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Energy oscillations of electrons in the beam (synchrotron oscillations) lead to a modulation of the spin tune and hence to sidebands (Bessel bands) around the main resonance frequency. These sidebands are equally distanced from the resonant frequency by multiples of the synchrotron tune . To distinguish the main resonance from its sidebands is varied.
The synchrotron tune and the RF voltage are related by :
Sidebands are identified by a change of the RF voltage which results in a change of synchrotron tune thus changing the distance between resonance dip and its sidebands noticeably. If the sweep is repeated with changed RF voltage one dip stays unchanged and the others are shifted. The unshifted resonance is the main resonance, the shifted resonances are sidebands.