In decade modulation, a number of frequencies in a multiple of 10 are generally used. The phase difference for each frequency and thus equivalent distance is determined separately and finally the total distance is computed.
Let us consider a modulation frequency of 15 MHz has been set up in the instrument, resulting in a half wavelength of 10 m.
Thus, a full sweep of the phase meter represents 10 m distance i.e., the phase-meter provides reading in unit meters and its decimal part and thus the range of the measured distance is from 0 to 9.999 m. For example, in a distance of, say, 4567.123 m, this frequency would provide the distance 7.123 m. Switching down to 1.5 MHz, the half wavelength is now 100m, which is resolved by the phase meter to give the tens of metres i.e., reading ranging from 10 to 90 m. In this example 60 (6 tens). The next frequency is then 0.15 MHz, which in conjunction with the phase meter, gives the hundreds of metres. In this instance it is 500 (5 hundreds). Finally, a 15-kHz frequency will give the number of thousand metres in the distance, which in this instance is 4000 (4 thousands). Thus, total distance is 7.123 + 60 + 500 + 4000 = 4567.123 m.