I am outputting a BIST sine-wave from the TX port of the AD9361. I am using the No-OS code in FDD mode. When programming the TX attenuation registers I get an output power of about -45 dB from a test sinewave that is half the full scale, when the attenuation setting is about 40 dB using registers 0x73 and 0x74. Register 0x77 is also set to 0. As I decrease the value of the 0x73 register the output power increases by about the right amount (ie 4 steps for 1 dB) until I get to an attenuation of about 20 dB after which the attenuation begins to increase again. The attenuation begins to increase again by about 0.25 dB per unit as I approach 0 dB. Are you able to provide an suggestion what may be going wrong ?