We have a long standing issue with phase alignment of channel A and channel B. It seems that this is a known issue as I found a discussion on this topic “Application for using the 2 RX channels simultaneously”, but it is not clear if a solution has been found for the 180 degree ‘random’ phase reversal between channel A and B, particularly as if we are using a custom board so the method is important to us.
I have partially resolved the issue of phase alignment by generating a CW signal using the Lime and activating the baseband loopback. Small differences in the phase alignment can be found and corrected, as the loopback signal seems to have very similar phase offset with the externally generated signal using a signal generator and 0 degree splitter to feed channel A and B simultaneously, except that the random 180 phase reversal (on power up or changing LO frequency, etc.) cannot be detected by baseband loopback as it seems to occur further down the chain (i.e. in the RF mixer).
The obvious solution to this is to activate the RF loopback, which also experiences the same 180 degree phase reversal. Unfortunately so does the transmitted signal, so this is not a reliable method of detecting it in order to compensate. Does anyone have a more reliable solution?