[time-nuts] The 5MHz Sweet Spot

Bob Camp lists at rtty.us
Sun Nov 3 14:15:21 EST 2013


Hi

Given that a full blown electroless BVA crystal typically  costs > $900 a piece, you are unlikely to find them in sub-$400 oscillators. There is pretty good evidence that they also can have jumps that need to be screened out….

Testing wise, looking at any part with a 2 to 5 ppt ADEV for sub 1.0 ppt max limit issues it not going to be very practical. You would simply reject all the product. The practical testing limits are going to be very much constrained  by the same things (wander / random walk / flicker noise)  that drive ADEV. If you want a 1 ppt test (resolution / limit / fit or what ever), then your ADEV probably should be 0.1 ppt or less. That’s a spec that will cost you some extra money. 

Bob

On Nov 3, 2013, at 12:33 PM, Said Jackson <saidjack at aol.com> wrote:

> Bob, et. al.,
> 
> Lots of opinions in this discussion, but none of it discusses the elephant in the room affecting todays' vendors:
> 
> Random crystal instability versus manufacturing techniques.
> 
> I can buy oscillators from multiple vendors that have -115dBc at 1Hz or better and noise floors of -182dBc. That technology is well understood and has been mature for a very long time and to me its boring. Recently Ulrich Rhode even had a great article in the Microwave Journal detailing how exactly to build one of those units.
> 
> But what does it help me to have -115dBc if the darn thing jumps 50ppt every two to three days??
> 
> Crystal jumps are the biggest menace facing users of crystals/oscillators today and so far I have never been given a reasonable explanation from any of the vendors out there what causes it and how to avoid it or how they plan to address it.
> 
> In fact no vendor we know tests for it to levels of sub-ppt over days which is what is necessary for any disciplined application as disciplining will clearly show even the smallest crystal jumps. Almost every vendor will do a frequency test only, where a phase test would be needed.
> 
> Users of crystals/oscillators are left with doing an exhaustive yield test during burn-in to find bad crystals. We test our boards for 3 days and more to weed out jumpy crystals, and its a pain and very expensive to have to do this on finished goods as rework is in order for units that fail.
> 
> The results are staggering, some vendors consistently have jumpy product, others consistently have excellent product, all have at least occasional batches that are worse to far worse than standard deviation. Some are so bad that one batch may yield 95% and the next batch of the same exact product will only yield 50% or less!
> 
> I think this is the area of Quartz processing that has the least amount of research invested into it, and as anyone that has seen their Z38xx unit jump up and down in phase can attest to its a menace and can ruin one's day. I wish there were something besides yield testing that can be done to avoid manufacturing and shipping bad crystals to integrators. BVA seems to be one of those solutions, but how many BVA's have we seen in products that cost $400 retail??
> 
> Bye,
> Said
> 
> On Nov 3, 2013, at 5:09, Bob Camp <lists at rtty.us> wrote:
> 
>> Hi
>> 
>> Rubiola is looking at resonators he can buy off the shelf. They are constrained by the commonly available packages. The Q x F product does not suddenly stop going up at 5 MHz. There is good documentation that it keeps on going as the frequency goes down. Is Q everything - of course not. However *if* you wanted a much higher Q crystal than the 5 MHz, it could be built if you had the time and the money.
>> 
>> Bob
>> 
>> On Nov 3, 2013, at 7:58 AM, GandalfG8 at aol.com wrote:
>> 
>>> Hi Bob
>>> 
>>> I've seen this topic discussed here before and it does seem to raise some  
>>> quite strong emotions and there does seem to be some confusion.
>>> 
>>> I can remember quite clearly, historically at least,  5MHz being commonly 
>>> promoted as the optimum frequency for crystal  oscillators on the basis that 
>>> 5MHz crystals had this "sweet spot",  presumably when it came to temperature 
>>> stability.
>>> An online search today hasn't been very forthcoming and unfortunately  most 
>>> of my reference library remains in storage, so I can't  provide instant 
>>> references, but I know I've also seen this in print  and can still envisage the 
>>> graphs used to demonstrate it.
>>> 
>>> The nearest to a current reference I've found is this comment from Enrico  
>>> Rubiola on page 156 of "Phase Noise and Frequency Stability in  
>>> Oscillators".........
>>> "The process described relies on the ability to estimate the resonator's  
>>> quality factor. Experience indicates that the product voQ is a technical  
