[time-nuts] Fwd: CS reservoir depletion

Sun Jan 16 20:27:23 UTC 2011

Hello Bob,

very nice summary. Here is a plot to support your list:

In terms of ADEV performance, a good GPSDO can come very close to and in  
some areas exceed the performance of a good Cs significantly.

See the attached plot, this was created by super-imposing two typical  
plots available on a prominent Time-Nuts members' website, and shows a 5071A  
compared to one of our Fury GPSDO units. Both compared against an even better  
standard I believe.

The red plot is the 5071A, the black plot is the Fury GPSDO.

We can see that the Fury actually out-performs the 5071A between 0.1s to  
10s with quite a bit of margin. Then it is less than one order of magnitude  
difference between the two from 10s to about 40Ks, at which time the Fury 
seems  to "catch up" to the 5071A. The GPSDO likely also out-performs the 
5071A in  terms of phase noise performance.

There is about a 33x price difference between the two units new,  and as 
you mentioned the GPSDO is maintenance-free, whereas the 5071A requires  care 
from time to time. There is also a massive difference in power-consumption  
and thus operating costs.

bye,
Said

From: Bob Camp <_lists at rtty.us_ (mailto:lists at rtty.us) >
Date: January 16,  2011 12:00:50 PST
To: Discussion of precise time and frequency  measurement 
<_time-nuts at febo.com_ (mailto:time-nuts at febo.com) >
Subject:  Re: [time-nuts] CS reservoir depletion
Reply-To: Discussion  of precise time and frequency measurement 
<_time-nuts at febo.com_ (mailto:time-nuts at febo.com) >

Hi

While I agree with the idea of  a "super GPS" as being a good standard, 
there are some reasons for needing  something else:

1) You need something to  compare *your* GPS gizmo to in your setting. 
Knowing that it might be working  ok is not as good as knowing that it is 
working  ok.

2) Without some fancy corrections, GPS  can indeed drift. The period might 
be hours, it could be days. The net effects  will cancel over a long enough 
time, but that time may be longer than your  loop can suppress.  

3) Anything that  creates a "fast" change in your local fly wheel standards 
will still show up  on the output. It will walk out with time. Things in 
this category are stuff  like a step change in the supply voltage creating a 
change in your OCXO  output. 

All of that can be worked on. Much  of it applies equally to having a 
single Cs in the basement.  

There are a couple of things the GPS gives  you that the Cs will not:

1) It will tell you  what time it is to within a few ns. Most of us 
"frequency guys" don't realize  quite how hard that is to do without GPS. 

2)  The long term stability (because it's steered) of the GPS is going to  
eventually beat anything else out there. If you do really long data runs  
(months, years) the GPS will always win.

3)  Low cost to run. Even if you have a working Cs, how long until the tube 
goes?  The GPS has essentially no wear out mechanisms. The Cs is full of 
strange  stuff. The parts in the GPS are pretty cheap / easy to find.  

4) Easy to duplicate. Everything you are  likely to use in a fancy GPS 
setup is commonly available in quantity. The  components are "known good" and 
cheap. No gamble on a tube or other hard to  replace stuff. 

5) Easy to run. There's not  much mystery about what's going on. Nothing is 
hidden in a vacuum bottle. No  hidden numbers in a rom. Everything is 
pretty much right in front of you. The  only exception would be the disciplining 
software in the GPSDO if you choose  to use it. 

I admit that the GPS makes a  *lot* of sense. I sure wouldn't turn down a 
nice shinny new HP cesium if one  showed up on my doorstep.  

Bob

On  Jan 16, 2011, at 7:01 AM, _EWKehren at aol.com_ (mailto:EWKehren at aol.com)  
 wrote:

Having two HP CBT's minus enclosure sitting on  my window sill, allow me to 

ad my two cents worth. Looking at the  assemblies I see more art than  

science and duplicating something like that  would most likely end in  
failure. 

Comparing that to the previous H Maser  discussions the collective know  
how 

and resources of the list could maybe result  in a  Maser.

As to refurbishing tubes some of us have  discussed that off line and  in  

my opinion with proper tools and equipment  cleaning and replacing  cesium 
is 

doable. Cynics say the manufacturers of tubes  could do it but rather  sell 

only new ones since they are the only source  of the much more expensive 
new  

tube. I think that is only half the  story.

The reason in my opinion why refurbishing the  tube is commercially not  

viable is you have to ask: when done what do  you really have? You have not 

eliminated some of the failure modes, in the  case of the 5071 the data in 
the  

EPROM does not necessarily reflect the tube  and who could say how long the 

tube  would last? A crap  shoot.

I have a HP 5061 B and a HP 5062 C but  eventually want to replace them  

totally with a Tbolt-Rb-HP 10811 combination  using two digital loops that 
are  

tailored to  the devices. I am now  focusing  on the thermal management  in 

order to get maximum performance. There is  work going on by some members 
of  

the list to develop more sophisticated digital  loops. Lets face it, with 
GPS 

properly  used, having a Cesium Standard  will give you the warm feeling  

that you have a primary  standard.

By the way that is why I repeatedly have asked  the list if there is any  

long term Tbolt data out there comparing the 1  PPS or the 10 MHz with a  
Maser.

I hope this is worth two  cents.

Bert Kehren

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