[time-nuts] Basic question regarding comparing two frequencies

[Neville Michie]

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 <vk2tfg at ozemail.com.au> 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.
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ 
> time-nuts
> and follow the instructions there.