Advantech AMT-70, Installation And Operation Manual, Page: 56 / 160

AMT-70/AMT-73/AMT-75 Installation and Operation
demodulator desiring to be tested. This test assumes that the modulator is off line. Place the
modulator frequency within an area that is supporting no carriers. An inline pad may be
necessary if the TX carrier is too high in level. The desirable level is where the modulated
carrier is approximately level with the received carriers off the satellite.
Use the S/N + N values from
Chapter 13B
to obtain conversion to Eb/No values.
This mode is entered by selecting TPRBS and selecting the modulator test pattern 2_23. Once the
modulator and demodulator is configured and passing data, accurate BER, raw BER, Eb/No and SNR
values can be taken by observing RBER, REBER, REBNO and RSNR results.
3.6.6
BER Measurement
Note that the AMT70 and AMT73L provides two means of analyzing link error rate performance
When the Modulator is operating with INTERNAL_PRBS of 2^15-1 then the front panel menu
MONITOR>RX>BER shows the real Bit Error Rate measured from errors detected on the PRBS
sequence. The BER can be reset by selecting FUNC>TEST>RESET>RX.STATS. This measure can
be accessed via the CLI command RBER. NOTE: THIS VALUE IS MEANINGLESS WHEN THE
MODEM IS RECEIVING REAL DATA TRAFFIC.
There is also an Estimated BER, which is useful when the modem is operating with real data. This is
displayed on the front panel MONITOR>RX>EST.BER. This can be accessed from the CLI using the
command REBER.
3.7 ASI Interfaces
The AMT70 supports two main ASI input modes of operation selected via the TX>INPUT field:
•
MPEG_VIA_ASI
•
VBR_MPEG_ASI
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MPEG_VIA_ASI does NOT change the incoming DVB MPEG ASI stream. Therefore the Modulator
symbol rate / bit rate must be configured to match the incoming bit rate otherwise the Input buffer will
overflow or underflow. The Modulator can operate with an incoming bit-rate up to +/- 200ppm
tolerance.
VBR_MPEG_ASI does allow the modulator to insert DVB NULL packets into the incoming MPEG-2
data stream. The modulator corrects any PCR values in the stream to compensate for the insertion of
null packets. In this mode the incoming data stream must never exceed the bit-rate set on the
Modulator.