ORBCOMM OG2 Modem Integration Manual
16
Figure 3-2 RF Output Power vs. input Voltage
3.1.5
Frequency Control
Frequency control is provided by the ORBCOMM System. The modem transmits an initial 7 bit (8
bits including the reference bit required by SDPSK modulation) burst to the satellite. Initial
specifications published almost 20 years ago required this burst to arrive within ±1500 Hz. This
frequency error budget includes oscillator uncertainty and Doppler effects. Current modem designs
routinely transmit this acquire burst within ±500 Hz.
Frequency and timing corrections are provided to the modem based on the offset of the 7 bit burst.
All subsequent transmissions for this message session are corrected to within ±50 Hz.
3.1.6
Transmit Frequency
The modem is capable of tuning to any 2.5 kHz channel between 148.0500 and 150.0000 MHz. Note
that ORBCOMM’s authorized spectrum is 148.0000 to 150.0500 MHz. Restriction of the channels
at the edge of the authorized spectrum insure no spectral content above the specifications occur
outside the authorized spectrum.
3.1.7
Occupied Bandwidth
Measured 20 dB occupied bandwidth is less than 4 kHz.
3.1.8
Transmitter Designator
The modem carries a 5K00G1D emission designator.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
RF
O
u
tp
u
t
P
o
w
er
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W
)
Input Voltage (DC)
RF Output Power vs Input Voltage
