[time-nuts] Some results of PRS10 and Trimble Resolution T

Robert Lutwak Lutwak at Alum.mit.edu
Thu Jun 29 06:44:08 EDT 2006

Thanks for the link to your report, Tom.  I was wondering what this 2 sec 
"bump" in the ADEV was that everyone was talking about.

The "bump" at 2 sec is just the loop tau of the main clock servo.  The 
physics package is performing at the (fairly typical) level of about 
2e-11/sqrt(tau).   The OCXO is somewhat better than that.  From Tom's plot 
it looks like the OCXO is good to about 3-4e-12 on tau=0.1 to 1 second. 
Because they've set the loop tau to about 1 seconds, the performance of the 
OCXO is degraded as it is steered to the atoms on that timescale.

Never mind fancy RS232 commands, the real thing that sets SRS apart from the 
competition is that they use a good OCXO for the local oscillator, rather 
than a TCXO.  This gives about 30 dB superior phase noise on the output and 
also contributes to the apparent "bump" described above.  In more typical Rb 
standards, a TCXO is chosen with minimally adequate ADEV to support the 1 
second stability goal, say 1-2e-11 on 0.1 to 1 second.  Then, rather than a 
"bump," you just see a flat line from 0.1 to 1 second which then turns 
downward onto the 1/sqrt(tau) line outside of the loop tau.  Don't sweat the 
"bump," you wouldn't see it if it weren't for the superior oscillator.

Note that, given a superior OCXO, SRS could have chosen to set the loop tau 
longer.  If the OCXO stays flat, they could set the loop tau way out at 10 
seconds.  Then the STS would look the OCXO, flat at 3-4e-12 from 0.1 to 10 
seconds and then turn gracefully downward onto the 1/sqrt(tau) line 
somewhere around 10-20 seconds.  The problem with this, of course, is that 
if something bad happens to the OCXO, such as a frequency hop, it takes 
10-20  seconds to steer it out, rather than 1 second.  Allowing a frequency 
hop, which could be as big as 1e-9, to persist for 10-20 seconds would 
destroy your medium and long-term stability records (and probably three or 
four other critical specifications).  I don't recall whether SRS gives you 
access to the loop tau over RS232 but, if they do, you could try turning it 
up to 5-10 seconds just to see your 1 sec stability improve.  Depending on 
the quality of the OCXO and the environment of your laboratory (temperature, 
vibration, power supply stability, etc.) you may be able to get away with 
running the rubidium at longer loop tau for months before you see an 
unpleasant frequency hop.

Of course, the real culprit here is Timing Solutions and their damn fine 
TS5110.  If it weren't for them, we wouldn't be having this conversation.


Robert Lutwak, Senior Scientist
Symmetricom - Technology Realization Center
34 Tozer Rd.
Beverly, MA 01915
(978) 232-1461   Voice           RLutwak at Symmetricom.com   (Business)
(978) 927-4099   FAX             Lutwak at Alum.MIT.edu  (Personal)
(339) 927-7896   Mobile
----- Original Message ----- 
From: "Tom Van Baak" <tvb at leapsecond.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Wednesday, June 28, 2006 11:16 PM
Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution T

>> It's exactly peaked at 2s, it's got to be something digital like
>> the DAC control of the OCXO etc. Any SRS designers among
>> this group? BTW: while I cannot prove it anymore, I am pretty
>> confident the PRS10 did the same thing while free-running
>> (without 1PPS input).
> I'm thinking it looks "exactly at 2s" simply because
> your plot only has data points for only 1s, 2s, 3s, etc.
> A high ADEV bump near tau 2 s suggests a phase
> modulation around 4 s. If you look closely you can see
> this in the phase plots; it's somewhere between 4 and 5
> seconds. It shows up in the power spectrum near 4.4 s.
> Many plots for you in this free-running PRS10 report:
> http://www.leapsecond.com/museum/prs10/
> /tvb
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