To measure AC PSRR of LDO, you can use LC summing node or a bias tee. This mixes the AC and the DC of your signal generator and DC power supply. When using a bias tee be sure to use high current driving capability..picosecond 5547 can do the trick if your ldo load is less than 500mA. Then you can measure the Input ripple which is supplying by your signal generator at a defined ampitude(lets say 500mV ptp)and frequency and measure its corresponding ripple across the input and output capacitor with a spectrum analyzer. The logarithmic ratio of output to input is your PSRR.