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AD8325ARUZ Fiches technique(PDF) 9 Page - Analog Devices |
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AD8325ARUZ Fiches technique(HTML) 9 Page - Analog Devices |
9 / 16 page AD8325 Distortion, Adjacent Channel Power, and DOCSIS In order to deliver 58 dBmV of high fidelity output power required by DOCSIS, the PA should be able to deliver about 61 dBmV in order to make up for losses associated with the transformer and diplexer. TPC 7 and TPC 8 show the AD8325 second and third harmonic distortion performance versus fundamental frequency for various output power levels. These figures are useful for determining the inband harmonic levels from 5 MHz to 65 MHz. Harmonics higher in frequency will be sharply attenu ated by the low-pass filter function of the diplexer. Another measure of signal integrity is adjacent channel power or ACP. DOCSIS section 4.2.9.1.1 states, “Spurious emissions from a transmitted carrier may occur in an adjacent channel that could be occupied by a carrier of the same or different symbol rates.” TPC 12 shows the measured ACP for a 16 QAM, 61 dBmV signal, taken at the output of the AD8325 evaluation board (see Figure 12 for evaluation board schematic). The transmit channel width and adjacent channel width in TPC 12 correspond to symbol rates of 160 KSYM/SEC. Table I shows the ACP results for the AD8325 for all conditions in DOCSIS Table 4-7 “Adjacent Channel Spurious Emissions.” Evaluation Board Features and Operation The AD8325 evaluation board (Part # AD8325-EVAL) and control software can be used to control the AD8325 upstream cable driver via the parallel port of a PC. A standard printer cable connected between the parallel port and the evaluation board is used to feed all the necessary data to the AD8325 by means of the Windows-based, Microsoft Visual Basic control software. This package provides a means of evaluating the amplifier by providing a convenient way to program the gain/ attenuation as well as offering easy control of the amplifiers’ asynchronous TXEN and SLEEP pins. With this evaluation kit the AD8325 can be evaluated with either a single-ended or differ ential input configuration. The amplifier can also be evaluated with or without the PULSE diplexer in the output signal path. To remove the diplexer from the signal path, leave R6 and R8 open and install a 0 W chip resistor at R7. A schematic of the evalua tion board is provided in Figure 12. Table I. ACP Performance for All DOCSIS Conditions (All Values in dBc) TRANSMIT CHANNEL ADJACENT CHANNEL SYMBOL RATE SYMBOL RATE 160 KSYM/SEC 320 KSYM/SEC 640 KSYM/SEC 1280 KSYM/SEC 2560 KSYM/SEC 160 KSYM/SEC –53.8 –55.6 –61.1 –67.0 –66.7 320 KSYM/SEC –53.1 –53.8 –56.0 –61.5 –67.6 640 KSYM/SEC –54.3 –53.2 –54.0 –56.3 –62.0 1280 KSYM/SEC –56.3 –54.3 –53.4 –54.1 –56.3 2560 KSYM/SEC –58.5 –56.2 –54.4 –53.5 –54.1 Noise and DOCSIS At minimum gain, the AD8325’s output noise spectral density is 10 nV/ ÷Hz measured at 10 MHz. DOCSIS Table 4-8, “Spurious Emissions in 5 MHz to 42 MHz,” specifies the output noise for various symbol rates. The calculated noise power in dBmV for 160 KSYM/SECOND is: Ê Ê Á20 log Á Á Ê ËÁ 10 nV ¯˜ ˆ 2 ¥160 kHz ˆ ˜ ˆ ˜ +60 = –48 dBmV ÁÁ Ë Ë Hz ˜˜˜¯¯ Comparing the computed noise power of –48 dBmV to the 8 dBmV signal yields –56 dBc, which meets the required level of –53 dBc set forth in DOCSIS Table 4-8. As the AD8325’s gain is increased from this minimum value, the output signal increases at a faster rate than the noise, resulting in a signal to noise ratio that improves with gain. In transmit disable mode, the output noise spectral density computed over 160 KSYM/SECOND is 1.0 nV/ ÷Hz or –68 dBmV. Overshoot on PC Printer Ports The data lines on some PC parallel printer ports have excessive overshoot that may cause communications problems when pre sented to the CLK pin of the AD8325 (TP6 on the evaluation board). The evaluation board was designed to accommodate a series resistor and shunt capacitor (R2 and C5) to filter the CLK signal if required. Transformer and Diplexer A 1:1 transformer is needed to couple the differential outputs of the AD8325 to the cable while maintaining a proper impedance match. The specified transformer is available from TOKO (Part # 617DB-A0070); however, MA/COM part # ETC-1-1T-15 can also be used. The evaluation board is equipped with the TOKO transformer, but is also designed to accept the MA/COM transformer. The PULSE diplexer included on the evaluation board provides a high-order low-pass filter function, typically used in the upstream path. The ability of the PULSE diplexer to achieve DOCSIS compliance is neither expressed nor implied by Analog Devices Inc. Data on the diplexer can be obtained from PULSE. REV. A –9– |
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