Performance of the algorithms which process intracardiac electrograms (IEGMs) highly depends on the accuracy of estimating the times that electrical waves pass the area under the electrodes. Estimating these activation times (ATs) from IEGMs during atrial fibrillation (AF) is extremely challenging as electrical activities of atria are very complex, non-stationary, and irregular. In this paper, we propose a new activation detector which is based on the test of the equality of variance of two sets of data. At any time t, we consider two sets of IEGM data: 1) data in a bounded interval around t, 2) data in bounded intervals around the first interval. We show that the activation zone can be extracted by comparing the variance of these two sets, i.e., we introduce a new preprocessing approach and show that it can effectively highlight activation zones of IEGMs. Our simulation results on bipolar atrial IEGMs gathered during AF confirm the efficiency of the proposed preprocessing method.