8.2 Timing Output Description
65
Figure 8.1: Rear Panel Description
Pulse; analog signals include modulated IRIG-B. Two of the outputs (3 and 4) can also source an
independent instance of IRIG-B or frequency of from 1 to 1000 pulses per second. Finally, each
digital output can be configured as either 5 Volt CMOS/level shift, or 300 Volt FET.
8.2.2
Digital Drivers – 250 mA Per Channel
Each of the four timing outputs uses a separate FDC6333C, N & P-channel Power Trench Logic
level MOSFET, for digital signals (e.g. unmodulated IRIG-B, 1 PPS and Programmable Pulse),
capable of 250 mA at 5 Vdc. Together, these four outputs together can provide up to one amp
of drive current at TTL/CMOS levels. This permits you to connect a large number of IED’s
that would draw up to that amount of current. For example, if the IED timing signal input (e.g.
IRIG-B003) requires 10 mA, one output channel should be able to support 25 identical devices.
8.2.3
Analog Drivers
Each of the four outputs also has a separate analog driver to be used exclusively for
modulated IRIG-
B signals
. The analog driver is an Analog Devices AD8532, single-supply, dual-output amplifier,
which supplies a 4.5 Volt peak-to-peak (Vpp) signal through a 19.6-ohm source resistor. As the
load current increases (by adding external IED’s), more voltage is dropped across the clock source
resistor and the drive voltage decreases. Due to this change in peak-to-peak output, it is sometimes
essential to match the modulated IRIG-B output voltage to the required IED input. Make sure
to check the equipment specification and verify the input voltage levels. Table 8.1 shows how the
actual drive voltage varies with increasing load current. For IED’s with a restricted input range, it
may be necessary to match the available drive voltage to the IED through a small dropping resistor.
Drive Current, mA
Actual Drive Voltage, Vpp
0
4.5
1
4.48
10
4.3
100
2.54
Table 8.1: Drive Current vs. Voltage
