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In this paper, the development of a pulse stability measurement device is presented. The measurement accuracy is better than 250uV, about 4.2ppm of a typical 60V input pulse. Pulse signals up to +/-80V peak can be measured. The device works together with an oscilloscope. The primary function of the measurement device is to provide a precision offset, such that variations in the flattop of the modulator voltage pulse can be accurately resolved. The oscilloscope records the difference between the pulse flattop and the reference for a series of waveforms. The scope math functions are utilized to calculate the rms variations over the series. The frequency response of the device is characterized by the measured cutoff frequency of about 6.5MHz.In addition to detailing the design and calibration of the precision pulse stability device, measurements of SLAC line-type linac modulators and recently developed induction modulators will be presented. Factors affecting pulse stability will be discussed.

Information concerning to a stability of LCLS RF linac modulators is allocated in this paper. In general a "pulseto-pulse" modulator stability (and RF phase as well) is acceptable for the LCLS commission and FEL programs. Further modulator stability improvements are possible and approaches are discussed based on our experimental results.

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