A burst signal is defined as a signal that has significant power for a known duration, and no power (dead time) outside of the interval. To demodulate such signals, the dead time must be avoided. Two ways to ensure that the intented part of the signal is used for demodulation and measurement are as follows.
Configure a trigger to align acquisition to the burst. Specify the number of symbols such that the measurement interval does not exceed the burst duration from start of trigger. The following equation shows the relation between the measurement interval and number of symbols.
The analyzer acquires an additional duration to account for filter delays in the measurement.
The error vector magnitude (EVM) at the two ends of the burst might be higher when the signal beyond the burst duration is acquired. Reduce the number of symbols and configure trigger delay so that only the intended part of the burst is used for demodulation and measurement.
Configure a trigger to align acquisition to the burst when you set the DDemod Signal Structure property to Bursted. The demodulator will detect the falling edge of the first burst, which can occur anywhere within the defined measurement interval. However, measurement will NOT be detecting the rising edge of the burst and hence the user needs to give appropriate trigger settings. The trigger point will be considered as the start of the burst.
The following equation shows the relation between the measurement interval, number of symbols, burst start exclusion symbols and burst end exclusion symbols.
Acquisition time is calculated using the following equation.
where the Burst Start Exclusion Symbol specifies the number of symbols from the start of the burst trigger that is excluded from the measurement and Burst End Exclusion Symbols specifies the number of symbols that is excluded from the measurement before the falling edge of the burst.
If the signal is bursty, NI recommends that you set the DDemod Signal Structure property to Bursted.
Symbols beyond the detected falling edge of the first burst are not used for demodulation.
The following images do not illustrate the extra acquisitions done for filter delays.
If the burst length is less than the acquisition length, then the burst analysis algorithm will detect the falling edge well within the acquisition length. In such a case, the number of symbols is coerced to a lower value such that the Burst Length = Burst Start Exclusion Symbols + Number of symbols + Burst End Exclusion Symbols, as illustrated in the following image.
If the burst length is greater than the acquisition length, then the measurement will not find the falling edge within the acquisition length. In such a case, the number of symbols is not coerced and specified number of symbols is used for demodulation, as illustrated in the following image.