User Manual DDS200 Digital Demodulation System 070-9952-00

Manual OperationWhen selecting the position of the central tap, the following should beconsidered:An equalizer with the central tap below 11 should be used for compensatinga signal subject to considerable post-echoes (for example, causedby reflections). In this case a maximum range is available for echocompensation. However, only in exceptional cases should the coefficientbe set to a value below 6. A value below 2 is not useful at all.A signal generated under laboratory conditions should be measured withthe central coefficient in position 11 or 12. Thus the equalizer can adaptsymmetrically to given conditions.If the receiving conditions are known and a shorter equalizer should besimulated, the central coefficient may be set to a position above 12.Generally, the yellow LED next to the SPECIAL FUNCTION key comes onif a central coefficient outside the range 6 to 18 is selected. The preset valueis 12.ROLLOFF FACTOR. The ROLLOFF FACTOR key is pressed to define the rootcosine pulse shaping of the digital input filter. For compliance with the firstNyquist condition, pulse shaping in the whole transmission path must besymmetrical to the –6-dB point at the Nyquist frequencies. According to DVBspecifications, the rolloff factor in the modulator is 0.15. The same rolloff factormust be set in the receiver. Another factor may be chosen for laboratory tests.MIN BER INTEGRATION. With the MIN BER INTEGRATION key, the BERmeasurement can be influenced. For details on operation and usefulness of thissetting, refer to Selection of Integration Time for BER Measurement onpage 2–79. For details on the BER EXTERN mode, refer to BER Measurementwith External Equipment on page 2–80. Note that in this case, the serial andparallel MPEG outputs are disabled and only the female connectors X6 (serialclock) and X5 (serial data) are active. Using the integrated PLL, any jitter can beremoved from the signals. When the MIN BER INTEGRATION setting ischanged, the ALARM register is cleared. The preset value is 10 samples.LOOP BANDWIDTH. When the LOOP BANDWIDTH key is pressed, the loopbandwidth for carrier recovery can be configured. The typical capture range forcarrier recovery is more than 5% of the symbol rate. This corresponds toapproximately 350 kHz in the case of standard DVB signals. This wide(permanently set) capture range is sufficient for any kind of application. Thedynamic characteristic can be changed with the aid of the LOOP BANDWIDTHfunction.The choices for changing the dynamic characteristic using the LOOP BAND-WIDTH function are as follows:DDS200 Digital Demodulation System User Manual 2–35