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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jul 5 03:23:18 EDT 2008


Poul-Henning Kamp wrote:
> In message <486EC7F0.10006 at xtra.co.nz>, Bruce Griffiths writes:
>   
>> Poul-Henning Kamp wrote:
>>     
>
>   
>>> Look at:
>>> 	http://phk.freebsd.dk/misc/L6/
>>>   
>>>       
>> Nonsense, although it may work in favourable cases where the signal 
>> levels arent too disparate, in the more general case this isnt true.
>>     
>
> Bruce, you are input-resistant, please read what I write:
>
> 1. I am not trying to solve the general case.
>
> 2. I am quite happy for it to work only in "favourable cases", because
>    most people in the western hemisphere live close enough to a Loran-C
>    for it to be a "favourable case".
>
>   
>> Thats obviously fine for some Loran applications but you haven't 
>> demonstrated that its also true when attempting to simultaneously track 
>> another signal of interest that is relatively weak.
>>     
>
> Yes I have, but you seem to refuse to read it:
>
> 	http://phk.freebsd.dk/loran-c/CW/
>
>   
That is restricted to the Loran-C case only.
>> 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?
> This is *not* an attempt to build a shiny complex, gold-plated contraption
> that can drag out a signal from half-way around the world in Hi-Fi
> quality, described as:
>
>   
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.

This is not true as it has been achieved many times at various locations 
around the world.
The phase tracking receivers of the 1960's had a sensitivity of 1uVrms 
or better.
For example see the performance data for the HP117A in the October 1964 
issue of the HP Journal.
>> In which case a dedicated ST32M103 for Loran and another for 
>> MSF/WWVB/JJYDCF77 etc is appropriate together with optimised analog 
>> front ends, bandpass filters, notch filters and orientation of the 
>> individual antennae to null strong interfering signals should cope with 
>> most problematic sites.
>>     
>
> That's exactly all the stuff I'm trying to avoid.
>
>   
Most of this is optional, only use it if it is actually needed for 
successful carrier phase tracking at one's site.
It is also relatively inexpensive in most cases, and easy to add if 
needed - the digital processing back end need not change although long 
averaging times may be required.
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,
> What I'll build is an <EUR100 single-chip receiver that will
> discipline an OCXO or Rb against the strongest Loran-C signal
> available.
>
> ... And make an excellent platform for experimenting with SDR
> on other VLF time/frequency signals.
>
>   
Bruce




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