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ADRF6518 Datasheet(PDF) 31 Page - Analog Devices |
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ADRF6518 Datasheet(HTML) 31 Page - Analog Devices |
31 / 39 page Data Sheet ADRF6518 Rev. A | Page 31 of 39 EFFECT OF FILTER BW ON EVM Figure 84 shows how changing the filter BW affects the EVM for signals at several different symbol rates. The x-axis is normalized such that it displays the baseband bandwidth of each respective signal to the set filter corner. For example, a filter corner of 10 MHz and a signal with a baseband bandwidth of 5 MHz yields 2 Hz/Hz. Similarly, a filter corner of 50 MHz and a signal with a baseband bandwidth of 25 MHz also yields 2 Hz/Hz. Baseband bandwidth is defined by the following: 2 1 ) ( Rate Symbol BW BB The general behavior shows that the higher the ratio of the filter corner to baseband bandwidth is, the better the EVM. This behavior starts to plateau at around a 2× ratio. This behavior affects the higher bandwidth signal more so than the smaller bandwidth signals. The primary reason for this behavior is that the noise is not flat across frequency (there is some interaction between the filters and all the gain stages). The noise shaping degrades EVM as the filter corner starts to encroach well outside of the signal bandwidth. Figure 84. EVM vs. Filter Corner/Baseband BW Ratio over Symbol Rate, Filter Corner = 63 MHz, QPSK, 1.5 V p-p Output Level, −20 dBm Input Power PULL-DOWN RESISTORS FOR DISABLE FUNCTION The ADRF6518 offers a disable function, by pulling ENBL low, that brings the supply current to approximately 1 mA. For the function to work correctly, a dc path to ground must be established on the output pins (OPP1, OPM1, OPP2, OPM2) to allow proper discharge of the postamplifier. Figure 85 shows how to properly place the pull-down resistors. The resistor value must be big enough so that it does not interfere with the output impedance that the postamplifer sees (for example, 400 Ω), but not so big that it prevents proper discharge, effectively becoming an open circuit. For most applications, a value of RPULLDOWN = 10 kΩ sufficiently satisfies these conditions. INSTABILITY AT HIGH GAIN IN FILTER BYPASS MODE The user must be cautious while operating the ADRF6518 at the highest of gains in filter bypass mode. Due to the high gain (up to 66 dB) and wide bandwidth (up to 350 MHz with maximum digital gains), the ADRF6518 is susceptible to oscillations when it is in filter bypass mode and its gain is set above 60 dB. The oscillation manifests itself with a broadband rise in the noise floor and significantly degrades the SNR and EVM. Orthogonal input to output signal paths on the printed circuit board (PCB) helps reduce this oscillation. This can be seen in the top layer silkscreen, shown in Figure 92. It is recommended that any posts or headers not be placed for measurement purposes on the signal paths, especially the output signal paths. Doing so causes the output signal to radiate back to the input and induce the oscillation at even lower gains than 60 dB. Figure 85. Pull-Down Resistors 0 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 010 9 8 7 6 5 4 3 2 1 FILTER CORNER/BASEBAND BANDWIDTH (Hz/Hz) 5MSPS 10MSPS 20MSPS 50MSPS VPSD COMD LE CLK DATA SDO/RST VICM/AC VPI OPP1 OPM1 COM VGN3 VOCM COM OPM2 OPP2 24 23 22 21 20 19 18 17 1 2 3 4 5 6 7 8 ADRF6518 I TO NEXT STAGE Q TO NEXT STAGE RPULLDOWN RPULLDOWN RPULLDOWN RPULLDOWN |
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