AD9361 Fast Attack AGC response when the ENSM Rx State is cycled

Tests were performed to characterize the AD9361 AGC response with the device configured for Fast Attack in FDD mode. The tests were conducted using two Analog Devices FMCOMMS3 and Xilinx ZCU102 development board setups. One test setup was used as the transmitter generating a -15 dBm CPFSK test signal. The second test setup was used as the receiver with an ILA used to capture receiver data at the AD9361/Programmable Logic interface.

Tests were performed to observe the AGC response when the Rx State was exited and then re-entered. Tests were conducted for each of the three gain configuration settings
using the ad9361_init_params variable "fagc_gain_index_type_after_exit_rx_mode" (Max Gain=0, Optimize Gain=1, Set Gain=2). Note, the gain configuration settings were listed in the ad9361.c file. 

In the Max Gain configuration, the AGC responded as expected. The ILA data collected showed the received signal saturated and then settled to a constant envelope level within 1 msec.

In the Optimized Gain configuration, the observed AGC response was not expected. The ILA data showed the received signal had a constant envelope level for the complete message reception; no gain change was observed. The envelope remained constant even when the Optimized Gain Offset parameter was varied from 0 to 15.

In the Set Gain configuration, the AGC responded similar to the Max Gain configuration. The ILA data showed the received signal saturated and then settled to a constant envelope. More confusing, the settling time was observed to vary as the Optimized Gain Offset parameter was varied from 0 to 15. With the Gain Offset set to 0, the data collected had a constant envelope. With the Gain Offset set to 15, the settling time was about 0.7 msec. The observed results were confusing because the AGC response was expected to remain constant in this configuration with no dependency on the Gain Offset parameter.

The responses observed for the Optimized Gain configuration and the Set Gain configuration were backwards to what was expected. Can you explain the discrepancy? Are the parameter settings listed above correct?

Is there an App Note available which provides a detailed discussion of the intended use and register setup for the different AGC modes?

  • Theoretically , In max gain mode , when you exit and re enter RX state it goes to max gain. So for a -15 dBm signal it will cross the high thresholds and AGC will act to bring it within specified range.

    In the case of optimized gain, when you re-enter Rx , since the input level is not changed and the Rx gain is the optimized one from last Rx  On period, you may not observe a change in amplitude of received signal in ILA.

    For set gain , it depends on what the set gain was and the thresholds. 

    UG-570 User guide is the only document available for details on same.