User`s guide

Chapter 10 261
GPS Modulation
GPS Concepts
User Files
Like the other data types, user files require a selection for the data mode (see “Data Modes
and Subframe Structures” on page 258 for more information on data modes). User files are
created either internally to the ESG or externally and downloaded, but file sizes are limited to
the amount of available ESG memory. If files are created externally and contain a structure
that incorporates parity bits, then the raw data mode should be used. This would enable the
transmission of the file in its original form. Conversely, if your file did not have parity bits and
you desired to have parity bits incorporated into the signal transmission, then you would
select the encoded (Enc) data mode. This enables the ESG to compute the parity bits for the
last 6-bits of each word.
NOTE If the encoded mode is selected with a user file that incorporates parity bits, the
parity bits will be recomputed and their values changed.
Externally developed user files let you define signal structures that may not be available
internally to the ESG. For example, if your needs require a fully coded signal consisting of
frames structured according to the GPS standards (1 frame consisting of 5 subframes and 25
pages), this structure could be developed externally and downloaded into the ESG, making it
available for use.
User files can be created and accepted by the ESG in either binary or bit format.
Bit Error Rate Test Measurements
Bit error rate test (BERT) measurements are accomplished using the ESG with Option UN7
(Baseband BERT) installed or a customer-supplied, external BER meter. The GPS signal can
be configured using either a PN9 or PN15 sequence. The Raw data mode is used so parity bits
are not computed for the subframe. If the encoded (Enc) mode was selected, 6 parity bits
would be computed for every 24-bits of data. This would disrupt the PN sequence.
When BERT measurements are made using an externally supplied BER meter, the ESG
provides the GPS signal to the UUT (unit under test) and the BER meter would compute the
error results. If the ESG with Option UN7 is used, the ESG will provide the signal and make
the BERT measurements. When the ESG is making the BERT measurement, it will accept a
clock and gate signal from the UUT along with the data. If the UUT does not have the
capability to provide a data clock signal, the ESG can provide a clock signal from the rear
panel Aux I/O connector (refer to “13. AUX I/O Connector” on page 24 for information on this
connector). When the clock signal from the ESG is used, a delay must be added to the signal
that corresponds to UUT’s processing time. For information on setting up BERT
measurements refer to “Setting Up a GPS Bit Error Rate Test” on page 266, and for further
information on BERT, refer to “Bit Error Rate Tester–Option UN7” on page 184.