AD9361 Tx Attenuation Setting

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 ?  

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    •  Analog Employees 
    on Mar 22, 2016 11:10 PM over 4 years ago

    I would recommend setting 0x077 to 0x40h. When bit D6 is set, Tx output power updates immediately after Tx attenuation word changes. Also, make sure 0x07C D6 (Immediately update TPC Atten) is set.

    As you sweep the attenuation of one Tx channel, set the other channel's attenuation to maximum. The isolation between channels is about 50dB, so depending on the attenuation settings you will get crosstalk.

    Tx power output vs attenuation should be a monotonic curve with 360 0.25dB steps resulting in 0-90dB attenuation range.

  • I set 0x077 to 0x40h and 0x07C D6 to 1 to update each of the attenuation settings in the 0x73 register. I also set 0x75 to 200 to attenuate the other TX channel. I still get the same problem, that at an attenuation setting of less than about 20 dB the attenuation increases rather than decreases. We didn't tie the unused TX outputs to 1.3V, could this be the problem ? Also are there any other register settings that affect the TX attenuation and potentially cause this type of effect ?

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    •  Analog Employees 
    on Mar 26, 2016 12:48 AM over 4 years ago

    I don't think unterminated unused Tx outputs are causing this issue.

    Can you provide a schematic of the Tx output structure you are using?

    Is there a way for you to verify DC bias on the Tx output pins (you should be seeing 1.3V +/-2.5% across the whole attenuation range). What is the signal amplitude at 0dB attenuation?

  • Thanks for your reply and helping us out with this issue !

    I measured the voltage at the TX output pins and the voltage dropped as the attenuation setting was decreased. Initially the 1.3V supply was actually not connected to the biasing inductors, then we had a resistor to the supply rail which was limiting the current, when this was replaced by 0 ohms, all the attenuation settings worked.

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    •  Analog Employees 
    on Apr 1, 2016 10:37 PM over 4 years ago

    Thanks for the update. Now that the attenuation is functional from the DC stand point, I would recommend sweeping the output power over frequency and making sure that you get the desired flatness.