Datum Systems M7 Series, Инструкция по установке и эксплуатации

Страница: 153 / 190

M7/M7L/M7LT Modular Satellite Modem Maintenance
M7/M7L/M7LT - Rev. 0.05 3-97
3.9.1. Modulator Bit Timing
When the modem is configured with any serial interface option, the modulator always outputs the
Send Timing signal on the baseband interface connector. In a typical network, this timing signal is
used to generate the bit timing for the transmission toward the satellite. This timing signal may be
selected from 1 of 4 sources:
1. Internal
– locked to a 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.
2. RCV Clock
– locked to the demodulator Receive Timing (i.e. Loop Timing)
3. External
– locked to an external timing input at the bit rate 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
4. Terminal Timing
– locked to the Terminal Timing input (pins 24/11)
NOTE: Bit timing options for optional interfaces are discussed in the applicable interface appendix.
A simplified block diagram of the Modulator clock sources for a Synchronous Interface is shown in
Figure 3-50.
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 3-50
– Modulator Clock Source Options