The strip chart recorder is, of course, a hangover of days when data-
logging
could not be done digitally. However I still plot the results out
from a data logger
or time stamped data so I can see what is happening. The scatter of
adjacent readings
shows the noise and measurement uncertainty, any periodicity is
visible as sign waves
and the drift is visible even through a considerable amount of noise.
The reason I still use a chart recorder is that I can see the data in
real time, and I can see
the effects of any adjustments that I make. One problem of chart
recorders is when they run off scale
so I often process signals so they fold back instead of running off
scale.
For setting a rubidium, a chart recorder showing the phase of the
difference between GPS and the rubidium
is very useful, you only have to make adjustments to steer down the
middle of the chart.
The recorder is more of an integrator than low pass filter.
Incidentally, I constructed an early data logger in about 1963. We
had an HP digital voltmeter attached to a
an HP printer. I constructed a 500Hz tuning fork time standard,
divided down with decatrons, to trigger the
voltmeter printer to record a measurement. I then had to punch the
numbers from the printout onto IBM punch cards
to calculate the process being monitored.
We have come a long way since then.
cheers, Neville Michie
On 30/07/2010, at 11:52 PM, paul swed wrote:
So on a 60 khz signal the long strip chart recorder is simply a
super long
low pass filter averaging out the doppler somewhat. It really
doesn't do
that well. The mark-1 eyeball does a better job. Right?
On Tue, Jul 27, 2010 at 4:53 AM, Geoff <[email protected]> wrote:
On Tue, 27 Jul 2010 09:08:49 am Chuck Harris wrote:
I suppose that you could always cheat? Since you know where the
transmitter is going to be, if you could get a timenut near to the
transmitter to give you a beacon to measure 24hrs prior to the
event,
you could use the diurnal variations that you observed (observe?) on
the beacon to predict the skywave offset due to Doppler at the time
of the event.
-Chuck Harris
Murray Greenman wrote:
You guys are trying to crack a nut with a sledgehammer!
For a start, as Didier says, you can't possibly read the
frequency of a
sky-wave signal to 0.01Hz in any short time frame since the
Doppler on
the signal can be as much as 1ppm (i.e. 10Hz at 10MHz). You can
only
infer it closer than that by studying the frequency in the very
long
term.
In addition, you'll never know how much of the daily variation is
ionospheric, and how much is due to thermal changes at the source.
snipped
There is one possible way of getting an accurate reading from a
sky wave
signal over a short(ish) period. Plot a doppler shift curve with
as fine a
resolution as you can manage. Then look for a point of inflexion
in the
curve, that is a point where the second derivative of the curve
function is
zero. The frequency at that time will be that transmitted as at that
instant
the path length is not changing. You may have to examine your data
set
visually and mathematically examine a much smaller section. Of
course if
you
don't get a point of inflexion you'll need much more data :-).
Cheers, Geoff vk2tfg.
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