Power　line　communication　is　attracting　increasing　attention　for　its　characteristics　of　easy　availability　and　low　cost.　Frequency　shift　keying　with　the　virtue　of　antijamming　capability　is　suitable　for　the　low　signal-to-noise　ratio　power　line　environment.　The　principle　of　the　frequency　shift　keying　communication　system　based　on　momentary　Fourier　transform　(MFT)　algorithm　is　provided　first　in　this　paper.　Theoretically,　the　twiddle　factor　in　the　MFT　algorithm　is　a　decimal　with　infinite　length.　However,　when　it　is　applied　on　digital　signal　processor　(DSP),　the　registers　with　finite　length　in　DSP　lead　to　the　inaccuracy　of　the　twiddle　factor.　Thus,　the　MFT　algorithm　with　recursive　form　will　cause　accumulative　error　as　a　result.　We　analyze　the　impact　of　the　inaccurate　twiddle　factor　on　the　MFT　algorithm　from　a　novel　point　of　view,　by　which　the　MFT　algorithm　is　considered　as　a　linear　time　invariant　system.　We　prove　that　the　accumulative　error　would　not　disperse　when　the　modulus　of　the　twiddle　factor　is　smaller　than　1.　Simulation　results　show　that　the　truncation　approach　is　more　effective　than　both　carrying　and　rounding　methods　in　a　practical　system.　In　addition,　the　number　of　significant　digits　of　truncated　twiddle　factor　must　be　chosen　larger　than　4　to　overcome　the　problem　of　accumulative　error.　Copyright　(C)　2012　John　Wiley　&　Sons,　Ltd.