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AD9361 gain table index vs SNR estimate

Question asked by on Feb 12, 2015



Below is a chart showing the average SNR estimate done in the BB processor on the received signal and the AD9361 gain table index.  AD9361 is in FDD mode and using automatic gain control with full table mode. Over the input signal power range (from - 8 dBm to -93 dBm with 1 dB steps),  RF Gain table index is increasing pretty much as expected and reaching the highest index 76 around input power level -75 dBm. It was expected to see the average SNR starting decreasing, around where gain index reaches 76 but SNR starts decreasing around -55 dBm. What could be the reason for this ~15- 20 dB difference? Although the gain index is increasing as expected the analog gain stages in the AD9361 cannot provide the expected amount of gain? Gain table programming is as described in the reference manual and I don't measure a loss on the HW more than ~3.5dB in between where the input signal injected and the AD9361 RX input.