Title: Frequency responses of the K-Rb-$^{21}$Ne co-magnetometer,
Abstract: The frequency responses of the K-Rb-$^{21}$Ne co-magnetometer to magnetic
field and exotic spin dependent forces are experimentally studied and simulated
in this paper. Both the relationship between the output amplitude, the phase
shift and frequencies are studied. The responses of magnetic field are
experimentally investigated. Due to a lack of input methods, others are
numerically simulated.