[time-nuts] WWVB PM Receiver

Attila Kinali attila at kinali.ch
Thu Sep 27 13:34:02 UTC 2012


Moin,

On Thu, 27 Sep 2012 08:53:03 -0400
paul swed <paulswedb at gmail.com> wrote:

> I am feeling a bit slow here.
> There is a carrier always. Thats how the AM works. So somehow we are
> speaking about a semi non coherent carrier perhaps??
> So whats the nickle solution and it is not squaring in a low s/n
> environment. Been there done that. Very bad results on the east coast.
> Regards

Yes, there is still a carrier left. A BPSK spektrum looks like a
sin(x)/x signal which is centered on the carrier. Yes, you can
sync on this signal without much difficult... if you use the right method.
And that's where the problem lies. Most old devices use a PLL to sync
to the signal, which tracks the phase. Now if the phase jumps 180°
every second, the PLL will jump back and forth every second (or actually
on every 0-1 and 1-0 transition). Ie you have a jump of the PLL error
signal every second, possibly unlocking the PLL until the PLL has locked
in again. This jumping will disturb any frequency normal.

A way to mitigate this problem is to use
1) a smaller phase step than 180°
2) a higher modulation frequency
3) a modulation that has an average of 0, even on the short term
(as in shorter than the time constant of a "standard" PLL)

1) and 2) allows the PLL filters to filter out the phase steps and 3) ensures
that the modulation pattern doesnt change the frequency, even in the short
term.

If you find yourself reminded of something... Yes, this is exactly what
DCF77 has done for over 20 years....[1]

I really wonder why they didn't copy the modulation scheme of DCF77 for
WWVB or at least take ideas from it...

			Attila Kinali

[1] http://remco.tk/handig/DCFp.pdf

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson



More information about the time-nuts mailing list