[time-nuts] Beginner's time reference

Fri Dec 11 13:10:20 UTC 2009

Bill,

he was planning to spend 100 USD on a used Rubidium, I think a Cs is way 
over his budget ;-)

But I agree that a Rubidium has a shorter life span and if you get one 
for 100 USD on eBay, I would expect it to be "worn out" already.

I would not spend 100 USD on a used oscillator, Rubidium or not, if you 
expect to get a -stable- (not -accurate-, full ack) reference.

Regards,
Heiko

WB6BNQ schrieb:
> Charl,
>
> If it is autonomous operation you want, then your only choice is a Cesium Beam
> frequency reference.  By definition it is absolute !  But that does not address
> resolution.  For instance, if the Cs is old tube and noisy, while it may be
> accurate, due to the noise you may not be able to resolve any better then, say, 1
> part in 10-9 (not good).  If you need more absolute resolution, then your only
> choice would be to have a new tube put in or buy another one.
>
> However, all this says nothing about accurate time, because there is no such
> thing !  Accurate time only means something with respect to a recognized
> reference point.  In this case USNO for the USA or another county’s primary lab.
>
> So, if you mean you want accurate time in addition to frequency you will need to
> have a method of comparing or setting your time device to a selected external
> time reference.  This is where the GPS comes into the picture.  It is the medium
> by which you can track and adjust your local time device to an external
> reference, USNO (for GPS), within a given time resolution (down to nanoseconds or
> better).
>
> Because time and frequency are related, having a time reference also gives you a
> frequency reference.  Having a very good local oscillator for the short term
> controlled by the GPS for the long term tracking is a good choice and relatively
> inexpensive compromise between having nothing and spending a lot of money for
> Cesium standard that may only last between 5 and 10 years if new.  Purchasing a
> used Cs is a crap shot, may be good, but could be bad.
>
> While a Rubidium oscillator looks good at first, you need to consider they are
> not a primary reference because they drift, albeit generally slower then a Quartz
> oscillator.  A really good Quartz oscillator will outperform the eBay run of the
> mill Rubidium for short term measurements.  A good Rubidium will prevail for
> medium term measurements and of course provide a much slower drift when in an out
> of lock condition (called holdover) with the GPS tracking.
>
> Another point to consider is the Rubidium has a short lifetime compared to a
> Quartz oscillator.  Things do fail, but the Quartz oscillator likes to have
> continuous operation and, actually, generally improves because of it.  There are
> many Quartz oscillators that are still ticking after 30-40-50 years !  The
> Rubidium, on the other hand, wears out faster when it is turned on with a life
> span of 5 to 10 years if new.
>
> So I would suggest doing some research like reading through the Timenut archives,
> reading various publications available from NIST off the web, perusing TVB’s web
> site at www.leapsecond.com and KO4BB’s site www.ko4bb.com under the timing and
> manuals subpages.  There are a variety of manuals on frequency and time
> equipment.  You should be able to find some hp app-notes on the subject as well.
> There are other worthwhile sites too.
>
> It doesn’t hurt to have a mix of items.  For instance a good oscillator (or two
> or three) tracked by GPS, a Rubidium (or two) for good measure and after you get
> your feet wet then decide if you really want to spend the money necessary to own,
> operate and maintain a Cesium.  Oh, by the way, unless you are damn rich, the
> wife will never agree to the Cesium when she hears the price.
>
> Bill....WB6BNQ
>
>
> Charl wrote:
>
>   
>> Dear Hal and others,
>>
>> Thank you for your suggestions. I suppose I should have emphasised that I
>> was looking to have my own reference, i.e. something not dependent on GPS,
>> LORAN, or other signals from the aether. Indeed I might get better accuracy
>> for less money by tapping the GPS time signal, but to me that's not as much
>> fun as building my own atomic reference.
>> Hal, is there much difference in quality for these rubidium tubes? Will I
>> get what I pay for? In any case, you make a good point about measuring the
>> accuracy. I'm a university student, so perhaps I can pay the physics
>> department a visit. Otherwise, maybe I could compare it to the GPS signal?
>>
>> Kind regards,
>> Charl
>>
>> On Fri, Dec 11, 2009 at 9:30 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>     
>>>> I'm hoping to build my own circuit around the device, which might cut
>>>> down the costs somewhat. Some rubidium sources on eBay go for less
>>>> than $100, but I'm not sure what quality to expect. Any advice or
>>>> suggestions are appreciated!
>>>>         
>>> Short answer:  Sure, get one of the $100 rubidium boxes and see what you
>>> can
>>> do with it.
>>>
>>> Long answer:  You just stuck you toe into a huge tar pit.  Pick a corner
>>> that
>>> seems like fun and dive in.
>>>
>>> How are you planning to measure if whatever you build is any good?
>>>
>>>
>>>
>>> --
>>> These are my opinions, not necessarily my employer's.  I hate spam.
>>>
>>>
>>>
>>>
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>
>
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