AW: BS-11 PLL clock source / calibration

[Andreas.Eversberg]

if you need clock signals, you can use the C4IO clock line of the ISDN cards. they clock 4096khz. you can use them to transfer clock from one card to another or to mesure the clock. the E1 card has this clock on one pin of the PCM connector. (see data sheet) the C4IO is also available on the HFC-PCI single port card. with an oscilloscope you can compare.

in order to retieve the clock from E1 interface in NT-mode, you need to:

modprobe hfcmulti port=0x200 debug=0x40000

watch the dmesg about init process of controller. this will override the default behavior: provide clock to the NT interface.

i would suggest to use a resistor of 1K to protect the e1 card. the signal must not look good, we just need the shift of both signals.

-----Ursprüngliche Nachricht-----
Von: openbsc-bounces at lists.gnumonks.org [mailto:openbsc-bounces at lists.gnumonks.org] Im Auftrag von Dieter Spaar
Gesendet: Freitag, 17. April 2009 14:25
An: Harald Welte
Cc: openbsc at lists.gnumonks.org
Betreff: Re: BS-11 PLL clock source / calibration

Hello Harald,

On Fri, 17 Apr 2009 10:17:13 +0200, "Harald Welte" <laforge at gnumonks.org> wrote:
> 
> Dieter, what I just found out is that the LI object has a user-configurable
> setting, where we can force it to be stand alone or locked to the E1.

You mean the "PLL synchonisation mode" attribute which is set to
"locked mode" ? I am not sure if we can change it for our version
of the BS-11. As far as I understand the documentation, this is
a setting which is only available with the CCU ("Combo Card Unit", the
universal interface which also allows to use a standard ISDN line).
The documentation mentions at one place that for the CCU variant
they use an oven controlled oscillator which fulfilles the GSM
clock stability requirements even without being snchronized to
the E1 line.

But we should of course try it.

> We could add this setting to the bs11_config program so people can try
> if their E1 clock or their [10 years uncalibrated] BS11 internal oscillator
> is better.
> 
> What do you think?

Yes, why not try it. Probably the result is similar to disabling
the master clock of the HFC-E1 card, this way the BS-11 clock
determines the E1 clock (the HFC-E1 card synchronizes to the
trasmitter E1 clock of the BS-11 and the BS-11 locks to the
transmitter clock of the HFC-E1 which is determined by the BS-11).
Surely the cleaner way is to adjust the clock setting in the BS-11
(if this settings works for our BS-11 variant).

> I'm quite sure there must be an option to do this externally.  The internal
> oscillator must be calibrated every two years, and I'm sure they do this
> without opening the case (in the field...)

Its possible to calibrate the PLL without opening the case. The attribute
is protected by some checksum, I have not looked at the details. And
"officially" out LMT version should not even allow to see the calibration
dialog ;-) The problem for me is how to measure the clock without opening
the case. I can measure an unmodulated carrier very accurate if it is
within the 10 MHz to 1 GHz range (the HP 8922 is accurate enough to do
this). However I don't have the equippment to measure the modulated and
pulsed signal of the BS-11. Maybe there is some hidden setting to send
just the unmodulated carrier, but I have not yet found anything in this
direction.

Another problem is that changing the calibration requires a restart of
the BS-11 (at least I only saw any effect when I restarted). Additionally
when you query the calibration, you get the "PLL Set Value" and the
"PLL Work Value", the "Set Value" seems to be used when starting up
and the "Work Value" changes, most certainly locking to an external
clock also influences it. I did not find anything in the documentation,
actually those settings are not indended for the end user, just the
factory.

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