In a passive atomic clock, an external Local Oscillator (LO), typically a high-quality quartz crystal at 5 MHz (Megahertz, or million oscillations per second, in the radiofrequency (RF) band of the spectrum), is multiplied up to the 9.2 GHz (Gigahertz, or billion oscillations per second, in the microwave band). These microwaves are used to interrogate the atoms. By detecting the state the atoms are in after the microwave interaction, it is possible to determine the frequency difference between the microwaves and that of the Cs atoms' natural transition. This error signal can then be fed back to the LO to steer it to the correct frequency.