>>> constant for piezoelectric quartz resonators, in the range from 1 x 10^11 to 2 
>>> x  10^13. As a matter of fact, the highest values are found in 5MHz  
>>> resonators.  ............."
>>> 
>>> I have no axe to grind on this, aside from willingly admitting to  being 
>>> one of the confused:-), but if this is a misconception is it possible to  
>>> address why such a situation might have arisen?
>>> 
>>> Regards
>>> 
>>> Nigel
>>> GM8PZR
>>> 
>>> 
>>> 
>>> In a message dated 03/11/2013 01:46:01 GMT Standard Time, lists at rtty.us  
>>> writes:
>>> 
>>> Hi
>>> 
>>> I believe that you are talking to two very different  groups, one who 
>>> actually design the crystals and the other who use the  products that are 
>>> designed. One is talking about what they can buy, the other  is talking about what 
>>> could / could not be done and why.
>>> 
>>> Bob
>>> 
>>> On  Nov 2, 2013, at 8:22 PM, Tom Knox <actast at hotmail.com>  wrote:
>>> 
>>>> From reading your past posts I must respect your opinion.  Your are 
>>> obviously extremely educated on the subject. So why is there some  disagreement 
>>> in two very knowledgeable groups? 
>>>> 
>>>> Thomas  Knox
>>>> 
>>>> 
>>>> 
>>>>> From: lists at rtty.us
>>>>> Date: Sat, 2 Nov 2013 18:44:18 -0400
>>>>> To:  time-nuts at febo.com
>>>>> Subject: Re: [time-nuts] The 5MHz Sweet  Spot
>>>>> 
>>>>> Hi
>>>>> 
>>>>> The only thing that  the 5 MHz 3rd crystal is “optimum” for is a holder 
>>> that will accept a 0.55”  max diameter blank.
>>>>> 
>>>>> Bpb
>>>>> 
>>>>> On  Nov 2, 2013, at 6:06 PM, Tom Knox <actast at hotmail.com>  wrote:
>>>>> 
>>>>>> Mike I think we must be talking to the  same smart people. I think 5MHz 
>>> was adopted over 3MHz simply because 5MHz  multiplies to other commonly 
>>> used frequencies with greater ease. I think the  top frequency standards have 
>>> evolved to 5MHz Third Overtone SC cut crystals  for a reason. The evolution 
>>> has gone on from the days of Tesla and improved  greatly during the glory 
>>> days of Quartz in the years leading up to atomic  standards with countless 
>>> hours of experimentation and research. Those lessons  learned are constantly 
>>> examined through the lens of the latest  science.   I may be wrong, but I have 
>>> not heard of any extreme  design prototype quartz oscillator with superior 
>>> Phase Noise and Stability.  Our house standard F1 a cesium fountain is used 
>>> roughly one month every few  months to characterize roughly 12 5071A cesium 
>>> standards steering about 5 MHM  2010 cleaned up with a number of 8607 option 
>>> 08 oscillator. (The equipment  choices are not a recommendation or 
>>> endorsement, and there are  po
>>>>>> ssibly m
>>>>>> any product that could meet or  exceed the performance of these fine 
>>> products.) But the oscillators selected  are 5MHz third Overtone SC cut. 
>>>>>> 
>>>>>> Thomas  Knox
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>> From: mfeher at eozinc.com
>>>>>>> To: n1hac at alum.dartmouth.org;  time-nuts at febo.com
>>>>>>> Date: Sat, 2 Nov 2013 09:50:50  -0400
>>>>>>> Subject: Re: [time-nuts] The 5MHz Sweet  Spot
>>>>>>> 
>>>>>>> Exactly - I mentioned this on  here about 3 years ago and all of the
>>>>>>> self-proclaimed  geniuses poo-pooded it.  I was told early in my  
>>> engineering
>>>>>>> career in the early 70's,  by a very  smart man, when I thought I had 
>>> all of
>>>>>>> the answers, that  considering all of the trade-offs regarding 
>>> performance,
>>>>>>> around 3 MHz for a crystal is best, operating in the 3rd overtone  
>>> mode,
>>>>>>> hence the slow progression from the 1 and 5 MHz  standards to 10 MHz. 
>>> Now,
>>>>>>> getting close to 70, I just see  what I can learn from all the "smart" 
>>> people
>>>>>>> on here, and  keep quiet most of the time. Regards - Mike
>>>>>>> 
>>>>>>> Mike B. Feher, EOZ Inc.
>>>>>>> 89 Arnold  Blvd.
>>>>>>> Howell, NJ, 07731
>>>>>>> 732-886-5960  office
>>>>>>> 908-902-3831 cell
>>>>>>> 
>>>>>>> -----Original Message-----
>>>>>>> From:  time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]  
>>> On
>>>>>>> Behalf Of David McGaw
>>>>>>> Sent:  Saturday, November 02, 2013 1:30 AM
>>>>>>> To: Discussion of  precise time and frequency measurement
>>>>>>> Subject: Re:  [time-nuts] The 5MHz Sweet Spot
>>>>>>> 
>>>>>>> This  all seems to be forgetting that the crystals are usually 
>>> operated  at
>>>>>>> 3rd or 5th harmonic.  The crystal in a 10811A is  10 MHz/3rd overtone. 
>>> A
>>>>>>> high quality 5 MHz/5th  overtone crystal is really a 1 MHz 
>>> fundamental, a
>>>>>>> large  piece of quartz.  Running at a harmonic greatly reduces the  
>>> influence
>>>>>>> of the package.
>>>>>>> 
>>>>>>> David
>>>>>>> 
>>>>>>> 
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