[time-nuts] is there a "best bet" advanced hobbyist buildable GPSDOdesign?

[Scott Burris]

On Dec 11, 2007 12:53 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz>
wrote:

> Tom Van Baak wrote:
> >> Hi,
> >>
> >> Like many, I've acquired a fair amount of surplus test equipment off of
> Ebay
> >> which could use the services of good master frequency standard.  So I'm
> >> looking to discipline an HP 10811 VXCO to provide this.
> >>
> >> Any general consensus about the best design for a hobbyist to build?
> >> I'm familiar with the Brooks Shera design, the G4JNT Jupiter-T design,
> >> the TAC-2 circuit, and the VE2ZAZ design.  I take it from discussions
> >> I've seen in the archives of this list that the VE2ZAZ design makes a
> >> number of simplification/performance tradeoffs.
> >>
> >> Is there a design I haven't listed which is "better" than the others?
> >> I'm quite familiar with microcontrollers, FPGAs, spinning my own
> >> PCBs, etc, so I'll roll my own if I have to, but I'd prefer to build
> >> a variation on someone's tried and true design.
> >>
> >> I'm aware of products like the Fury, but I'd like something I could
> tinker
> >> with, and the cost is hard to justify for a hobbyist.
> >>
> >> Scott
> >>
> >
> > Scott,
> >
> > Hard to say which is better at this point; there are a number
> > of variables, not the least of which is the intrinsic short-term
> > stability of the OCXO you use.
> >
> > Do have a close look at James Miller's GPSDO:
> >
> > http://www.jrmiller.demon.co.uk/projects/freqstd/frqstd.htm
> >
> > I recently tested one and it makes it to 1e-13 at one day, which
> > is really nice for a simple, cheap, homebrew GPSDO.
> >
> > /tvb
> >
> >
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> >
> Tom
>
> What about the short term performance?
> Its relatively easy to achieve a stability of 1E-13 for an averaging
> time of 1 day, achieving good short or medium term stability is more
> difficult.
>
> If you want simplicity and higher performance you can do far better with
> fewer parts,
> An expensive high resolution DAC can be replaced with a software
> sigma-delta DAC that has higher resolution.
> The complex phase detector can be replaced with a D flipflop.
> Add a microprocessor plus an opamp or 2 to filter and scale the EFC
> voltage and thats about all thats required in addition to a good GPS
> timing receiver.
> For improved performance a hardware circuit to correct the PPS sawtooth
> error will improve the medium term stability significantly when using a
> high performance GPS timing receiver that provides an estimate of this
> error.
>
> Both the Brooks Shera and the James Miller designs have inadequate phase
> error measurement resolution to achieve good short and medium term
> stability.
> However, this is only noticeable when using high performance GPS timing
> receivers (M12+T, M12MT etc) and a high quality OCXO (10811A etc).
>
> Bruce
>
> _______________________________________________
>

OK, so for the DAC piece, why not just use an NXP LPC ARM chip for the
microcontroller, and use a 32bit PCM output followed by a low pass filter as
the VXCO EFC?  The DAC just needs high resolution, not accuracy, right?
Or would the switching noise from the processor modulate the control
voltage?
I would hope the filter would clean any such noise, but I'll be the first to
admit
that the farther we get into the analog domain, the more I'm out of my
comfort zone.

I'm still trying to wrap my brain around the phase detection piece of this.
I've studied the Shera controller with it's 24Mhz oscillator and divided
down
sample of the VXCO and I'm can't get past thinking that this ends up
adding jitter.  With more modern parts can't the phase be measured more
directly?  What about sampling both the VXCO and 1PPS at a 200MHZ rate?
That should determine the phase difference within no more than a 10ns
inaccuracy.

Or use a pulse stretching technique to amplify the short time intervals into
something
more easily measured, although that's beyond what I'm familiar with.

I've read the PTTI presentation about using a DS1020 delay line to
de-sawtooth the
1PPS signal -- that's a pretty interesting idea.  At least the chip is
available in Qty 1,
at $30!

It just seems that the designs I've seen could use a refresh with some more
modern
circuitry.  At the very least the Shera controller could have much of its
logic put into
a single CPLD these days.

Scott