  Light-induced frequency shifts can be a key limiting contribution to the mid
and long-term frequency instability in atomic clocks. In this letter, we
demonstrate the experimental implementation of the combined error signal
interrogation protocol to a cold-atom clock based on coherent population
trapping (CPT) and Ramsey spectroscopy. The method uses a single error signal
that results from the normalized combination of two error signals extracted
from two Ramsey sequences of different dark periods. The single combined error
signal is used to stabilize the atomic clock frequency. Compared to the
standard Ramsey-CPT interrogation, this method reduces the clock frequency
sensitivity to light-shift variations by more than one order of magnitude. This
method can be applied in various kinds of Ramsey-based atomic clocks, sensors
and instruments.
