Appendix C
IRIG-B Primer
C.1
Introduction
IRIG-B is a complete serial time code of 74 bits that occurs once per second. It be subdivided into
various forms, including modulated (analog), unmodulated (CMOS/TTL/Level-Shift) and with the
C37.118 specification bits enabled or disabled (IEEE-1344 mode turned ON or OFF). The Model
1133A transmits only unmodulated IRIG-B, and allows you to also set the time zone to Local or
UTC.
C.1.1
Modulated and Unmodulated IRIG-B
Figure C.1 illustrates the primary differences between modulated and unmodulated IRIG-B. You
will notice that the while modulated IRIG-B is distinctive because it has a sine wave carrier signal of
1 kHz, it is similar to unmodulated IRIG-B since the peak values of the carrier follow the same form
as the digital waveform. IRIG-B is considered to be pulse-width modulated, in that the information
is contained in the percent of time that the analog peak-to-peak value, or TTL level, is maximum.
Notice the three states in both forms of the IRIG-B waveform (with times in parentheses): IRIG-B
Reference (8 ms); IRIG Zero (2 ms); IRIG-B One (5 ms). Reference bits are also placed within
each instance that the time code is transmitted.
Figure C.1: IRIG-B Waveforms
The IRIG-B time code consists of 74 bits produced every second, and contain various time,
