Advantech AMT-70, Installation And Operation Manual, Page: 55 / 160

AMT-70/AMT-73/AMT-75 Installation and Operation
3.6.2
Data System Near End/Far End Loop-back
A data system internal loop-back is in between the framer (if so equipped) and the FEC
encoder/decoder. Activation of the near end internal loop-back therefore tests the interface card
(clocking and framer) of the local modem, and testing from the far end checks the modulator and
demodulator (including FEC, scrambler, and differential coder) of the distant end modem, plus the
entire modem at the testing end.
The near end mode is entered by selecting IFLOOPMODE
system local.
The far end mode is entered
by selecting IFLOOPMODE
system far.
3.6.3
IF Loop-back
Data can be clocked into and out of the Doppler Buffer via the four clocking modes described below:
This mode is entered by selecting TLOOPBACK
on,
and then selecting RINPUT
loopback.
3.6.4
Pure Carrier Mode
A pure carrier can be generated which allows testing of the modulator without modulated data. Care is
needed before using this mode since this could cause interference with other traffic on the same
satellite transponder. If the modulator is configured and operating in any mode, then select TCWT
on.
The CW power is equivalent to the normal modulated power.
3.6.5
Internal BER Test Pattern Generator and Tester
The AMT-70/73L provides the means of testing the modulator and the demodulator circuitry. By
activating one of 10 test patterns (5 non-inverted; 5 inverted) from the modulator, a demodulator
equipped with a like BER tester (or a BER tester itself) can test the link integrity and obtain a
reasonable Eb/No and BER of the satellite link – assuming that the demodulator is on the other end of
a satellite link. Or it can be used to test a new modem prior to putting it online. The test pattern
polynomials generated by the modulator are standard BER test patterns provided by most BER testers.
Note that the AMT70/73L demodulator has a built-in true BERT measurement function. This ONLY
operates on 2_23 true PRBS mode and the measured BER can be viewed using the
RX>MONITOR>BERT display on the front panel. The BERT statistics can be reset using the TEST
menu> FAULT.RST function.
•
Example 1:
Set the far end modulator of an AMT-70/73L to one of the test patterns, then set
the near end AMT-70/73L demodulator to the same test pattern. By observing RSNR, or
REBNO, and or REBER (estimated BER), a fairly accurate link performance can be obtained.
Raising or lowering the TLEVEL at the sending end will increase/decrease Eb/No readings.
RBER can also be used if the polynomial pattern is PRBS 2_23.
Both the modulator and demodulator need to be on the same frequency, data rate, FEC type,
FEC code rate, etc.
Both ends of the link can be tested merely by switching the transmitting source and the
receiving end.
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•
Example 2:
A local test with noise would be to receive a downlink feed, mix it through a power
divider with the modulator output, then feed the mixed output from the power divider to the