[time-nuts] Cs standard stability

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 4 20:14:37 EST 2017


Bob

You should to use the same measurement bandwidth when comparing ADEV measures.

The measurement bandwidth of a Timepod is usually less than 1kHz whereas the 5370A has a trigger system bandwidth of 100MHz or so. If the high phase noise of your GPDSO isnt attenuated by a filter it may cause significant measurement noise.

Bruce

> On 05 March 2017 at 14:03 Bob Stewart <bob at evoria.net> wrote:
> 
>     OK, but how does the phase noise of the GPSDO make the ADEV for the Cs worse than that for the GPSDO?  I find it hard to believe that the phase noise from two separate units would cancel out for two GPSDOs but add for the Cs.
> 
>     Bob
>      
> 
> 
> 
>     ---------------------------------------------
>     From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>     To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
>     Sent: Saturday, March 4, 2017 6:03 PM
>     Subject: Re: [time-nuts] Cs standard stability
> 
>     Bob
>     We are certainly not off track.
>     The phase noise of the GPSDO affects the trigger jitter of both the 5370A and the jitter of the GPSDO PPS output and consequently the ADEV measured by your setup.
>     NB ADEV depends on the measurement bandwidth which is relatively small when using a Timepod. 
>     Bruce
> 
>         > > On 05 March 2017 at 12:31 Bob Stewart <bob at evoria.net> wrote:
> > 
> >         Bruce,
> > 
> >         Are we getting a bit off track?  The question I asked is in regards to the Cs, not the purity of the output of the GPSDO.  The two ADEVs were done under the same conditions, yet the ADEV of the Cs is obviously not as good as that of the GPSDO.  Also, the ADEV of the GPSDO pretty much tracks what is to be expected when run on a good 5370, isn't it?
> > 
> >         Bob
> > 
> > 
> > 
> >         ---------------------------------------------
> >         From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> >         To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com>
> >         Sent: Saturday, March 4, 2017 5:22 PM
> >         Subject: Re: [time-nuts] Cs standard stability
> > 
> >         Bob
> >         The Phase noise of that GPDSO seems quite high and some form of filtering (bandpass or low pass) of its output may be necessary to reduce the resultant jitter when connected to the 5370A input.
> >         There are commercial TDCs that claim that sort of performance (~10ps) but a comprehensive test of their actual performance appears as yet unavailable.
> >         HP used to sell a TDC with 5ps resolution but detailed specs are unavailable.
> >         Higher resolution and lower noise should be achievable but would require a lot of number crunching for each measurement. 
> >         Bruce
> > 
> > 
> > 
> >     > 
> 
> 



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