Hold your horses
Above 900MHz the 5th overtone is used so at 1500MHz the actual port 0 output is a square wave of 300MHz and at port 1 this is mixed with the 7th overtone of a 214286428Hz to produce the 5000Hz input signal for the DSP stage
This works because a VNA controls both the output signal and the mixer LO.
For using as a SA please read my post on using the nanoVNA as spectrum analyzer.
