There are several register settings used for the Tx quadrature calibration that occurs on the AD9361. How do I set these registers? What is an NCO frequency and why do I need to set the Rx NCO when I'm calibrating the transmit path?
The AD9361 NCO Configuration for Tx Quad Cal document describes the process of setting up the Tx Quadrature Calibration. There are three aspects to it and a few comments are worth highlighting as described below.
1. For the Tx Quadrature Calibration to succeed, the Rx Quadrature Calibration Tracking algorithm cannot be running. This is accomplished by setting 0x169 to its default value of 0xC0.
2. The calibration uses two numerically controlled oscillators (NCOs). The Tx NCO produces a test tone while the Rx NCO is used with the receive path to receive the test tone and process it. The looping of the test tone occurs on-chip. It is important that the two tones be at the same frequency as mentioned in the user guide.
3. It is also important that the Rx NCO and Tx NCO be aligned in phase. A phase delay value in register 0x0A0 allows the two NCOs to be aligned but bench testing is required to determine the optimum setting for these bits. This test would use the actual configuration of digital filters and sampling rates that will be used in operation. This process is described in detail in the user guide mentioned above.
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