Result Length (Digital Demod)
determines the number of symbols that the VSA displays. For example, if the result length is set to 10, the VSA displays 10 symbols in time-domain traces (such as vector, eye, and constellation diagrams).
For the Offset QPSK digital demodulation format, if Low SNR Enhancement is enabled, the maximum result length is about 40000 symbols, otherwise it is 2048. In both cases this maximum may be limited by "Search Time" or by hardware capabilities.
Result Length and Carrier Lock
The result length for some demodulation formats affects carrier lock. The following table shows the minimum result length as well as the maximum frequency offset for reliable carrier lock:
Digital Modulation Format |
Minimum Result Length (0 Hz frequency offset) |
Maximum frequency offset (Result Length = 1000 symbols) |
---|---|---|
50 symbols |
9.6% symbol rate |
|
50 symbols |
4.8% symbol rate |
|
32-QAM |
75 symbols |
3.15% symbol rate |
64-QAM |
150 symbols |
4.65% symbol rate |
128-QAM |
250 symbols |
0.3% symbol rate |
256-QAM |
400 symbols |
0.3% symbol rate |
512-QAM |
1000 symbols |
0.15% symbol rate |
1024-QAM |
1000 symbols |
0.15% symbol rate |
2048-QAM |
4000 symbols |
0.1% symbol rate* |
4096-QAM |
4000 symbols |
0.1% symbol rate* |
|
|
|
DVB 16- QAM |
50 symbols |
4.65% symbol rate |
DVB 32- QAM |
75 symbols |
2.85% symbol rate |
DVB 64- QAM |
150 symbols |
4.65% symbol rate |
DVB 128- QAM |
250 symbols |
0.45% symbol rate |
DVB 256- QAM |
400 symbols |
0.15% symbol rate |
|
|
|
Star 16-QAM |
50 symbols |
4.95% symbol rate |
Star 32-QAM |
75 symbols |
4.95% symbol rate |
*Using Low SNR Signal-to-Noise Ratio Enhancement is required to make frequency locking robust for this QAM format.
QAM formats with 512 or more states are very sensitive to noise. Even an EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. of approximately 2% can cause symbol detection errors in the algorithm. This can cause a positive bias on EVM (an EVM appears better than it really would be for that signal). Analysis of these higher-order QAM formats is not intended with a Signal to Noise Ratio of less than 40dB.
Result Length for Pre-Demodulated Time Data
The trace data) is 20% larger than the for demodulated data so that the pre-demodulation traces include the transition regions before and after the demodulated data. This can be used to verify the expected RF Radio Frequency: A generic term for radio-based technologies, operating between the Low Frequency range (30k Hz) and the Extra High Frequency range (300 GHz). envelope of a pulsed signal in the time domain and ensures that frequency domain spectrum measurements also include these transition regions.
for pre-demodulated data (such as the , , andSee Also