[time-nuts] E1938A source code/ firmware

[Achim Gratz]

Glen English VK1XX writes:
> My intention is to use the average of multiple stationary mode GPS
> 1PPS signals to drive a single OCXO, the idea to be a better 1pps
> estimate. I'll upsample the inputs to get the control sample rate up.

I'd expect a lot of correlation among multiple GPS receivers, especially
when they're fed from the same antenna and are otherwise sharing the
same environment.  Averaging will only deal with the non-correlated
errors in the signal.

> Eventually I want to explore the use multiple OCXOs, but not until I
> think of a good way to take an average of multiple OCXOs, or, even if
> that is a useful idea.

I think you'll have trouble isolating these well enough to not (at least
occasionally) injection lock on each other.

> FPGA based,  I'll  put the OCXO drive and the 1ppS to the FPGA
> differentially into maybe  8 FPGA inputs (that is each signal into 8
> different FPGA pin pairs) , and use IDELAY blocks to delay the 8
> different inputs to provide more edge resolution for each signal.

There are a few recent papers on that topic and the IDELAY blocks are
indeed used in some of these.  Getting them to track over voltage and
temperature is apparently a problem, so different methods of
implementing the TDC seem to be more useful depending on how far down
you want to push the resolution.

> The IDELAY blocks can be dynamic but I'll probably use then
> fixed. output of the FPGA can be sigma-delta converter, which can
> provide almost arbritary number of bits. LVDS output of the 1 bit FPGA
> converter signal will go to an outboard LVDS buffer with its own power
> supply so bumps on FPGA  VCCIO dont affect the output.

Getting the residual noise down on that signal will still be a
challenge, so you might want to (optionally) consider a proper DAC there
depending on how good an OCXO you intend to use.


Regards,
Achim.
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