Pulse Search (Digital Demod)

Pulse Search demodulates pulsed (burst) transmissions. Selecting the Pulse Search check box turns on Pulse Search; clearing it disables Pulse Search.

When the Digital Demodulation measurement type is first selected, the default setting for Pulse Search is disabled (cleared).

Digital communication systems that use pulsed transmissions send and receive information in bursts (such as those in mobile units). In other words, carrier power is switched on and off. The carrier switches on when the transmitter transmits data and switches off when the transmitter finishes transmitting data. If Pulse Search is on, the demodulator searches within the defined Search Length to locate the first complete pulse, which can occur anywhere in the Search Length. Pulses beyond the first complete pulse are disregarded and are not demodulated. In order for the Pulse Search algorithm to detect a pulse edge, the pulse must be at least 15 dB above the noise floor. The resultant pulse is then demodulated. If Constellation Sync Search is selected in addition to Pulse Search, the demodulator performs Pulse Search first then performs sync search on the resultant demodulated pulse as illustrated below.

$image\pulsesearch_wmf.jpg$

The Result Length, which includes both the offset and the sync word, must not violate the boundaries of the demodulated pulse.

Using Pulse Search

There are two ways to use Pulse Search:

Pulse Search with Constellation Sync Search OFF
Pulse Search with Constellation Sync Search ON

Pulse Search with Constellation Sync Search OFF

When Pulse Search is used and Constellation Sync Search is disabled, these parameters determine how the demodulator searches for the pulse and the data that is demodulated:

Search length determines the length of time over which the demodulator searches for a complete pulse (off-on-off).

The trigger signal determines when the demodulator begins its search.

Result length determines how much information is demodulated. Only the first pulse detected in the search region is demodulated.

The demodulator automatically centers the Result Length with respect to the pulse. In other words, the demodulator positions the Result Length around the center of the pulse, as shown below.

$image\pulsedmd_wmf.jpg$

Pulse Search With Constellation Sync Search ON

When Pulse Search is used and Constellation Sync Search is enabled, these parameters determine how the demodulator searches for the pulse and the data that is demodulated:

When Constellation Sync Search is ON, the demodulator searches for a pulse within the Search Length. The demodulator demodulates the entire pulse to locate the synchronization pattern (also called sync pattern or sync word). If there are multiple pulses within the Search Length, the demodulator demodulates only the first pulse.

The demodulator uses the Search Pattern and the Search Offset to position the Result Length¾the Result Length is NOT centered around the center of the pulse.

The following example shows the operation of Pulse Searchwhen sync search is ON. The example uses a positive (+) offset (for details about offset, see Search Offset).

$image\pulse2_wmf.jpg$

Tips When Using Pulse Search

Here are some additional tips for using Pulse Search:

Determining Search Length when performing a Pulse Search:
Search Length = (2*MaxOn) + MaxOff
where Search Length is in seconds.
MaxOn is the maximum on time of a pulse in seconds.
MaxOff is the maximum off time between pulses in seconds.
If a single measurement contains more than one pulse, the demodulator examines only the first pulse. The demodulator ignores all other pulses in the measurement¾even when sync search is used and the first pulse does not contain the sync word. This situation can occur for large search lengths. For example, if the pulse consists of 100 symbols and the Search Length is 500 symbols, a single measurement may contain up to 4 pulses. Use external triggering to align the desired pulse near the beginning of the search record.
Increasing the Span may improve pulse-position repeatability. Narrow spans may distort the pulse shape, whereas wider spans offer better time resolution in the pulse detector. For spans below twice the symbol rate, you will see about 1 symbol of jitter Variation in the amount of time that it takes packets to traverse the network and arrive at a destination.. For more stable results, set the Span equal to or larger than three times the symbol rate.
It is often helpful to view the Search Time and Time trace data when setting up or troubleshooting pulsed measurements. The Search Time trace data shows data before applying Pulse Search; the Time trace data shows data after applying Pulse Search.

Status Bits: The status bits for "Pulse Not Found" only apply to Digital Demodulation Vector Modulation Analysis (Option AYA) and is not meaningful for any other measurements.