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 <[email protected]> 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 <[email protected]> wrote:
Hi
On Jan 19, 2017, at 3:03 PM, Scott Stobbe <[email protected]>
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 <[email protected]> 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 <[email protected]>
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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