Hi Uwe, Geri, Larry and all,
a while ago, I had posted some observations on the frequency stability of
DBF39 (or DCF?) on 138.83 kHz (pasted below). Maybe I should try to explain a
little more.
what are the exact (0.001Hz) qrgs of mark and space (in the mark)?
There is no extra narrowband modulation. The widening of the mark spectral
line is simply caused by the normal FSK bursts (200 bd, 340 Hz shift) which
are sent at irregular intervals, at least once in every 12 seconds.
As the frequency shift is no integer multiple of the baud rate, each "space"
bit accumulates a phase shift of 340/200 * 360deg = -108deg . After a
telegram with an arbitrary number of "1" bits, the carrier does not return to
its old phase. In a fast spectrogram, you can see the telegrams separated in
time, the typical blooms Larry mentioned. But in a high-resolution FFT like
Uwe's waterfall, they merge into a noise-band about 1/12 Hz wide, without any
coherent central line.
The appended little image also shows this DBF "band", compared to several
atomic-clock driven LF carriers. During an hour of recording, I left Argo
(120s dots slow) running while switching the synthesized local oscillator.
The small downwards steps are an audio deviation proportional to carrier freq
(about 10 mHz max), caused by my 10 MHz OCXO reference being 0.05 ppm low.
Both broadcast carriers are accompanied by distant, less accurate stations.
Larry brought up an interesting point:
Just a short note, I am receiving DCF39 most every day, ...
My bucket size is less than 1.22 milliHz and I can see the day/night
frequency change on DCF39 as well, variation is very small but it is visible
most days.
Could that be ionospheric doppler shift? How large is the effect?
73 de Markus, DF6NM
______________________________________________________________
I wrote on Feb. 12:
There are two reasons why DBF39 may be of limited use for very precise
frequency calibration:
- Due to the phase-continuous FSK, the phase has shifted arbitrarily after
each data burst. On Argo with 21mHz or finer resolution you will not see a
narrow line but rather an irregular strip several ten mHz wide.
- Even in the intervals between the telegrams, the freq appears to be
slightly higher than nominal. Using a scope to directly measure the rate of
phase advance relative to an OCXO reference derived from terrestrial TV
"ZDF", I got 138830.030 Hz for DBF39 and 128930.014 Hz for DCF49, but these
offsets may even be variable. On the other hand, 122.5 (DCF42 pilot), 153.0
(DLF radio) and of course 77.5 kHz (DCF77) seem to be very accurate.
73 and good luck
Markus
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