[time-nuts] GPS vagaries and binary interface

Didier Juges didier at cox.net
Sun Oct 29 00:44:42 EDT 2006

Dr Bruce Griffiths wrote:
> 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.

OK, I follow even if I am not sure where this is leading...
Anything magic about 32, other than it's probably the smallest division 
that may not immediately result in rollover when the OCXO is cold?
Based on yesterday's experiment, my OCXO rolled over once while warming 
up with a division ratio of 128. A few more chips are not a real 
problem. I like the 74F161, they are fast and synchronous. If I could 
find 74F162's, that would be the best, or I can program the 161s as 
decade counters, but it's more work.

> Use ACMOS flipflops in the ring counter so the sine wave output 
> amplitude is reasonably stable.

Of course, TTL outputs are anything but 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.
That would be a software interpolator?

I like that approach because it reduces the hardware to a relative 
minimum, compared to the Brooke Shera approach, and puts the complexity 
in software.

Regarding your next message recommending to use a dual channel ADC, I 
agree, even though it may be simpler to use S&H devices with the 
built-in multiplexed ADC of the microprocessor. I have a few monolithic 
Burr Brown devices that have a small aperture gate, I forgot how much.

This sounds very interesting, but it won't be an evening project :-)
I don't do PICs (no development tools, no code bank). I do not have the 
tools to do PLDs either. My favorite uCs are 8051s, particularly the 
Silabs parts. I also have access to a good C compiler and I have written 
a lot of 8051 code. I do not know how these parts fare as timing chips. 
They are plenty fast though, some run at a 100 MHz clock, with many 
instructions taking one clock.

> Bruce
> _______________________________________________
Thanks again for many thought provoking ideas.


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