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P300H

P300 Series Modem Installation and Operating Handbook

Page 92

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 142
8.2 "Determining Clocking Schemes & Buffer Size" on page 143
and "P1440 in X.21 Mode" on page 176

Buffer/Clocking: 1:Station clock input

2:Rx clock source 3:Buffer Size

Change, Rx, Buffer/Clocking Menu

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

Station clock connection [?]:

1=None 2:Via BNC 3:RS422

Change, Rx, Buffer / Clocking, Station Clock, Available Menu

Provided

Ò±²»

is not selected, the display continues to the next question where the frequency of the

clock is specified.

Station clock frequency: [0]kHz

(1-10MHz) ????? (YES)

Change, Rx, Buffer / Clocking, Station Clock, Frequency Menu

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:

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

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.