So instead of adding a rotary encoder to change frequencies, I added a joystick. However, I coded the joystick in such a way that the user does not have to move it multiple times to switch from, for instance, 91.0 to 93.7. 

Instead, as soon as the user pushes the joystick to the right once, a loop will begin and the frequency will increase by 0.1 every second. Then, by pushing down on the joystick button, the user can stop at the specific frequency. Similarly, if the user pushes the joystick to the left then the frequencies will start decreasing by 0.1 every second. 

The frequency control only uses the x-axis and button of the joystick. This leaves me with the y-axis which I intend on using for volume control in the future.