[time-nuts] Near-perfect chip for Loran-C frequency receiver

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jul 5 04:54:25 EDT 2008


Poul-Henning Kamp wrote:
> In message <486F2166.4080907 at xtra.co.nz>, Bruce Griffiths writes:
>   
>> Poul-Henning Kamp wrote:
>>     
>
>   
>>> 	http://phk.freebsd.dk/loran-c/CW/
>>>   
>>>       
>> That is restricted to the Loran-C case only.
>>     
>
> You have not actually looked at that page have you ?
>   
Not for a few years anyway.
> Hint: "CW" is, as usually, used as an abbreviation for "Continous Wave".
>
>
>   
Plots without scales are not particularly useful.
Given the relatively large field strength (>100uV/m ??) for DCF77 in 
Denmark perhaps a receiver with an optimised (for DCF77 or other LF 
station) front end can do better.

>>>> Thats only a 33 dB difference in signal level what happens when the 
>>>> signal strengths differ by 60, 80 dB?
>>>>         
>>> Then we don't care to track them, because we will not be able to
>>> derive a sensible frequency signal from them.
>>>   
>>>       
>> So how come NIST at WWVH in Hawaii still manges to do this then?
>>     
>
> I am happy tell you that NIST has taken great care to NOT inform
> me, that they want to replace their very expensive high-end gear
> with my <EUR100 gadget.
>
>   
Their commercial?? VLF receiver uses a chart recorder and may need 
replacement eventually.
> Likewise, I can inform you that I have neither the budget or level
> of ambition of NIST in this area, wherefore a <EUR100 receiver that
> will track a couple of loud loran-C signals is plenty to make me
> happy.
>
>   
As there is the option of adding a high performance front end and 
disabling the Loran-C code (where its not sensible/practical to use it), 
then there will be much wider interest in this.
>> In the case of LF transmissions you would have us believe that anyone 
>> beyond 500km (theoretical limit within which the DCF77 ground wave 
>> dominates according to the PTB- excludes Denmark) from the DCF77 
>> transmitter hasnt any hope of usefully accurate (1E-9 or better) 
>> frequency comparisons against the received signal.
>>     
>
> I can get 1e-12 from DCF77, as long as I integrate for about a day.
>
>   
Which perhaps indicates that the effect of the skywave interacting with 
the groundwave isn't too critical as long as you integrate for long enough.
> I can get 1e-14 from DCF77, as long as I integrate for about a year.
>
>   
Given the frequency of European power blackouts and transmitter 
maintenance the latter integration period is perhaps a little impractical.
> Please don't argue any more until you understand that.
>
>   
>> If the system is split appropriately into 2 modules the analog front end 
>> and the DSP backend, customisation to suit a particular site is easy to do,
>>     
>
> If you had read what I wrote originally, you would have realized instantly
> that it is perfectly possible to use as complex an analog frontend as
> you want.
>
> The goal of my project is to use as simple a frontend as possible,
> and there is nothing you can say to persuade me to make it more
> complex than it needs to be.
>
> Poul-Henning
>
>
>   

Your original loop antenna should have a lower phase tempco 
(particularly if a lower TCE coil former than a plastic lid is used) 
than any ferrite antenna, particularly so if the antenna is resonated.
Quartzlock have abandoned the use of ferrite antennas and substituted a 
balanced shielded loop antenna for this reason for their VLF phase 
tracking receivers.

Bruce




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