User Manual DDS200 Digital Demodulation System 070-9952-00

Manual OperationFor further information, refer to BER as a Function of C/N, Calculation ofFurther Parameters on page 2–82. The noise power is always derived from thenoise power set in the instrument in connection with the selected channelbandwidth (refer to SAW Filter on page 2–31).BER Measurement. During normal reception, the BER is continuously calculatedin the DDS200. The raw BER (BER before any error correction is made) isindicated. The BER measurement is fully automatic. The instrument selects therequired integration rate, depending on the error rate measured. The integrationrate is indicated in brackets after the measured value. It is always specified insamples; the duration of a sample depends on the selected order of QAM and onthe symbol rate.With 64 QAM and 6.900 MSPS, the BER events occurring in 241.55 ms arecombined to one sample. The second value in brackets states the number ofsamples collected by the instrument after the measurement is completed (lastcount). The first value indicates the recorded samples. The indication (133/1000)means that 133 of the 1000 desired measurement results (last count) have alreadybeen collected and stored. Of course, the BER can be evaluated now, but theaccuracy is not of optimum value. The first value is regularly increased until thelast count is reached.At this stage a switch over is made to a running BER calculation: the last 1000samples are combined to one measured value so that a new set of values isobtained every 241.55 ms. The great advantage of this method is that indicatedBER is always the latest value, since the measured value is continuouslyupdated. The system is flexible enough to immediately reduce the number ofintegrated samples at a sudden deterioration of the input signal quality, such aswhen a noise generator is connected.The running BER calculation can best be explained with an example using 10 asthe last count (see Figure 2–38). The other counts can be deducted.NOTE. BER, as measured by the DDS200, may be limited by system noise to10 -6 in 256 QAM.2–48 DDS200 Digital Demodulation System User Manual