Apogee 2349 DLTM4, Instructions Manual

Page: 28 / 33

210 South 3
rd
Street, North Wales, PA 19454
Tel: 215.699.2060 Fax: 215.699.2061
22
09.21.2017
A
POGEE
L
ABS
, I
NC
.
6.1.2.2 Insert Known Error Rate
This feature is useful when testing the DLTM4 receiver and display or for testing the error recovery
6.1.3 BLANKING
The Blanking feature forces a program-selectable number of data bits to the zero state after bit coding is applied. This
simulates line dropouts and provides a measure of the ability of a bit synchronizer to achieve or maintain Lock during and
after data periods without transitions. The DLTM4 allows the number of blanked bits to be set and it provides two
operating modes. The first is a ‘free-run’ mode where the blanks are inserted after a programmable number of normal bits.
For example, blank 100 bits then output normally for 900 bits. The second mode is ‘frame synchronized’ where the
blanking begins a selectable number of bits after the start of the PCM (or PRN) frame and occurs only once per frame.
This feature is handy when it is desired to erase a certain field in the PCM data stream.
6.1.4 SYMMETRY TESTS
Data symmetry is the ratio of data ‘1’ bits to data ‘0’ bits. PRN data begins with symmetry of 50%, an equal number of
1’s and 0’s in the long run. Thus if a link has a biasing problem, for example if the receiver discriminator is offset from
the transmitted frequency, there may be a greater tendency to change 0’s into 1’s or vice versa. The DLTM4 detects and
reports this bias.
The AL6300 DLTS provides a count of 1’s in error and a symmetry calculation to support this kind of investigation. With
random errors, the symmetry should show 50% with PRN data making the evaluation straightforward. While displaying
the number of bit errors and 1’s in error, FREEZE the AL6300 display updates and compare these numbers. Normally the
1’s in error should be about half of the total bit errors. Any bias becomes apparent. An alternative method is to display the
calculated SYMMETRY value, which is constantly updated by operating the AL63001 receiver in the periodic mode (vs.
accumulate). The display is updated with the results of tests conducted every measurement period (10^6 bits, for example).
This allows the operator to make adjustments in the link transmission and reception equipment while monitoring its effect
on the symmetry.
Many times, a link may test good when using PRN data due to the large number of data transitions and the 50% data
symmetry, only to have problems when PCM mission data begins to flow due to A.C. coupling. Pattern effects are detected
by using the DLTM4 PCM data mode with a programmed frame format that resembles the data to be encountered. The
unit allows data word values to be specified so that fewer transitions and data symmetry characteristics can be accurately
modeled.
6.1.5 PULSE CODED MODULATION
Using PCM data allows more of the equipment in a link to be verified in addition to just the serial data link functions
shown in Figure 19. Frame synchronizers, computer ingests and display systems, strip chart recorders, etc. can all be
tested. The error insertion capability of the DLTM4 allows equipment performance to be evaluated under noisy path
conditions. For example, a test might be conducted to determine how random errors in the data affect a strip chart display.