Constellation Sync Search (Digital Demod)

Menu Path: MeasSetup > Digital Demod Properties > Search tab

Constellation Sync Search uses a synchronization pattern to isolate a segment of the signal for display and analysis. The demodulator searches through demodulated data to find the sync pattern and then uses the Result Length to determine how much data to display and uses Search Offset to display data relative to the sync pattern.

Synchronization words (or patterns) are often used to resolve carrier phase ambiguity on non-differential modulation formats. It is important to realize that synchronization words are optional and are not necessary in order to achieve carrier locking.

The sync pattern must be a multiple of the number of bits-per-symbol. For example, if the number of bits-per-symbol is 4 (as with 16 QAM Quadrature Amplitude Modulation), the number of bits in the sync pattern must be a multiple of four. Sync search specifies any number of bits for the sync pattern, however, bits that are not a multiple of the bits-per-symbol are truncated. In this example, if 6 bits is entered for the sync pattern, sync search would only use the first four bits.

With Pulse Search off, triggering determines when the demodulator starts demodulating data and Search Length determines when the demodulator stops demodulating data. Sync Search locates only the first match and ignores any subsequent matches within the Search Length. The sync pattern and the Search Offset must fit within the search length.

Use Constellation Sync Search with or without Pulse Search, although many measurements require both features.

Sync Search uses the following parameters:

Using Constellation Sync Search

There are two ways to use sync search:

The following paragraphs describe Constellation Sync Search with Pulse Search OFF. To learn how Constellation Sync Search works with Pulse Search ON, see Pulse Search.

Constellation Sync Search with Pulse Search OFF

The following illustration shows how sync search works when Pulse Search is OFF. The illustration shows a positive (+) offset.

 image\syncdmd1_wmf.jpg

Trigger determines the start of search length.

Offset determines the position of result length with respect to the start of the sync pattern.

Result Length determines how much data is displayed.

Search Length determines how much data is demodulated.

Triggering

Selecting an appropriate trigger is critical when using Constellation Sync Search. Basically, the VSA must be triggered such that the sync pattern and Result Length fit within Search Length.

If the Search Lengthis set less than the Result Length, the VSA sets the Result Length equal to the Search Length.

The trigger signal determines when the VSA begins accumulating data for Search Length. The VSA begins to demodulate the signal when it receives a valid trigger and continues to demodulate the signal for the duration of Search Length. If Search Length does not contain the sync pattern, the VSA shows the SYNC NOT FOUND trace indicator.

The location of the sync pattern and the value of Search Offset determine the location of Result Length within Search Length. Because the sync pattern can occur anywhere within Search Length, it is possible to specify a Search Offset such that the Result Length doesn't fit within the Search Length. If this situation occurs, the VSA shows the SYNC NOT FOUND trace indicator.

If the demodulator finds two or more sync patterns in the Search Length, the VSA ignores all but the first sync pattern.

External trigger can be selected and a signal connected to the VSA's EXT TRIGGER BNC. In this case, a valid external-trigger signal causes the VSA to capture the amount of data specified by Search Length.

If Free Run trigger is used for continuous transmissions, care must be taken when setting the Search Length. For best results, set Search Length large enough to include at least two Result Lengths.

For additional information for troubleshooting digital demodulated measurements, see Troubleshooting.