Hi flyhigh,
1.measure LO power level nLO using spectrum analyzer
2. connect LO to mixer, terminate IF port
Why is that the LO power level need to be measured , Could i just straightaway connect the LO input to the signal generator and key in the respective power level that I want to measure?
Yes, you can in principle, for the first test. Just make sure your power can drive mixer to reasonable extent. Almost all mixer characteristics, e.g. coversion gain-loss, noise, intermodulations are very dependent on LO power. For final test you should connect the real LO to be sure about pulling of its frequency by the mixer, real power due to LO signal reflection etc.
3. measure level of LO frequency sigal on RF port nRF_LO using spectrum analyzer
4. terminate RF port
5. measure level of LO frequency sigal on IF port nIF_LO using spectrum analyzer
Then LO-RF isolation is nLO - nRF_LO
LO-IF isolation is nLO - nI
flyhigh