Could you fire me some summary comments on
what frequency stability should be achieved by transmitters and receivers to
join the game, for respective modes?
Cheers,
Bob
ZL2CA
Hi Bob
-
I've
copied this reply to the LF Group as well for general
information...........
QRSS (
'SLOWCW', why invent meaningless Q codes for the sake of it) as usually
practiced now, calls for a resolution bandwidth of 0.3Hz or wider in order
to see the 3s dot period. But as this mode is always viewed on
a spectrogram display generated from an FFT, we are actually seeing a large
chunk of spectrum, typically many 10s to 100Hz simultaneously and a drift
of a 1Hz or so probably won't be noticed. Thus the frequency stability
needed for this mode can be a very poor 10 parts per million - which even the
cheapest and nastiest crystal can manage - hence one of the popularities of
the mode. A 10s dot peroiod is occasionally used calling for a 0.1Hz bin
size or less but still a few ppm accuracy will suffice.
For
BPSK, a frequency stability of the order of a tenth the bit rate is needed,a
lthough a figure of better than this speeds initial lock up. Once locked,
the frequency can drift outside this range, and manual assistance ina
chieving initial lock by 'nudging' the tuning process can allow worse
initial frequency setting accuracy. So for the 100ms (10B/s)
rate we normally use, 1Hz stability is ideally needed, better to
aim for 0.2Hz and not have to play around with initial acquisition - still well
within the capabilities of any crystal oscillator.
So to summarise, by recommending crystal
oscillator stabilities you won't be giving any wrong steers - 1 part per million
should be the stability people should really aim for, but a factor or so worse
than this can still be useable with Spectrogram type modes.
I
myself use a Direct Digital Synthesizer source driven routinely by a TCXOo
scillator adjusted up for a repeatable 0.1ppm. The DDS
tunes in 0.001 Hz steps and without accurate calibration achieves 0.03Hz
accuracy, with calibration against off air signals (eg MSF in the UK) 0
.001Hz is possible. For times when some distant stations actually
get set up for some serious experimentation with very narrow band modes, I have
Droitwich and TV sync locked frequency sources - both hangovers from microwave
operating, (all home station operators and several /P have constructed one
or the other of these). All these give a frequency setting accuracy of a
few parts in 10^-9 - about 1mHz at 137k.
For
calibration purposes, a GPS receiver giving 1s pulses drives the external clock
of a frequency counter allowing 1000s / 10ks (2.78 hour) or 100ks (just over 1
day) gating periods. Also have a Caesium standard but prefer
to only turn it on once in a while when needed as the tube is nearing the end of
its life and its usefulness for LF is debateable. For calibration the GPS
pulses are traceable to international standards and can be used immediately,
whereas the Cs standard would need to be set up each time and run for several
days to guarantee its few parts in 10^-11 accuracy.
Sorry
to sound a bit cynical about LF frequency accuracies and stabilities, but having
spent over 10 years with narrowband operation on the microwaves bands,o
ften from exposed portable locations, where the 1ppm accuracy that very
few LF operators bother to aim for is several SSB bandwidths, I am rather
surprised at the difficulties many seem to have and the poor stabilities still
being seen on LF now. At least the disgusting LC oscillator has faded out,
but the norm now seems to be two VCXOs mixed, which I suppose is comparable
to an LC VFO on the HF bands, not something seen too often now,
fortunately. Up on 10GHz a low cost crystal heater is the minimum
anyone ever uses and 1ppm is usually the result. Operators routinely
calibrate with beacons and other stations throughout an operating period and
everyone knows at least when they are within a 3kHz SSB bandwidth tuning
range.
Ironically, HF
has the lowest requirement for frequency stability. With ionosphericp
ropagation causing several Hz shifts over minutes, as well as multipath with
several millisecond delays, who needs better than 0.2 ppm frequency
accuracy?
Andy
G4JNT
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