Apogee 2349 DLTM4, Instructions Manual

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09.21.2017
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The DLTM4 provides the facility for a user to measure the time delay introduced by a data path. This provides an easy
solution for determining the path delay of a data communication system, such as through a satellite path.
Link delay is measured directly on the DLTM4 (Model 2349). The DLTM4 is configured to run PRN data through the
system with the data being looped back to the DLTM4. To measure the link delay, an error is automatically inserted into
the PRN pattern. The time it takes for that error to propagate through the link and back is measured; this value represents
the full loop link delay. In the diagrammed example shown in Figure 21, the measured time represents the time to propagate
twice the length of the link. The delay measurement is repeated to determine the average delay to account for any system
variance in the data path. The DLTM takes into account internal delays caused by data rate and internal circuitry. Thus,
the actual delay through the link is reported.
Because the delay is measured by inserting a known error into the data, link delay can only be measured in an error-free
link. The value of the link delay is measured and displayed to an accuracy of one tenth of a microsecond.
5.4.10 SETUP & CONFIGURATION
The following steps describe the setup for the DLTM4 to configure it to measure the link delay on a full duplex system:
1. The system that is being tested is configured in full loop-back mode. Data from the DLTM4 passes through the
system to be tested and is looped back at the remote end and returned back to the DLTM4.
2. The Transmit Side (Output Settings) is set up to run in PRN mode. The rest of the transmitter settings are
configured to be compatible with the data being sent. Data and clock from either the TTL TX or RS422 TX are
connected to the input of the link to be tested. On the Output Settings page, set the data rate.
3. The Receive Interface (RX Settings) is configured to be compatible with the data and interface electronics at the
return link.
4. Observe the MEASURE screen and verify that the system is running and that it is passing error-free data through
the system. If errors are present, check the ERRORS screen to verify that No Errors are programmed to be
introduced into the transmit stream. If errors are still being received, verify that the interfaces are configured
correctly. The Link delay cannot be accurately measured on a corrupted data link.
5.4.10 MEASURING THE LINK DELAY
After an error free link is established, the link delay can be measured by using the Status display.
1. In the right column, change the actions field to Test Link delay.
2. Verify that the data is running error free, press F5 (CLEAR) to clear the error counter. If data is running error
free, press the Send button to initiate the measuring of the link delay.
3. The measured delay is displayed (in microseconds) in the 2LNK DLAY data field. Verify that no errors occurred
in the ERRORS field. If an error occurs it may trigger the link delay measurement prematurely, which renders
the measurement invalid.
4. Steps 3 and 4 are repeated to take subsequent measurements of the link delay.
6.4 DOPPLER SHIFT WITH THE DLTM4
The DLTM4 Data Link Test Module is programmable so as to simulate data rate changes due to Doppler Shift caused by
the relative motion between the data source and the listening station. This is commonly seen when receiving a signal from
a spacecraft. The DLTM4 mimics Doppler Shift by increasing and decreasing the output bit rate according to a
programmed model. The shift in output bit rate is user set to follow one of three patterns: SINGLE, SAWTOOTH, or
CYCLE output. Refer to section 5.4.14 for complete definitions of these modes. The bit rate change curve that is followed
is determined by the setting of the setting of the DOP MODE field. Setting this field to Linear produces a linear change
in output bit rate that follows the setting of the cycle time value in bits per second per second. Setting the DOP MODE to
CURVE causes the rate of change of the output bit rate to follow a curve that emulates that which is caused by the motion
of a satellite. The example setup below causes the output bit rate to follow the satellite curve.

Program the DLTM4 to generate a PRN data pattern of whatever length that is appropriate for the system under
test.