[time-nuts] Home made GPS disciplined atomic clock

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jan 27 15:09:12 EST 2009


Esa

Wenzel has a introduction to low cost phase noise measurement at:

http://www.wenzel.com/documents/measuringphasenoise.htm

Its relatively easy to assemble such a system.
A PC sound card can be used as a spectrum analyser for measuring phase
noise to within a few Hz of the carrier.
There are low noise amplifiers with lower flicker noise than Wenzel's
low noise FET input audio amplifier
<http://www.wenzel.com/pdffiles1/pdfs/lowamp.pdf>.
It is also possible to build a variant of Wenzel's amplifier that doesnt
require selection of JFETs.

If one wishes to measure the Allan variance of an oscillator, the 3
cornered hat technique can be used with 3 oscillators having similar
performance.

For longer tau (1000sec or more depending on the OCXO being
characterised) the PPS output of a good gps timing receiver can be used
as a reference.
However a clear view of the southern sky is required,

Bruce

Esa Heikkinen wrote:
> Hello again...
>
>   
>> Right, it all depends on what stability you're after. The OCXO
>> will have much better short-term stability than the LPRO -- the
>> LPRO is close to ten times worse. So do not replace the TBolt
>> OCXO with a LPRO if short-term stability is your goal. See:
>>     
>
> I'm wondering what could be the cause of this. According to operating 
> manual LPRO's output should be crystal oscillator (VCXO) generated 
> signal, which is synchronized to rubidium. So why it is so much worse 
> than any other crystal oscillator (or other Rd oscillators). Are there 
> any schematics for LPRO available anywhere?
>
> I cannot see the any phase noise difference between Trimble's OCXO and 
> LPRO with spectrum analyser. Measured with different spans from 500 kHz 
> to 200 Hz, using resolution bandwiths 300 Hz to 6 Hz. So the noise which 
> is causing bad short term drift must be very close to fundamental.
>
> It seems that only way to see this noise is to use phase detector 
> circuit but my problem with that is that I haven't got any good 
> reference for it and this kind of equipments are quite hard to find here 
> in Finland. It would be nice to see what kind of noise there are, to 
> design the filter bandwith for external OCXO lock circuit.
>
> Other idea to bring that noise visible could be multiplying it with some 
> kind of comb generator circuit (might be hard to build one). Then it 
> would be possible to measure it's harmonics. Not sure if there's enough 
> level present anymore at GHz frequencies...
>
> What kind of test setup did you use when getting this result:
>  > LPRO plots:
>  > http://www.leapsecond.com/museum/lpro/
>
>   





More information about the time-nuts mailing list