Assessing respiratory impedance (Zrs) in ventilated patients over a wide frequency band, ranging from breathing rates to typical forced oscillation frequencies, during end-expiratory pauses at different positive end-expiratory pressures (PEEP) is of potential interest to assess a patient's respiratory mechanics. Zrs measurements under these conditions are not possible with the present variants of the forced oscillation technique. The aim of this work was to design a forced oscillation generator operating from spontaneous breathing frequencies whilst withstanding PEEP. To this end, we constructed a generator based on a servocontrolled loudspeaker. This allowed the loudspeaker cone to remain at its resting position regardless of the external PEEP applied. The system was optimized by using a mechanical analogue. The clinical applicability of the servocontrolled generator was assessed by measuring Zrs in mechanically-ventilated chronic obstructive pulmonary disease (COPD) patients during end-expiratory pauses at different transrespiratory pressures. The forced oscillation generator designed may be easily applicable in practice since it is small and light. The system is able to withstand transrespiratory pressures of up to 17 hPa and allows the application of forced oscillation of sufficient amplitude ( > 2 hPa peak-to-peak, 0.25-26 Hz) to obtain reliable respiratory resistance and reactance data. The servocontrolled generator permits the assessment of respiratory mechanics over a wide frequency band ranging from breathing frequencies to the most typical forced oscillation frequencies during end-expiratory pauses at PEEPs within the conventional range.