[time-nuts] GPS vagaries and binary interface

[Dr Bruce Griffiths]

Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>
>  
>
>   
>> Good timing antennas have built in ceramic or equivalent bandpass 
>> filters to minimise the effect of interference.
>> A patch antenna is not as satisfactory as a quadrifilar helix or a choke 
>> ring ground plane antenna for accurate timing purposes.
>>
>> If GPSDO did some statistical filtering instead of just blindly 
>> accepting all PPS signals as valid and usable such dropouts would cease 
>> to be much of a problem.
>> There's no substitute for a a correctly engineered design with an 
>> appropriate tracking loop bandwidth and statistical filtering of outliers.
>> A good crystal will drift very little over half an hour or so when the 
>> GPS derived PPS signal may be unreliable.
>>
>> Bruce
>>
>> _______________________________________________
>>   
>>     
> That's the impression I am getting. I do not know if any of the GPSDO 
> that I have seen described in recent literature take care of this properly.
> It seems when the GPS goes nuts, the 1 PPS goes quite a bit out of 
> normal range, so it should not take too much processing power to 
> determine if it's in range or not.
> Of course, an analog solution would require many more parts to do that 
> determination, filtering and switching, so it seems the most *practical* 
> way to implement a GPSDO is with a uC of some sort.
> The uC could even monitor what's coming out of the GPS receiver's serial 
> port and open the loop if there are not enough satellites in range.
>
> Now, about the Jupiter receiver, it seems the only way to set a mask 
> angle is through the binary interface that is not well documented at all 
> in the Navman documentation. They do not even say if the mask is saved 
> to flash or EEPROM, or if it has to be reloaded each time the GPS is 
> powered up. That would also require a uC.
>
> I would be really grateful if someone had done that research and could 
> help me.
>
> In the mean time, I may just put the spectrum analyzer at the output of 
> the patch antenna (with proper biasing of the LNA of course) and see if 
> I am getting junk other than the GPS signal. I know the GPS signal will 
> be too low for the spectrum analyzer, but if I see anything else between 
> 1 and 2 GHz, I know that would be a problem. I live about 5 miles from 
> the largest US air base (Eglin AFB) and I am sure they have L-band 
> radars running there, and probably a bunch of other ether pollutants. 
> Assuming the radar signal is not swamping the LNA, a good filter could 
> not hurt.
>
>
> Didier KO4BB
>
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>   
Didier

Alternative GPSDO solution
Divide the 10MHz reference by 32 resync the output to 10MHz with a fast 
D flipflop and then divide the D flipflop output by 4 using a 2 bit 
switchtail ring (Johnson) counter.
Low pass filter the outputs of both divide by 4 counter flipflops with 
identical filters.
Use ACMOS flipflops in the ring counter so the sine wave output 
amplitude is reasonably stable.

This should produce 2 nominally quadrature sinewave outputs at (10/128) MHz.
Use 2 12 bit ADCs (eg AD7942) to sample the 2 quadrature sinewaves on 
the leading edge of the GPS receiver PPS signal.
The ADC readings can be processed to derive the phase angle at the PPS edge.
A resolution of 10ns or better is readily achieved. This is more than 
adequate for most current GPS receivers.
If you are worried about the stability of the low pass filter phase 
shifts just use another pair of ADCs to sample the 2 sinewaves at 10/128 
MHz.or a submultiple thereof.
The difference between the 2 phase angles will be independent of the 
filter phase shifts.

Bruce