone, fax, data and other services to broadband. The backbone network, long-distance switching network and inter-office relay network of the telecommunications network have basically achieved digitization and broadband. However, at the end of the high-speed information network (i.e., the user access end), most of them are still connected to it using narrowband copper ring telephone lines, which has become one of the biggest bottlenecks restricting the development of the Internet. Therefore, ADSL technology has attracted widespread attention as a low-cost solution to achieve high-speed access at the user end. At present, the standards and implementation technologies of ADSL are developing steadily, and product development has also entered a period of rapid development. Under this premise, more and more developers realize that the high performance of the ADSL Modem analog front-end will win a huge market for the product. In other words, the performance of the analog front-end also determines the success or failure of the development. 2 Technical Features of ADSL ADSL (Asymmetric Digital Subscriber Line) technology has three basic characteristics: first, it can transmit high-speed data and low-speed voice signals simultaneously on a copper ring telephone line; second, it is asymmetric, with the ratio of upload speed to download speed being about 1:10, that is, the download speed is about 6Mbps, while the upload speed is about 640kbps; third, it has a long transmission distance, and the relationship between the speed and transmission distance is shown in Table 1〔3〕. The technical characteristics of ADSL are very suitable for high-speed access to the Internet user end. ADSL uses frequency division multiplexing (FDM) technology to achieve asymmetric high-speed transmission on the same telephone copper ring line, dividing a copper ring line into three channels: (1) ordinary telephone service (POTS) channel, with a frequency band of 0 to 4kHz, separated from the digital channel by a front-end data separator; (2) upstream digital channel, about 25kHz to 150kHz, used to transmit response and control signals; (3) downstream digital channel, about 150kHz to 1.1MHz. ADSL modulation generally uses DMT (discrete multi-tone modulation), which divides the modulator into many (usually 256) small modulators with a bandwidth of 4k. DMT uses many carriers to establish subchannels. Each subchannel carries a fragment of the total information. The subchannel is independently modulated and processed in parallel according to the corresponding carrier center frequency. The subchannel uses QAM modulation technology and can carry 0 to 15 bits/symbol/Hz (maximum). When the noise of a subchannel is too large, the information is transferred to a channel with less noise and the subchannel is closed. 　 The theoretical maximum upload bandwidth is: 25 channels × 15 bit/symbol/Hz/channel × 4kHz = 1.5Mbps The theoretical maximum download bandwidth is: 249 channels × 15 bit/symbol/Hz/channel × 4kHz = 14.9Mbps〔3〕 The actual bandwidth of the subchannel depends on the characteristics of the line. 3 Determination of preamplifier design indicators ADSL uses the DMT scheme to process signals. This specification makes the design of ADSL linear interface circuits quite difficult. It requires that the signal input to the codec has low distortion and good linearity. In order to compensate for the distortion of the signal, the digital signal processor must constantly change its sub-channel to find the channel with the lowest noise, which greatly reduces the signal transmission rate. The noise that causes distortion mainly comes from inside and outside the ADSL modem. The external noise mainly includes: line impact noise, white noise, attenuation, crosstalk noise, and intermodulation distortion; the internal noise mainly comes from the inside of the device. Each circuit in the signal path has an impact on the distortion: linear driver, analog filter, ADC, DAC, gain amplifier, etc. Therefore, it is particularly important to carefully consider and analyze the noise and distortion of each component for the design of ADSL. The attenuation of high-frequency signals by copper twisted pair cables can reach 100dB〔1〕, and the amplifier must have sufficient sensitivity to detect weak signals at the μV level. More importantly, such weak signals will inevitably be drowned by the external and internal noise of the amplifier, and their spectrum may also overlap with useful signals. It is difficult to achieve the purpose of noise suppression using only conventional filtering methods. Designing a high common-mode rejection ratio and low-noise preamplifier with excellent performance and trying to suppress the common mode and amplifier noise will be an important solution. For weak signal amplifiers, the CMRR (common-mode rejection ratio) is generally required to be at least higher than 120dB, which is generally difficult to achieve with only a single op amp. ADSL allows the maximum line distortion (THD) to be less than -65dB〔6〕 and the attenuation to be less than 35dB〔5〕, so the amplification amplitude will also reach 35dB. Since the post-stage codec generally provides a second-stage amplification〔9〕, its total gain reaches 20dB, so the preamplifier gain can be taken as Gain≤20dB. ADSL standard ANSIT1.413 requires that the noise power spectrum density be -140dBm. In a system with a 100Ω line impedance, the equivalent input noise En should be less than ≤-150dBm/Hz in the actual system, or the input equivalent noise voltage En≤10nV [4]. Generally, it is required to select low-noise and low-resistance resistors. Based on the above discussion, the main technical indicators of the preamplifier are as follows: Noise: En≤10nV Common mode rejection ratio: CMRR ≥ 120dB; Distortion: THD ≤ -65dB; Bandwidth: Bandwidth ≥ 50MHz; Gain: Gain ≤ 20dB 4 Design of low noise preamplifier circuit 4.1 Introduction to THS6062 THS6062 is a new generation of ADSL differential amplifier receiver developed by TI, which has the characteristics of low noise, high speed, high common mode rejection ratio, etc. Its main technical indicators are as follows: Noise: Voltage En≤1.6nV Current In≤1.2pA Bandwidth: 100MHz〔－3dB, G＝2（－1）〕 Slew rate: 100V/μs; Driving capability: 90mA (typical) 　 Harmonic distortion: THD = -72dBc (f = 1MHz, RL = 150Ω) THD＝-90dBc（f＝1MHz，RL＝1kΩ） Voltage supply: 5V, ±5V, ±15V Common mode rejection ratio: 80~100dB Power supply rejection ratio: 80～95dB Input impedance: 2MΩ Output impedance: 13Ω 4.2 Circuit Design The ADSL preamplifier circuit is shown in Figure 2. 1) Selection of circuit component parameters According to the noise figure 〔9〕 It can be seen that when Rs = 1kΩ, NF is relatively small. The selection of other resistance values is as follows: R2 = 100Ω, R1 = 200Ω According to the gain Gain = Rf/R = 10, R = 100Ω, Rf = 1kΩ can be selected. 2) Calculation of CMRR Common-mode rejection ratio for three-op-amp amplifiers〔8〕 　in: Among them, CMRR1 = CMRR2 = CMRR3 = 80dB, which are the common mode rejection ratios of op amps A1, A2, and A3. In general, the pairing accuracy of A1 and A2 in the same package is much greater than 1%. CMRRR is the common mode rejection ratio generated when the resistors are asymmetric. When the resistor accuracy is 0.01%, CMRRR = 80dB, so: CMRR = 200dB. 5 Conclusion The amplifier designed in this paper has the characteristics of simple circuit and high performance index. Tests show that various technical indicators meet the expected goals and have been successfully applied in the developed home server external network interface module.

This article is excerpted from EDN Electronic Design Technology