use a mixer as a phase detector (u need on with a DC coupled IF output). Line length L2 is much longer than L1, so it forms a delay line frequency discriminator. You might need to trim L2 or L1 slightly to get the mixer into quadrature. Drive the power splitter with >14 dBm. The oscilloscope shows the voltage out of the mixer, which stops changing when the DDS reaches final frequency and phase.
Syncronized zero span SA can be used to measure locking time any kind of PLL.External Sweep Triggering is coupled to Latch Enable signal-for instance- when sweep is triggered zero spanned ( Time Domain Mode) SA will show you ringing and locking time of the carrier.