Datasheet AD8325 (Analog Devices) - 9

AD8325 Distortion, Adjacent Channel Power, and DOCSIS Evaluation Board Features and Operation
In order to deliver 58 dBmV of high fidelity output power required The AD8325 evaluation board (Part # AD8325-EVAL) and by DOCSIS, the PA should be able to deliver about 61 dBmV control software can be used to control the AD8325 upstream in order to make up for losses associated with the transformer cable driver via the parallel port of a PC. A standard printer and diplexer. TPC 7 and TPC 8 show the AD8325 second and cable connected between the parallel port and the evaluation third harmonic distortion performance versus fundamental board is used to feed all the necessary data to the AD8325 by frequency for various output power levels. These figures are means of the Windows-based, Microsoft Visual Basic control useful for determining the inband harmonic levels from 5 MHz software. This package provides a means of evaluating the to 65 MHz. Harmonics higher in frequency will be sharply attenu­ amplifier by providing a convenient way to program the gain/ ated by the low-pass filter function of the diplexer. Another attenuation as well as offering easy control of the amplifiers’ measure of signal integrity is adjacent channel power or ACP. asynchronous TXEN and SLEEP pins. With this evaluation kit DOCSIS section 4.2.9.1.1 states, “Spurious emissions from the AD8325 can be evaluated with either a single-ended or differ­ a transmitted carrier may occur in an adjacent channel that could ential input configuration. The amplifier can also be evaluated be occupied by a carrier of the same or different symbol rates.” with or without the PULSE diplexer in the output signal path. To TPC 12 shows the measured ACP for a 16 QAM, 61 dBmV signal, remove the diplexer from the signal path, leave R6 and R8 open taken at the output of the AD8325 evaluation board (see Figure and install a 0 W chip resistor at R7. A schematic of the evalua­ 12 for evaluation board schematic). The transmit channel width tion board is provided in Figure 12. 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.”
Table I. ACP Performance for All DOCSIS Conditions (All Values in dBc) TRANSMIT ADJACENT CHANNEL SYMBOL RATE CHANNEL SYMBOL 160 K RATE SYM/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 Overshoot on PC Printer Ports
At minimum gain, the AD8325’s output noise spectral density is The data lines on some PC parallel printer ports have excessive 10 nV/÷ Hz measured at 10 MHz. DOCSIS Table 4-8, “Spurious overshoot that may cause communications problems when pre­ Emissions in 5 MHz to 42 MHz,” specifies the output noise for sented to the CLK pin of the AD8325 (TP6 on the evaluation various symbol rates. The calculated noise power in dBmV for board). The evaluation board was designed to accommodate a 160 KSYM/SECOND is: series resistor and shunt capacitor (R2 and C5) to filter the CLK signal if required. Ê Ê ˆˆ Á 10 nV ˆ 2
Transformer and Diplexer
20 logÁ Ê ¥ ˜˜ + Á 160 kHz 60 = –48 dBmV Á A 1:1 transformer is needed to couple the differential outputs of Á ËÁ ¯˜ Ë Ë Hz ˜˜˜¯¯ the AD8325 to the cable while maintaining a proper impedance match. The specified transformer is available from TOKO (Part Comparing the computed noise power of –48 dBmV to the # 617DB-A0070); however, MA/COM part # ETC-1-1T-15 8 dBmV signal yields –56 dBc, which meets the required level of can also be used. The evaluation board is equipped with the –53 dBc set forth in DOCSIS Table 4-8. As the AD8325’s gain is TOKO transformer, but is also designed to accept the MA/COM increased from this minimum value, the output signal increases at a transformer. The PULSE diplexer included on the evaluation faster rate than the noise, resulting in a signal to noise ratio that board provides a high-order low-pass filter function, typically improves with gain. In transmit disable mode, the output noise used in the upstream path. The ability of the PULSE diplexer spectral density computed over 160 K to achieve DOCSIS compliance is neither expressed nor implied SYM/SECOND is 1.0 nV/÷ Hz or –68 dBmV. by Analog Devices Inc. Data on the diplexer can be obtained from PULSE. REV. A –9– Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS LOGIC INPUTS TIMING REQUIREMENTS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDERING GUIDE PIN FUNCTION DESCRIPTIONS Typical Performance Characteristics APPLICATIONS General Application Operational Description SPI Programming and Gain Adjustment Input Bias, Impedance, and Termination Output Bias, Impedance, and Termination Power Supply Decoupling, Grounding, and Layout Considerations Initial Power-Up Between Burst Operation Distortion, Adjacent Channel Power, and DOCSIS Noise and DOCSIS Evaluation Board Features and Operation Overshoot on PC Printer Ports Transformer and Diplexer Differential Inputs Single-Ended-to-Differential Input Differential Input Installing the Visual Basic Control Software Running the Software Controlling the Gain/Attenuation of the AD8325 Transmit Enable, Transmit Disable, and Sleep Memory Section EVALUATION BOARD FEATURES AND OPERATION EVALUATION BOARD BILL OF MATERIALS OUTLINE DIMENSIONS Revision History