Abstract: The effect of Digital BaseBand Converter (DBBC) sub-channel delay on bandwidth synthesis precision is analyzed under the condition of the consistent receiving system channel delay in order to improve the accuracy of bandwidth synthesis. Through a theoretical derivation, it is for the first time to discover that the sub-channel delay could induce phase step phenomenon among the different sub-channels in the group delay measurement of a single station. And in the delay difference measurement of different stations, the phase step phenomenon could occur when the frequency differences of different stations corresponding sub-channel Local Oscillator (LO) are different. The phase step could deteriorate the precision of bandwidth synthesis. The impact domain of the sub-channel delay is discussed for different DBBCs, suggesting that the phase step can be eliminated by a sub-channel delay compensation. The simulation results show that the sub-channel delay compensation can effectively remove the phase step and the precision of bandwidth synthesis can be improved by at least one order of magnitude or more.