User Manual DDS200 Digital Demodulation System 070-9952-00

Manual OperationFrom the standard deviation N , the RMS value of the noise power (N) andthe signal to noise ration (SNR) are obtained directly as follows: According to the definition of the DVB Measurement Group, the SNR valueis a pure baseband quantity. This means that the calculated SNR cannot becompared directly with the C/N value of the transmission channel. Thedifference between SNR and C/N lies in peaking in the constellation diagram(3 dB) and in the root-cosine roll-off filtering (0.441 dB for r = 0.15).Phase JitterAs a last step, the RMS value of the phase jitter must be determined. Thefour outer corners of the constellation diagram are assessed, since this iswhere phase jitter has the strongest effect. Now the frequency distribution isdetermined for each decision field along the four circular paths farthest awayfrom the center point, which is the coordinate origin of the constellationdiagram. Here, too, the standard deviation PJ+N can be calculated, which isin addition affected by noise. The effect of a sinusoidal interferer, which maybe present at the same time, is eliminated by way of calculation prior todetermining the RMS phase jitter. According to the addition theorem of theGaussian distribution, the following applies: The RMS phase jitter can thus be obtained by means of trigonometricconversion.With an ideal QAM signal, all measured I/Q values lie exactly in the centersof the decision fields. If a signal is distorted, eg through amplitude imbalance,a phase error or a residual carrier, the measured I/Q values will not bein the centers of the decision fields but still concentrated on a single point ineach field. The measured values are available in discrete form as a result ofdigital processing of the I/Q coordinates. The resolution of the measuredvalues can be considerably increased by superimposing a small noisecomponent (SNR < 45 dB) on the signal to be analyzed and, using themethod described above, forming an average over several discrete values andthe above-named frequency distribution. This effect is shown in Figure 2–56for a signal distorted by amplitude imbalance.2–72 DDS200 Digital Demodulation System User Manual