TM 11-5820-803-12
in the installation. Cable selection may be simplified by
(but only one) of the following outputs may be connected.
using the length requirement to determine the maximum
(1) 75-ohm balanced cable driver.
value of the loss characteristic (a) at 5 MHz which can be
(2) 75-ohm unbalanced cable driver.
used. The maximum value of a can be determined from
(3) 50-ohm unbalanced cable driver.
the relationship:
(4) LOS microwave driver.
max = 3000
2-10. Test and Monitor Capabilities.
(2) Cable classification. The transmission of
a. Link Test. The PSK modem provides the
baseband data over an interconnect facility cable results
capability, with adequate coordination between earth
in unequal phase distortion at the output. The PSK
terminal sites, of measuring the quality of a digital
modem LOS/cable receiver and decoder circuits contain
communications link. The transmit link test setup (para
equalization networks to compensate for this phase
distortion.
Detailed instructions for adjusting the
transmit an internally generated 2047 bit sequence over
equalizers are given in paragraph 2-21.
Further
the satellite link. The receive link test procedure (para 3-
information on configuring the LOS/cable receiver and
decoder circuits to provide the appropriate equalization is
receive this same sequence from a satellite link, perform
given in paragraph 2-20, depending on whether the cable
an internal data comparison, and determine the error
is classified as long, medium, or short. Although the
rate introduced in the link. If necessary, the test may be
phase distortion is not directly related to the 5 MHz cable
performed for communications transmitted from a site
loss, the parameters are so closely related that cable
without disturbing the communications being received,
loss may be used to determine the equalization
and vice versa.
requirements.
To determine cable classification,
b. Terminal Test. If an earth terminal is configured
calculate the cable loss based on the cable length and
to receive its own transmissions, the link test capabilities
of the PSK modem may be used to determine the
the appropriate classification in accordance with the
operational status at the terminal.
following list to ensure proper alinement (para 2-21).
c. Self-Test. The PSK modem contains internal
self-test circuits which play a major role in the fault
Cable loss at 5 MHz
Classification
isolation of the modem.
In addition, this self-test
0 dB to 3 dB
short
capability provides a means of rapidly verifying modem
3 dB to 12 dB
midrange
operation to aid in isolating a communication system
12 dB to 30 dB
long
malfunction.
d. Signal-to-Noise Indication. The PSK modem
c. Input Signals. The desired input from the ICF
contains a circuit which monitors the operation of the
modem is internally selected at installation. Details for
baseband conditioner in the receiver during normal
the LOS/cable receiver and decoder input selection are
communications.
The resulting front-panel meter
given in paragraph 2-20f. The modem must also be
indication may be used to estimate the input signal-to-
configured to derive internal timing from and transmit the
noise ratio to the PSK modem (fig. 2-15) or the error rate
selected ICF input. Any one of the following input
occurring in the received data (fig. 2-16). For the most
sources may be selected:
accurate results, the operator should be instructed to plot
(1) 75-ohm balanced cable.
calibration curves at the operational data rate of the
(2) 75-ohm unbalanced cable.
receive link. The data for calibration curves is collected
(3) 50-ohm balanced cable.
by performing the noise test (para 4-6) and noting the
(4) LOS microwave receiver.
error rate and meter reading for each value of (Eb/No)
tested.
NOTE
e. On-Line Fault Monitoring. The on-line fault
If a cable is used for the ICF interface,
indicators monitor various characteristics of the signals
the required equalization procedure
being processed by the system. It is recommended that
is given in paragraph 2-21.
the function of the indicators be reviewed and utilized in
d. Output Signals. All outputs available to the ICF
the site operating and troubleshooting procedures.
system are present at the PSK modem rear panel and
their levels are controlled by a single internal switch.
Detailed instructions for setting this switch are given in
2-14
