[ntp:questions] Re: ntp & adsl

Martin Burnicki martin.burnicki at meinberg.de
Thu Feb 2 15:39:08 UTC 2006


Folkert van Heusden wrote:
> Well if I look in the comments of my ntp.conf it seems I looked into
> that a while ago:
> # DCF77 receiver
> # "Der DCF77 Sender steht in Mainflingen (50 0 Nord, 09 00 Ost), etwa 25
> # km s�döstlich von Frankfurt/Main."
> # http://www.maporama.com/
> # Mainflingen: 50 1' 54" , 9 1' 33" 50.0318 , 9.026
> # Gouda/2804 RR: 52 1' 37" , 4 42' 15" 52.0272 , 4.7044
> # http://www.fcc.gov/mb/audio/bickel/distance.html
> # -> 374.766km
> # speed of light: 299 782.458 km/s
> # 1.25012652ms delay
> 
> so I need to add an offset of 1.25ms somehow? still less then this 17ms
> offset I saw.

This is only the pure signal propagation delay. You must also take into
account the signal delay due to the limited bandwith of the transmitting
antenna, and the signal delay due to the limited bandwith of the antenna in
the receiver. 

The latter should be higher than the first since normally the DCF77 AM
receivers have a narrow bandwith in order to suppress noise.

The narrower the receiver bandwidth the slower the AM signal amplitude
decreases after the start of the second marker. The slower the amplitude
decreases the longer it takes until the second marker is detected by a
pulse shaper, and the more jitter is introduced due to uncertainty of the
pulse shaper's trigger level. 

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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