[time-nuts] 10MHz to 25MHz

Alex Pummer alex at pcscons.com
Fri Jan 20 12:57:23 EST 2017


once upon the time there was an other crystal material -- NOT quartz ! 
--  the Russian came up with it, perhaps Bernd [Neubig] remembers on 
that, what happened to that story?
that crystal could be run at higher drive level, therefore it would be 
possible to make some better oscillators....
73
KJ6UHN
Alex



On 1/20/2017 8:38 AM, Bob Camp wrote:
> Hi
>
> If you think about it, current through the crystal is at least as important as
> “drive level”. They are related by the crystal resistance. As the overtone
> goes up, the resistance (in general) goes up. There are size constrained
> designs where other things get in the way of this. There are also tricks
> that might be used to degrade the fundamental.
>
> Since the resistance is higher at the 5th than at the fundamental, 1 mw of crystal
> dissipation (drive level) is going to be less current through the crystal. At some
> (possibly a bit removed) point that gets you less current into your buffer amplifier at
> a given impedance level. Less current / same impedance gets you to worse signal
> to noise broadband.
>
> Is this really that big a deal? As always … that depends. ADEV usually degrades
> as drive goes up. Phase noise gets better. At some point this or that crystal explodes
> (the electrodes fly off). It is uncommon to get to the damage level on a crystal. You
> normally massage the design in the tradeoff region.
>
> Bob
>
>> On Jan 19, 2017, at 7:31 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
>>
>> Is there any reason why you wouldn't be able to run the same drive level on
>> say the fifth overtone versus the fundamental? I would guess at 100 MHz it
>> may be 3rd or 5th, or are they fundamental?
>>
>> The comments one drivelevel are simply based on snr, larger signal with
>> same noise, better snr
>>
>> On Thu, Jan 19, 2017 at 7:06 PM Bob Camp <kb8tq at n1k.org> wrote:
>>
>>> Hi
>>>
>>>
>>>
>>>
>>>
>>>> On Jan 19, 2017, at 3:03 PM, Scott Stobbe <scott.j.stobbe at gmail.com>
>>> wrote:
>>>
>>>> Wouldn't crystal drive level be one of the important specifications for
>>> far
>>>
>>>> out phase noise?
>>>
>>>
>>> It would, but you can get the same floor at 10 MHz as you can get at 100
>>> MHz.
>>>
>>>
>>>
>>> Bob
>>>
>>>
>>>
>>>> On Thu, Jan 19, 2017 at 1:33 PM, Bob Camp <kb8tq at n1k.org> wrote:
>>>>> HI
>>>>> A lot of your evaluation of the term “better” will depend on your
>>> intended
>>>
>>>>> use. One of the limits on phase noise
>>>>> is the thermal noise floor. Because of that, starting at a higher
>>>>> frequency will always give you an edge on broadband
>>>>> phase noise. ADEV / short term stability is linked to the Q of your
>>>>> resonator. In a quartz crystal, maximum Q is
>>>>> roughly proportional to frequency. The other limit on Q is blank
>>> geometry
>>>
>>>>> (size). One other limit is practicality -
>>>>> is a $250,000 OCXO that is 1 cubic meter in size appropriate for your
>>>>> application? The answer to that one is
>>>>> universally - NO :) Somewhere along the line of larger size and cost,
>>>>> other technologies make more sense.
>>>>> So, if better = phase noise floor, 100 MHz is better than 10 MHz. If
>>>>> better = ADEV, 5 MHz in a large package is
>>>>> likely better than 100 MHz. Indeed these are only two variables. There
>>> are
>>>
>>>>> *many* others you could look at.
>>>>> Lots of fun
>>>>> Bob
>>>>>> On Jan 19, 2017, at 7:13 AM, Charles Steinmetz <csteinmetz at yandex.com>
>>>>> wrote:
>>>>>> Chris wrote:
>>>>>>> I have always wondered why we build our "standard" with such a low
>>>>>>> frequency.   Why not a 100MHz GPSDO?   Why 10MHz
>>>>>> Quartz crystals work better at lower frequencies, predominantly because
>>>>> they have higher Q.  10MHz was chosen because it is low enough for
>>>>> excellent performance but high enough to be directly useful (since an
>>>>> accident of biology gave us ten fingers, we've created a base-10 world
>>> and
>>>
>>>>> powers of 10 are favored in almost everything).
>>>>>> In prior times, 5MHz crystals held this position, and before that,
>>>>> 1MHz.  There is a good argument even today that the best 2.5MHz or 5MHz
>>>>> crystals are better than the best 10MHz crystals, but not by enough to
>>> make
>>>
>>>>> 2.5MHz or 5MHz standards popular any longer.
>>>>>> One lonely data point, which proves nothing:  My best crystal
>>> oscillator
>>>
>>>>> is a Symmetricom clone of the double-oven HP 10811s (it came out of an
>>> HP
>>>
>>>>> GPSDO, so apparently HP at one time used them interchangeably with the
>>>>> 10811).  That OCXO uses a 5MHz crystal and a frequency doubler to
>>> produce
>>>
>>>>> its 10MHz output.
>>>>>> Best Regards,
>>>>>> Charles
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