Comtech EF Data SDM-650B Especificaciones descargar pdf (Pagina 239)

100

101

SDM-300L3 Satellite Modem Revision 0

Clocking Options MN/SDM300L3.IOM

6–13

6.5 Plesiochronous

The stability of station reference clocks is normally 1 x 10

-12

(derived from a cesium

standard). While the stability is exceptionally high, the two clocks are not in

synchronization with each other and will eventually pass by each other.

The clock used for the transmit signal is passed over the satellite, but will not be used at

the receive earth station where a national network derives its time locally. A buffer will

fill up with data using the clock from the satellite and will empty using the local clock.

The object of the buffer is to ensure that the buffer overflows or underflows at regular,

determinable intervals (typically every 40 days).

The buffer depth required (from center to end) will be:

• Minimum slip period (seconds)

[stability of far end (transmit) clock +

stability of local clock]

For example:

Far end (transmit) clock stability

1 x 10

-9

Local (buffer) clock

1 x 10

-11

Minimum clock slip 40 days

Buffer Depth = (40 x 24 x 60 x 60)

x (1 x 10

-9

1 x 10

-11

) = 3.49 ms

Because the buffer will either fill or empty (depending on the frequency relationship of

the two clocks), the total buffer depth will be 2 x 3.49 ms = 6.98 ms.

1 2 ... 234 235 236 237 238 239 240 241 242 243 244 ... 433 434

Comentarios a estos manuales

Sin comentarios

Comtech EF Data SDM-650B Especificaciones Pagina 239

Comentarios a estos manuales

Relacionado con productos y manuales para Redes Comtech EF Data SDM-650B