Creating a Constant Envelope Signal Using the ADL5331 RFVGA and AD8319 Log Detector (CN0082)
Overall performance of a transmitter, wired or wireless, is a strong function of the output power of the amplifier. If the signal is weak, bit error rate (BER) or modulation error rate (MER) will degrade due to low SNR. If the signal is too strong, distortion will cause the same issues. This circuit uses the ADL5331 VGA, the AD8319 power detector, and the AD5621 low power nanoDAC to generate output power control accurate to 12 bits. The AD8319 has very high temperature stability to compensate for any gain variation over temperature of the VGA, resulting in very accurate power control over a wide temperature range. Because the AD8319 control input VSET and the output VOUT are related to the RF input on a volts/dB scale and the AD5621 nanoDAC has a linear transfer function, the resulting output power control will be a linear-in-dB vs. DAC input code.
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