[time-nuts] Home made GPS disciplined atomic clock

[Ulrich Bangert]

Esa,

> 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 it is! But the detection process of the "atomic lock" is "noisy"
itself. That makes it necessary to have low pass filters in this pll
that make the overall noise dependend from the xtal oscillator's noise
at short observation times. If you consider that a typical "normal" xtal
oscillator (no oven, no temperature compensation) has an Allan deviation
of 5E-8 @ 1 s then you see that 1E-11 @ 1 s IS ALREADY a BIG improvement
above the "normal" case. 1E-12 @ 1 s is near the best that amateur money
can buy. A lot of high grade OCXOs will be >1E-12 and <1E-11 @ 1 s and
VERY FEW will be <1E-12 @ 1 s

73s Ulrich, DF6JB  

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Esa Heikkinen
> Gesendet: Dienstag, 27. Januar 2009 17:34
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Home made GPS disciplined atomic clock
> 
> 
> 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/
> 
> -- 
> 73s!
> Esa
> OH4KJU
> 
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