Abstract: An atomic beam resonator, particularly a cesium beam resonator, suitable for producing an extremely stable reference frequency. The resonator comprises, arranged in series in the beam direction, a source (1) of the atomic jet (2), a first state selector (3), an interrogating region (4) with a Ramsey-type cavity resonator having two arms (17, 18), a second state selector (5) an atom detector (6) for producing the useful output signal of the resonator, and a device for injecting an interrogation signal into the cavity resonator to generate a microwave interrogation field in each arm of said cavity. Said interrogating region (4) is surrounded by a magnetic shield (10). The resonator is characterized in that the device for injecting microwave energy (25) into the the cavity resonator is a coupling device which forms an odd number of modes in the cavity so that both interrogating regions are of opposite phase and the resonator response is at its lowest at the resonant frequency.