Some notes about BS-11 clock calibration

[Dieter Spaar]

Hello,

I did a few tests with the BS-11 clock. I wanted to
find out how accurate the clock of the BS-11 is and how
the calibration value influences the clock. This is only
about one BS-11, but I guess the others are similar.

To control the BS-11 clock I used a slightly modified HFC-E1
card, instead of the 32.768 MHz oscillator of the card an
external signal from an accurate and stable clock source is 
used.

The BS-11 "PLL Mode" is set to "Locked" so that it locks to the
E1 clock. I am observing the PLL "Work Value" which is the actual
value used by the PLL. I also measured the RF frequency of the BS-11.

  HFC-E1 clock is 32.768 MHz:             PLL Work Value is 1024
  HFC-E1 clock is 32.768 MHz + 0.1 ppm:   PLL Work Value is  941
  HFC-E1 clock is 32.768 MHz - 0.1 ppm:   PLL Work Value is 1116

So a lower PLL calibration value means a higher frequency.

Another observation is that the actual RF frequency is following the
E1 clock slowly. For example if the E1 clock is changed by 0.1 ppm,
the RF frequency is slowly changing until after several minutes it
has also changed by 0.1 ppm. The PLL "Working Value" is following
even slower, it takes nearly an hour until is has settled to a
stable value.

The clock of the BS-11 is very stable, if set to "Stand Alone"
the RF frequency is not changing very much, its much less than
the required 0.05 ppm from the GSM specification for a BTS.

Another interesting thing is that the factory PLL calibration of
this BS-11 is 1016, very close to 1024 of the measured value.

What does that all mean ? 

If you want to calibrate the clock of your BS-11 by using a HFC-E1 
card which is synchronized from a stable and accurate clock (Andreas 
explained how this can be done with a second ISDN card) I would 
recommend to do the following:

  - let the BS-11 warm up for at least one hour

  - set the "PLL Mode" to "Locked" with bs11_config

  - start bsc_hack and let it run for an hour so that the PLL can
    adjust to the E1 clock and the PLL "Work Value" settles (you
    can use bs11_config to watch the value).

  - set the "PLL Mode" to "Stand Alone" with bs11_config, this
    makes sure that the PLL "Work Value" will no longer change and
    is used next time, even if the HFC-E1 clock is no longer 
    synchronized. The BS-11 oscillator should be stable enough so 
    that there is no need for continuous synchronisation with an 
    external reference.

Best regards,
  Dieter
-- 
Dieter Spaar, Germany                           spaar at mirider.augusta.de