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P300H P300 Series Modem Installation and Operating Handbook Page 89
Buffer/Clocking: 1:Station clock input
2:Rx clock source 3:Buffer Size
Change, Rx, Buffer/Clocking Menu
Station clock connection [?]:
1=None 2:Via BNC 3:RS422:
Change, Rx, Buffer / Clocking, Station Clock, Available Menu
Station clock frequency: [0]kHz
(1-10MHz) ????? (YES)
Change, Rx, Buffer / Clocking, Station Clock, Frequency Menu
Use station clock as IF synth ref [No]:
1=No, use normal int' ref 2=Yes
READ
Change, Rx, Buffer / Clocking, Station Clock, Frequency Menu
6.7.3 Change, Rx, BUFFER / CLOCKING Menu
Refer also to the following applications sections for helpful notes:
8.1 "Doppler & Plesiochronous Buffering" on page 140
8.2 "Determining Clocking Schemes & Buffer Size" on page 141
and "P1440 in X.21 Mode" on page 173
These three options lead onto the following three Menus
6.7.4 Change, Rx, Buffer / Clocking, STATION CLOCK Menu
Firstly the operator is asked to specify is a station clock is available
Provided None is not selected, the display continues to the next question where the frequency of the clock
is specified.
This screen allows the operator to set the frequency for the Station Clock if one is available. This may be
any integer value of kHz (eg 1544k, 2048k, 5000k, 10000k, or arbitrary values such as 2500k or 4321k).
Values down to 1kHz (not 1MHz) are accepted, a frequency of 0 indicates there is no station clock. When
a Station Clock is present the Modem will use it in preference to its internal frequency reference for clock
generation of any backup clocks required. See the notes in the Rx Clock section on the following page as
to where the modem sources the Rx clock from under fault conditions.
The station clock frequency can be any integer multiple of 1kHz, and the 75
BNC will accept any sinusoid
> 0dBm or a square wave > 1Vp/p (eg G.703 Para 10 synchronising clock). The operator should be aware
that the losses associated with the coupling transformer on this BNC input increase below 1MHz, and at
frequencies below this higher levels of input signal may be required. The RS422 input is located on the
`Async ESC` connector and requires standard RS422 levels at any frequency.
If the station clock is specified as 10MHz, then a final screen is displayed as follows:
This allows the IF synthesisers to be locked to the station clock instead of the internal 1PPM reference. This
is normally not a requirement as the internal reference provides good enough phase noise performance
for the lowest data rate the modem can support and at 1PPM results in a worst case IF error of 180Hz
when the IF is tuned to 180MHz. However if this option is used please be aware that the transmit IF
spectrum purity is entirely dependent upon the quality (phase noise performance) of the supplied reference.