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

[Bruce Griffiths]

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