User Manual DDS200 Digital Demodulation System 070-9952-00

Manual Operationare taken into account automatically. To refer C’ to the Nyquist bandwidthB N of the DVB signal, the signal power C is to be calculated as follows:C C10lg B N C 10lg(symbol rateHz) dB [dBm]Switch the signal of the DVT 200 Digital Video Transmitter off, switch thenoise generator on, and measure the noise power density on the spectrumanalyzer. For this, set a marker again to the same frequency as the QAMcarrier and select the NOISE marker mode. The noise power density N’ isindicated by the analyzer. To refer N’ to the channel bandwidth B C of theDVB channel used, the noise power N is to be calculated as follows:N N10lg B C N 10lg(channel bandwidthHz) dB [dBm]From this the C/N ratio is obtained as follows:C N[dB] C [dBm] –N [dBm]The noise level and thus the BER can be changed by modifying the attenuator.The relationship between the BER and the signal-to-noise ratio C/N will beexamined in the next section.BER as a Function of C/N, Calculation of Further Parameters. If a QAM signal isdisturbed by additive Gaussian noise, error-free demodulation is not possible.The error frequency is indicated by the bit error rate. Figure 2–61 shows thetheoretical relationship between the C/N ratio and the bit error rate. Anyadditional errors that may occur cause a shifting of the displayed curvescorresponding to the magnitude of the error and thus an increase of the bit errorrate.2–82 DDS200 Digital Demodulation System User Manual