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<h1>BTS Time Base Reference from PCM</h1>

<p>The Master Clock Unit of the BTS can be adjusted according
to the frequency of the incoming PCM signal. A 2 MHz signal is extracted from
the PCM, and it is used then as a reference when adjusting the output of the
oven oscillator producing 13 MHz for the BTS internal timing. This is not
a phase-locked loop but a longer term averaging type of adjustment, the 13
MHz output is adjusted every 20 minutes based on the 2 MHz. The requirement
for the PCM is ± 0.015 PPM in order to meet the GSM requirement 0.05 PPM for
clock accuracy in the Air interface.</p>


<p>If the relation between the output of the master clock
and the 2 MHz shows a big difference, an alarm is generated. There can be
two reasons for this alarm, either the incoming 2M signal is not correct,
or the output of the master clock is not correct. The tuning, however, is
continued since the master clock cannot know which one of the signals is incorrect.
When the tuning is completed and the relation between these two frequencies
is correct, the alarm is cleared.</p>


<p>The tuning can be controlled per site with the hardware
database. If tuning is not used, the master clock output is based solely on
the oven oscillator. Also, if the 2 M PCM is disconnected, the tuning is stopped
since the 2 MHz may not be accurate any more. When the PCM is reconnected,
tuning is continued. The disconnection / reconnection is detected from the
state of the O&amp;M LAPD signalling link. If the link is down, the 2 MHz
is not used.</p>


<p>Because the tuning period is so long, a special fast tuning
method is available in 2nd generation BTSs for commissioning purposes. With
a command from BTS-MMI, the MCLP tunes itself according to the incoming 2-Mb
signal. The accuracy of this adjustment is not quite as good as for the long-term
adjustment, but it is sufficient for the commissioning. </p>
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