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List:       timekeepers
Subject:    Re: [Pool] Background info on DCF77- was: Re: Trouble with DCF77 device
From:       "David Lord" <david () lordynet ! org>
Date:       2012-09-08 23:15:58
Message-ID: 504BD1AE.4512.9E89F2 () david ! lordynet ! org
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On 9 Sep 2012 at 0:21, Andreas Krüger wrote:

> 
> Hello, all,
> 
> here comes some background information on DCF77, for those of you
> interested in such. I want to expand on what Martin has written.
> 
> How precise is a DCF77 clock? It depends...
> To summarize how most DCF77 clocks work:  At the start of each second,
> the transmitting power is reduced to a quarter of the normal
> amplitude.  That power reduction lasts for either 100 ms or 200 ms.
> The time information is coded via these duration differences. For the
> record: Changing transmitter power is also called "amplitude
> modulation" or, in short, AM.
> 

I have a Conrad module modified for MSF and the filter 
appears to be about 25 msec which is ok for a unit I 
assume is meant for wall clocks. 

My attempt at a homebrew AM receiver used a filter with
RC of around 1 msec and initially offsets were much better
than 35 usec. That was until some days later atmospherics
blocked reception twice a day such that use for ntpd 
wasn't wothwhile.

Reception of MSF since relocation of transmitter from 
Rugby to Anthorn seems now much more likely to be blocked
by atmospherics although mostly my server syncs to MSF
coincidently with good reception at the moment but 
offset 245 ms and jitter at 274 usec vs GPS when it was
connected mostly giving  offset of under 3 usec.


David

- -- 
David Lord <david@lordynet.org>
<ftp://ftp.lordynet.org/pub/pgpkeys/lordynet.org/david/pubk
ey.asc>
<http://www.lordynet.org/pub/pgpkeys/lordynet.org/david/pub
key.asc>



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