Development of reliable field emission devices requires detailed characterization of variability over the device lifetime. Silicon tip manufacturing processes and test equipment limitations are theoretically predicted to cause similar to 5% variability in field emission currents. Long-duration emission current measurements at both constant voltage and constant current, controlled with a software feedback loop, were made. Power spectra from the two operational modes were significantly different. The software feedback loop response was determined; it is expected to contribute similar to 1% of the standard deviation in emission current when used to control emission by modulating the gate voltage. Convolution of constant voltage time series with the software feedback loop response reproduced the spectral characteristics of constant current time series, indicating that many of the differences between the two operating modes were caused by the operating mode itself rather than the field emitters tested. This has implications for the operating mode of the application for which the device is intended. (c) 2006 American Vacuum Society.