To create a linear sweep, you would enter in the start frequency - end frequency = duration to sweep in seconds.
So to use your example you would enter in the frequencies box after selecting File, Create Program:
This sweeps the 20 Hz range over 30 minutes, giving us a sweep speed of 0.01111 Hz per second.
It is important to calculate the sweep speed to ensure you don't skip large chunks of frequencies by moving to fast.
For more details, please check the link: