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Digital Loopback Issues in Ad9361 Custom board


We have a custom AD9361 board using Zynq and No-Os software from ADI. Everything is working fine in this board, including the Rx and Tx tuning (in fact this board is in production). But when we enable digital loopback, we see that the digital loopback actually sends a pattern data on Rx, instead of looping back Tx data to Rx. We have tried sending constants on Tx path, including all 0s and all 1s, but we still see this pattern (see figure below). We are enabling register 0x3f5 (D0) bit right after "AD9361 successfully configured" message is displayed. 

My questions are:

1) Are we doing anything wrong ?

2) Why does digital loopback not work even though the board is working good in all normal operations ?

3) However, when we enable BIST by setting (0x3F4 = 0x5B), we see a sinusoidal signal though. Why only BIST is working but not loopback ?

PS : We are testing digital loopback on this board which is working good because we to use the same firmware on a newer version board wherein the Rx and Tx tuning are both failing.