Datum Systems M7 Series, Инструкция по установке и эксплуатации

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Maintenance M7/M7L/M7LT Modular Satellite Modem
3-98 M7/M7L/M7LT - Rev. 0.05
3.9.2. Demodulator Timing
When the modem is configured with any serial interface option, the modem will always output receive
timing synchronous with the receive data and is a source of the timing for the local network. If the
system requires a different timing standard then provisions are made to buffer the data in a
programmable FIFO buffer. The receive timing signal that is derived from the satellite is always used
to clock the data into the FIFO buffer. The FIFO buffer output clock must be the same average rate as
the demodulator receive timing otherwise a buffer over/under flow will occur. The FIFO buffer clock
output may be selected to 1 of 4 sources
1. RCV Clock
– the demodulator receive timing (buffer bypass mode) Meaning that the input
and output clocks are the same, disabling the FIFO
2. Mod Clock
– uses the selected modulator data rate clock as the output clock and requires
that the modulator and demodulator data rate be identical
3. Internal
– an internally generated bit rate NCO locked to the 2.0 ppm Internal Reference. If
bit rate timing requirements require a better reference, the internal oscillator may be phase
locked to an External Reference applied at the rear panel (J1). Refer to Section 3.7.4 for
details on how to activate the External Reference.
4. External
– an external clock at the bit rate applied on pins 9 and 21 of the overhead channel
interface connector J10. This option allows a station-derived standard clock rate to be used
to clock data out of the FIFO
A simplified block diagram representation of the demodulator clock sources are shown in Figure 3-51.
Transmit Clock Sources
Receive Clock Sources
Demodulator
Modulator
Receive FIFO
Buffer
DATA
CLOCK
DATA
IN
IN OUT
OUT
DATA
Optional Reed-Solomon Decoder
& IBS Multiplexer
CLOCK
Demodulator RCV
External
Reference Input
(Rear Panel)
From Modulator
bit timing
CLOCK
Internal
Reference
Oscillator
External
Reference PLL
Terminal
Timing
Terminal
Timing
From
Interface
Demod output
clock is phase
locked to receive
bit timing
FIFO output clock selected from
"RCV Clock", "Internal", "External"
or "Mod Clock". Selection of "RCV
Clock" bypasses the FIFO buffer.
Bit Rate NCO
From
Receive
Clock
Send Data
From
Interface
Send
Timing To
Interface
Modulator bit clock source is
selected from "Internal", "Terminal
Timing", "External" or "RCV
Clock". The Send Timing is
always an output from the modem.
Receive
Data To
Interface
Receive
Timing To
Interface
External
FIFO Clock
From
Interface
Bit Rate NCO
"Internal"
External Send Timing Input
(Rear Panel)
Figure 3-51
– Demodulator Clock Source Options