User's Manual

Operation
RFI-148 & RFI-900 High Output Power Paging Transmitters User Manual Page 28 of 134
HARDWARE PTT EDGE OR LEVEL DETECTION
The transmitter keys up due to the rising or falling edge of the hardware PTT signal – it is based on edge
detection rather than sampling. However, there are three exceptions to this case where the hardware PTT signal
is sampled to check for key up:
• When the unit powers up.
• When the hardware PTT configuration is changed from Disabled to Enabled.
• When the unit comes out of PTT Override.
4.3.4 External Reference
The transmitter supports an external reference for channel frequency generation.
To use the external reference, a 5 or 10 MHz sine or square wave -20 dBm to +15 dBm signal must be applied
to the “External Frequency” input BNC connector on the back panel. The Reference Mode supports four
options:
• Internal: The internal reference is used. External reference is ignored and will not cause faults.
• External With Failover: The external reference is used where possible. If the external reference is
disconnected, has out of spec amplitude, or drifts too far from the internal reference (or vice versa)
then the PTX will switch to the internal reference immediately. If the unit is transmitting at the time of
reference switchover, there may be data loss. The switchover is latched, and therefore the Clear All
Faults routine must be executed (through Cruise Control or AT command) before the PTX will attempt
to switch to the external reference. This mode is intended for use with reliable reference sources that
fail rarely, since user intervention is required to restore normal PTX behaviour.
• External When Available: The external reference is used where possible. If the external reference is
disconnected, has out of spec amplitude, or drifts too far from the internal reference (or vice versa)
then the PTX will switch to the internal reference immediately. If the unit is transmitting at the time of
reference switchover, there may be data loss. The switchover is not latched and, therefore, restoration
of a quality external reference source will cause the PTX to revert to using the external reference
without user intervention. To minimise data loss, use of the external reference will not be restored until
the PTX has stopped transmitting. This mode is intended for use with less reliable reference sources
that fail more frequently, or for installations that are difficult or impractical to remotely monitor and
control, since user intervention is not required to restore normal PTX behaviour
• External Only: The external reference becomes a pre-requisite for transmission. If the external
reference is disconnected, has out of spec amplitude, or drifts too far from the internal reference (or
vice versa) then the PTX will disable all PTT sources thereby stopping any transmission. When the
external reference is restored the PTX re-enables PTT sources and continues transmission – without
any intervention.
Radio -> Reference