I have some question when reading UG1727 chapter SYSTEM CONSIDERATIONS FOR INITIAL CALIBRATIONS,staring on page 85.
1. In External Transmitter LO Leakage Initial Calibration,staring on page 86.It says "The purpose of this calibration is to obtain a good estimate (gain/phase) of the external loop channel conditions prior to operation.".
But I still can't understand why external loop has something to do with LO leakage. In my understanding, LO leakage is because of the mixer inside, not external channel, besides, the nonlinear cross-tuning frequency of external PA will not appear at the LO position.
May I ask if my above understanding is wrong? If there is any problem, can you help me point it out? I can understand using the estimate (gain/phase) of the external loop channel to do DPD, but can you help me understand the relation between external loop channel and LO Leakage?
2. Contrary to “External Transmitter LO Leakage Initial Calibration” mentioned in question 1,the UG does not mention the "External Transmitter QEC Initial Calibration".I take it that,image is because of the mismatch of phase or amplitude of I/Q signals, the process happens internal, so there is no meaning of external calibration.May I ask if my understanding is wrong?
3. As mentioned several time in UG, such as in page 91, "QEC tracking uses an offset LO on the feedback path during tracking. This ensures that the quadrature errors of the transmitter path are not aligned with those of the observation receiver path.".
I think not only TX has the LO problem, RX and ORX also have DC offset and LOL caused by mixers in RX/ORX channel. I don‘t understand why there is no "Internal/External Receiver/Observation Receiver LO Leakage Initial Calibration"? I understand that since it is RX, we can't generating a signal that can cancelling LOL, but can we do it in the digital domain?Is there already this mechanism, just didn't mention it?