TM 11-5820-803-12
CHAPTER 2
SERVICE UPON RECEIPT AND INSTALLATION
Section I. SYSTEMS PLANNING
2-1.
General
allocated for the transmission of data must be increased
The PSK modem can be used in a variety of
proportionally. Because of this variance, the available
applications. The 70 MHz IF interfaces are compatible
bandwidth of the network used to interface the
with several types of earth terminals. The PSK modem
modulated 70 MHz signal with the earth terminal must be
operates without error correcting coding, or with either an
taken into consideration during systems planning.
internal or an external coder/decoder. The PSK modem
(2) As shown in figure 2-3, most of the signal
interfaces directly between an earth terminal and a digital
energy is contained in the frequency band at 70
user (fig. 2-1). Optionally, the PSK modem can interface
MHz+Rs, (Rs, = symbol rate). However, the modulation
with a remotely located digital user via a shielded cable
spreads signal energy into the sidebands. When the
or a line-of-sight (LOS) microwave link. The PSK
PSK modem is used in frequency division multiple
modem includes circuits that aid in testing the
access (FDMA) satellite communications systems, these
performance of a digital satellite communications link, as
sidebands may interfere with other signals present at the
well as providing a self-test function. The operating
satellite at nearby frequencies. For this reason, it may
configurations of the PSK modem are controlled by front
be necessary to filter the transmitter output to reduce the
panel and internal switches.
amplitude of these sidebands.
The PSK modem
transmitter output contains internal filters reduce
2-2.
Earth Terminal Interface Signals
sideband amplitudes.
The internal filters are
automatically selected by the setting of the INPUT DATA
RATE switches and the TRANSMIT ERROR CODING
a. 70-MHz Output Signal Characteristics.
switch. The filters selected for the various switch
(1) The transmitter output is provided to the
settings are listed in table 2-1.
In some system
terminal via TNC connector AT1J1 located on the rear
applications, it may be necessary to add additional
panel of the modem. This output is a biphase modulated
filtering between the 70 MHz modem output and the
phase-shift-keyed (PSK) signal with a center frequency
upconverter.
of 70 MHz. A PSK signal is one in which the modulation
(3) The output power of the 70 MHz
is accomplished by switching the phase angle of the
transmitted signal is +10 dBm.
The input power
output carrier. When biphase PSK is employed, the
requirements of the upconverter used in the earth
phase angle of the output carrier is switched between
terminal must be reviewed for compatibility.
If
only two states to represent the data pattern. This type
necessary, an appropriate attenuator should be added to
of signal modulation is illustrated in figure 2-2. In the
the system in series with the 70 MHz output of the PSK
case shown, a ZERO data bit results in a 0-degree
modem.
carrier phase angle. When the data state b. changes to
b. 70 MHz Input Signal Characteristics.
a ONE, the carrier phase angle is switched to 180
(1) The 70 MHz input to the PSK modem is
degrees and maintained until the data state reverts to
received from the output of the downconverter in the
ZERO. The resulting spectrum for this type of signal is
earth terminal. Connection of this signal to the modem is
shown in figure 2-3 for a random data pattern. The width
made via TNC connector AT2J1 located on the rear
of the spectrum is proportional to the symbol rate of the
panel. This interface is a biphase-modulated PSK signal
modulation (symbol rate is twice data rate if error
correcting encoding is used). Therefore, as the symbol
spectrum characteristics of this signal are the same as
rate is increased, the system bandwidth
the 70 MHz output.
The major differences in
characteristics are the 70 MHz input signal is degraded
2-1
