I have one doubt regarding how to analyze a constellation diagram. It is from a QPSK transmitter. The constellation diagram looks typically as books (as a square as you can see in Figure 1), but for shorts periods it changes as you can see in Figure 2. We don't know if it is a problem with the transmitter, a problem with the data that is being modulated, or if it is a bad configuration on the Spectrum Analyzer. What do you think about that? How can we interpret the constellation diagram and the transitions? Thanks in advance.
UPDATE: 27/02/2018 Please see Figure 3 that shows how we set up of the Vector Analyzer.