Result Length (Digital Demod)

Result length 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)
QPSKQuadrature phase shift keying	50 symbols	9.6% symbol rate
16-QAMQuadrature Amplitude Modulation	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 SNRSignal-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 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 Result Length for pre-demodulated data (such as the Time, Spectrum, and Inst Spectrum trace data) is 20% larger than the Result Length 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 envelope of a pulsed signal in the time domain and ensures that frequency domain spectrum measurements also include these transition regions.