Advantech AMT-70, Инструкция по установке и эксплуатации

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AMT-70/AMT-73/AMT-75 Installation and Operation
3.5
Clock Options and Buffers – Standard Interface Card
The AMT-70 and AMT-73L includes many clocking options that can be selected to satisfy a multitude
of satellite conditions. The standard AMT-70 modulator includes three different options, and the
standard AMT-73L includes five different options. Both the standard AMT-70 demodulator and the
standard AMT-73L have four different options.
Figure 7
is a simplified block diagram of the modem
that shows the clocking options.
Figure 7: Functional Block Diagram of AMT-70/73L modem
The terminology used herein is based on the RS-530/422 mnemonics, which includes such
abbreviations as TT for Terminal Timing, ST for Send Timing, RT for Receive Timing, SD for Send
Data, and RD for Receive Data.
The modem is always a DCE (Data Communications Equipment, which normally supplies the clock to
the DTE (Data Terminal Equipment). Crossover cables for interfacing to equipment that supports only
the DCE interface is provided in
Chapter 8
.
The modulator incorporates a transmit de-jitter buffer that reduces the amount of jitter from the DTE
terminal. It is always in the transmit path, and has a fixed size. This size is varied depending on the
FEC system. A command (TXJTRST) allows the variation of the de-jitter buffer to be measured.
The demodulator incorporates a Doppler Buffer that can be bypassed or activated. The Doppler Buffer
length is operator programmable for length and is automatically centered. The lowest limit is 32 bytes
and the maximum upper limit is 65536 bytes (approx 0.5Mbits). Length is related to data rate; the lower
the data rate, the smaller the buffer.
The Doppler Buffer can also be used as a Plesiochronous Buffer, where the near end modem and far
end modem are operating with dissimilar (but accurate) clocks. Using the buffer in this fashion, clock
slips occur and should be expected. When used as a Plesiochronous Buffer, the size of the buffer
51
DATA IN
Far End
Internal
Loopback
RS-530
or
HSSI
Interface
TX
Jitter
Buffer
Encoder &
Scrambler
Turbo
Encoder
TX Reed
Solomon
Framer/
Deframer/IBS
Scrambler/
Descrambler
Turbo
Decoder
Viterbi
Encoder
RX
Doppler
Buffer
RX Reed
Solomon Viterbi
Decoder
Modulator &
IF
Encoder &
Scrambler IF &
Demodulator
Near End
Interface
Loopback
Demod
Clock
Demod D
RT
RD
SD
TT
Internal
Timing
ST
Write
Clock
Write Clock
Serial Data
Far End
Interface
Loopback
Near End
Internal
Loopback
IF
Loopback
IF OUTPUT
IF INPUT
DATA OUT
TT
ST
EXT CLK