Arbiter Systems 1073A, Руководство по эксплуатации

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12 Physical Configuration
from 0.6 to 5 Vpp. DC bias levels of up to 25 volts are rejected by a series capacitor (1 microfarad)
and will have no effect on operation.
Lower signal levels may also work properly if the input has sufficient slew rate. The AC input
limiting amplifier has high gain, and will oscillate if no input is present, or if the applied signal
crosses too slowly through the transition level. To prevent this, apply 0 dBm at 100 kHz or above,
sine wave or square wave.
In DC-coupled mode, the input is a 562-ohm resistor in series with a 6N137 optocoupler LED,
with a 6.8 V Zener diode (1N4736A) in parallel with the input for protection against overvoltage,
overshoot and ringing, or reverse voltage. Drive the DC-coupled input with a CMOS-compatible,
0 to +5 volt signal. The input will draw approximately 5 mA when driven at 5 volts. DC-coupled
mode may be used with any signal up to 5 MHz.
Both AC and DC coupled inputs are isolated from the instrument chassis and from each other
with isolation of 2000 Vrms minimum.
3.4.4 AC/DC Coupling of Outputs
Each output BNC connector may be individually selected for AC or DC coupling. AC coupling is
normally used for time-base signals, such as 5 or 10 MHz. The output signal will be a leveled square
wave, even if the input signal is a sine wave (a limiting amplifier is used to control output level).
DC coupling is normally used for signals such as unmodulated IRIG time codes, and asymmetric
pulse outputs such as 1PPS. DC coupling may also be used with square wave signals, if signal levels
of 0 to 5 Volts (CMOS-compatible levels) are required.
To select DC coupling for an output, set the associated jumper to the ‘A’ position. For AC
coupling, set the jumper to the ‘B’ position. Table 3.4 on page 13 lists the correspondence between
each jumper and the corresponding output connector; however, each jumper is located adjacent to
the respective output connector.
3.4.5 Output Characteristics
Each output is driven by four drivers of a 74HC125-bus buffer in parallel. This gives each output
the capability of sourcing, or sinking, up to 75 mA. In DC-coupled mode, the driver has a 10-ohm
series resistor, which provides some current limiting in the event of a short circuit, reducing driver
power dissipation. The output impedance of the buffer itself is around 5 ohms at the rated current
levels of 0 to 75 mA, for a total source impedance of approximately 15 ohms. DC-coupled mode
may be used for any signal.
In AC-coupled mode, the driver has a 45-ohm resistor and 0.1 microfarad capacitor in series,
providing approximately 50 ohms source impedance to match 50-ohm coaxial cables. The driver
will deliver a signal level of +10 to +13 dBm, nominal, into a 50-ohm load. AC-coupled output
mode is recommended for use at 100 kHz or above.