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Why does adjusting the AD9361 RX NCO phase help with TX quad calibration?

Category: Hardware
Product Number: AD9361

I've read through the reference manual, EngineerZone posts, the FAQ but don't understand why adjusting the AD9361 RX NCO phase would help the quadrature calibration for TX.

Does "RX NCO phase offset" refer to phase adjustment of the RX NCOs in the RX chain? Or does it refer to a different/separate knobs that adjusts TX NCOs directly in the TX chain?

If "RX NCO Phase offset" refers to controlling the RX NCOs in the RX chain, I'm concerned that adjusting the RX NCO for calibration isn't valid when the RX and TX links aren't connected. The FAQ linked above says "The Rx NCO Phase Offset compensates for the delay of the signal through the signal path.", but in our application the output of the RX path goes to a modem that decodes bits then encodes bits for the TX path. The path delays on TX can't be compensated by the RX chain so I want to understand how the RX NCO phase helps with the TX IQ imbalance in more detail. I've seen the block diagram linked above but don't see a connection between the RX NCO path and the TX path.

Can someone provide insight, and is there a more detailed block diagram showing the RX NCO phase control and how that connects to the TX chain?

  • The RX NCO and the TX NCO that you are referring to in the block diagram are the analog mixers that are used to down convert the  input signal to DC. 

    The TX Quadrature cal uses internal calibration tones and there is an internal loopback path through which these cal tones are sent to compensate for the gain and phase errors. The RX NCO that is being referred to in the document are the digital NCO's. 

    What is your requirement? Can you please run the TX Quad cal using ad9361_tx_quad_calib API and then check the performance at the TX output?

  • Hi  , thanks for the quick response.

    You're right the place I indicated for the NCO in the picture is incorrect, thanks for the callout. Is there a block diagram showing the digital NCOs in the signal chain? I haven't seen this in the reference manual or block diagrams but may have missed it.

    I'm able to run the quad calibration sequence, but I want to understand how it works to make sure it'll still work when we have the modem.

  • but I want to understand how it works to make sure it'll still work when we have the modem.

    Yes TX  quad cal will work for all the applications. It will work for modem as well.. Pls use the API for running the cal. Note that the image and the LOL performance is only limited to 50dBc w.r.t the tone level.

  • Thanks, I'm using the API so no worries there.

    Is there a block diagram showing the NCOs in the chip?

    I'd also like to understand how the convergence algorithms work in more detail, is there anything that provides more detail?

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