IRIG-B is sent and received as a digital signal. It is transmitted either as a 0-5Vdc signal over a DC cable, or as an optical signal over a fiber optic cable. The transmitted data is modulated in one of the three ways listed below depending on the configuration set up by a user.
Direct Current Level Shift (DCLS) –
Typically this is a 0-5 Vdc pulse width modulated square wave signal, where the different pulse widths represent coded data. This is the most common modulation method used today due to its high accuracy (< 0.1 µs at the port). This is often referred to as unmodulated IRIG-B, as it is composed of DC pulses and has no carrier frequency modulating the signal unlike AM IRIG and Modified Manchester IRIG.
Amplitude Modulated (AM) –
This is modulated with a 1 kHz sine wave carrier signal - Tekron uses the default 10:3 low to high ratio for this, but a range of 3:1 to 6:1 is allowed in the standard. AM IRIG-B is sometimes simply referred to as modulated IRIG-B due to its 1kHz carrier signal. The signal has no DC content. This made AM popular in the past as it allowed the signal to be transmitted over long distances. Due to the low signal accuracy (< 2 µs at the port) AM is no longer the signal of choice.
Modified Manchester Modulation –
Modified Manchester modulation is the least common modulation type for IRIG-B. It uses a 1 kHz square wave with phase modulation rather than DC level shift. This signal contains no DC bias. This allows for transmission over long distances, while maintaining a high accuracy (<0.1 µs).
Distances and accuracies for each modulation type:
|
|
Modulation Type |
Maximum Transmission Distance |
Accuracy (at clock port) |
|
0 |
DCLS |
<100m |
<0.1µs |
|
1 |
AM |
<300m |
< 2µs |
|
2 |
Modified Manchester |
<300m |
<0.1µs |