CHAPTER 2 SERVICE UPON RECEIPT AND INSTALLATION

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

(fig. 2-1)

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.

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

with a center frequency of 70 MHz. The waveform and

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