User Manual DDS200 Digital Demodulation System 070-9952-00

Manual OperationThe term “modulation error ratio” and the prescribed method of calculation weredeclared international standard by the DVB Measurement Group. Some testsystems, however, specify the “error vector magnitude,” so the conversion rulefor the two quantities is given in Table 2–7. The two quantities differ only in thereference used. The modulation error ratio is referenced to the RMS value of thebaseband signal; the error vector magnitude is referenced to the peak value.Table 2–7: Limit values for modulation errorOrder MER EVM [%] EVM MER [%] MER EVM [dB] EVM MER [dB]416EVM = MEREVM = MER / 1,342MER = EVMMER = EVM * 1,342EVM = MEREVM = MER – 2,56dBMER = EVMMER = EVM + 2,56dBÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ3264EVM = MER / 1,304EVM = MER / 1,527MER = EVM * 1,304MER = EVM * 1,527EVM = MER – 2,31dBEVM = MER – 3,68dBMER = EVM + 2,31dBMER = EVM + 3,68dBÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ128 ÁÁÁÁÁÁÁÁEVM = MER / 1,440 ÁÁÁÁÁÁÁÁÁMER = EVM * 1,440 EVM = MER ÁÁÁÁÁÁÁÁ– 3,17dB MER = EVM + ÁÁÁÁÁÁÁÁ3,17dBÁÁÁÁÁÁÁÁ256 EVM = MER / 1,627MER = EVM * 1,627EVM = MER – 4,23dBMER = EVM + 4,23dBOther ErrorsÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁThis subsection describes the effects of errors on the QAM signal that havenot been investigated before.Impulse noise:Impulse noise is caused by switching and ignitionprocesses. The pulses are very short and of widelydiffering shapes. Since the analysis of the constellationdiagram is based on approximately 1,000,000 symbols,the time characteristic of the QAM signal and theinformation regarding any superimposed noise is lost.Moreover, there is no information as to what symbolswere incorrectly demodulated. It is, therefore, notpossible to evaluate impulse noise separately fromGausian noise.Nonlinearities Nonlinearities of amplifiers cause amplitude and phaseof amplifiers: distortion of modulated signals. Since linear QAMmodulation is employed mainly in cable systems wheresignal levels are relatively low, feedback amplifiers canbe used. These amplifiers are sufficiently linear so thatthe effect on the QAM signal is negligibly small.2–76 DDS200 Digital Demodulation System User Manual