During checking the 11MHz demod boards I found that the I-Q relative phase showed funny LO power dependence.
It is now under investigation.
In the plot above the green curve represents the I-Q phase of a 11MHz demod board (see here).
It showed a strong dependence on the LO power and it changes from -60 deg to -130 deg as the LO power changes.
This is not a good situation because any power modulation on the LO will cause a phase jitter.
For a comparison I also took I-Q relative phase of a 33MHz demod board, which hasn't been modified recently.
It shows a nice flat curve up to 5 dBm although it looks like my rough measurement adds a systematic error of about -5 deg.
- to do -
* check RF power in every point of LO path on the circuit
* check if there is saturation by looking at wave forms.