User Manual

System Description RF-Orchestra!™ Transmitter Installation and Operation
2-8 6880493G02-C July 2000
• Module monitoring: The power amplifier, power supplies, exciter, circulator load, and
OCM are monitored continuously for error conditions.
• Alarm reporting: Any error or operational condition that may degrade transmitter
operation is logged and reported.
• Power cutback: Power is cutback when an error or operational condition requires a
reduction in power level.
• Carrier feedthrough nulling: The carrier feedthrough of the modulation spectrum can be
periodically removed from the center channel to maintain adjacent channel performance
requirements and achieve accurate phase training.
For more details on power control, see Chapter 5, "Operation".
Timing Accuracy
To ensure optimum simulcast performance, each RF-O! transmitter uses GPS to synchronize
all transmitters throughout the simulcast network. Transmitter throughput delay variations
are also minimized. Digital Signal Processing (DSP) technology performs the modulation
waveshaping, significantly reducing traditional analog delay variations.
The timing chain shown in Figure 2-2 controls carrier and baseband frequency. Timing
accuracy is maintained by GPS which disciplines a 10 MHz UHSO to 1 ppb or less. Launch
time is synchronized to the GPS using the 1-PPS signal and time stamp information. When
GPS is temporarily lost, the 10 MHz operates free running at 5 ppb. The synthesized GPS
1-PPS signal is not corrected by the raw GPS 1-PPS signal, but runs freely until the satellites
are reacquired. The base station operates in free-run mode for 96 minutes before disabling
paging. This value defaults to 96 minutes, but it is user-configurable.
The FPGA in the RF-B! transmitter controller compares the 10-MHz signal from the UHSO to
the 1-PPS signal coming from the GPS. The host processor reads the difference (called the
phase report) and steers the UHSO with a DC voltage to a corrected frequency. The UHSO is
accurate to 1 ppb or less with GPS correction.
The 1-PPS signal from the satellite can dither or fade, so the FPGA checks to make sure the
1-PPS is within an expected window of 7.62 us. The FPGA reconstructs 1-PPS and places it in
the middle of the expected window, whether 1-PPS is there or not. This reference signal is
checked in the RF-O! transmitter, and similarly checked in the RF-B! transmitter controller.
The baseband signals, RF frequency, and page launch time are all disciplined by the 10 MHz
reference. In both the RF-B! transmitter controller and RF-O! transmitter, a 16.8 MHz crystal
oscillator coupled with a synthesizer are used as phase-locked VCOs to generate the baseband