Historically, the instability of the high voltage of TEM systems had been one of the major problems behind the resolution improvement. To achieve atomic resolution, one needs the stability in the range of one part per million (1 ppm). For this purpose, Cockcroft–Walton voltage generator has been used to supply the high voltage to the electron guns and a negative feedback system is also used. In addition, discharge in the high voltage area should be prevented. However, the most common phenomena in high-voltage TEMs is still discharging, for instance, discharging often occurs when the high voltage is turned on. This discharge can induce damage to the insulators even though it also cleans the inner surface of the electron gun.
In STEM, the fluctuations in accelerating voltage or the current flow in the probe-forming electromagnetic lenses contributes to the chromatic aberration that leads a cutoff to the highest spatial frequency. In HRTEM, the cut-off frequency corresponds to the information limit. In STEM, the effect of the fluctuations induces additional contribution to the probe size ,
Cc -- The chromatic coefficient,
E0 -- The energy of the electron beam,
I -- The current in the probe-forming lens,
ΔE -- The spread in energy of the beam,
ΔE0 -- The fluctuation in the accelerating voltage,
ΔI -- The fluctuation of the lens current.
The high-tension power supply and beam-current in modern electron microscopes can achieve 10-7 stability so that the main source of the chromatic aberration is due to energy spread of the electrons within the probe.
The practical (real) energy resolution of EELS in a TEM depends not only on the energy spread of the electron source, but also on instabilities in accelerating voltage of the electron beam, spectrometer energy dispersion and stray electromagnetic field.
 Spence JCH. High resolution electron microscopy. 3rd ed. Oxford: Clarendon Press; 2003.