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How does the 89400 series vector signal analyzer (VSA) lock to the symbols when it demodulates a vector-modulated signal?

There is no coherent carrier or symbol clock timing signal required for demodulation. The actual demodulation algorithm is proprietary, but we can say in general that it expects random data and a significant subset of states. The functional sequence is:

  1. The VSA looks for amplitude and phase changes to lock to the symbol clock.
  2. The VSA looks for the largest magnitude symbol to map to the outermost state and scales the rest of the symbols.

Therefore for any vector modulation format that changes in both amplitude and phase like QAM, the algorithm must see a change in amplitude and phase from one symbol to the next such as would be found in a random sequence. Fixed patterns may not be demodulated to the bit level correctly because they lack the necessary variety of amplitude and phase changes.

For constant-amplitude formats such as QPSK or standards such as GSM or NADC, fixed patterns work as long as there is some change in the data. For best results, use the VSA sync search function and enter a pattern of up to 32 bits. Look for more information in the built-in Help when in demodulation mode by pressing Time (hardkey) > sync search (F3) > Help (hardkey).

To see the data sequence in the symbol table that matches your particular signal source, it may be necessary to remap the constellation in the VSA. This process reassigns data sets to symbol states. Refer to "To View and Change Display State Definitions" in the chapter called "Analyzing Digitally Demodulated Signals" in the 89410A / 89441A Operator's Guide.

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