A novel robust encoding mechanism for spectral-domain chipless tags is proposed. The idea is based on storing information in a passive resonator by using a differential backscattering coding. This is achieved by subtracting multi-resonant slightly frequency-shifted amplitude responses obtained along vertical and horizontal polarization. The encoding mechanism is demonstrated by using a tag formed by a periodic surface patterned on a conductor-backed substrate. The unit cells of the periodic surface are formed by three concentric square loops loaded with asymmetric stubs. The resonant peaks are shifted finely and independently from each other by simply changing the length of the stubs. The reliability of the proposed encoding mechanism is experimentally verified by prototyping some samples of the tags on low-cost FR4 boards. © 2015 IEEE.