To provide additional details: I connected my VS60A to my SA124B option 1 (OCXO) and generated a 1GHZ -20dBm CW. Initial offset was -293 Hz. I let them run for 1 hour. And then the offset was -223 HZ (so around 70Hz drift over 1 hour)
But then provided a reference input to VSG60A. Initial offset: -284Hz. Already after 10 minutes, the offset is -224Hz (so, 60Hz drift in 10 minutes)
So, there’s no way for me to tell if the external clock is correctly detected and in-use by VSG60A based on these 2 scenarios