  We have developed a universal method to form the reference signal for the
stabilization of arbitrary atomic clocks based on Ramsey spectroscopy. Our
approach uses an interrogation scheme of the atomic system with two different
Ramsey periods and a specially constructed combined error signal (CES) computed
by subtracting two error signals with the appropriate calibration factor. CES
spectroscopy allows for perfect elimination of probe-induced light shifts and
does not suffer from the effects of relaxation, time-dependent pulse
fluctuations and phase-jump modulation errors and other imperfections of the
interrogation procedure. The method is simpler than recently developed
auto-balanced Ramsey spectroscopy techniques [Ch. Sanner, et al., Phys. Rev.
Lett. 120, 053602 (2018); V. I. Yudin, et al., Phys. Rev. Appl. 9, 054034
(2018)], because it uses a single error signal that feeds back on the clock
frequency. CES universal technique can be applied to many applications of
precision spectroscopy.
