10 MHz to 32.768 kHz converter
every injection locked oscillator has jitter, since the oscillator
slowly --depend on the "tank circuit"s Q will return to it's original
frequency, what will be interrupted by the next injection, therefore the
oscillator will have a larger phase jump during the interaction with the
pulse.
"how fare the oscillator could get" is depend on the "tank circuit"-'s
Q, therefor the jitter in worst case could be a whole period of the
oscillator, which is 1/100kHz = 10usec
in case the locking pulse does not let the oscillator go to fare away
the jitter will be less, but the jitter will be always a dependence of
the Q of the oscillator's "tank circuit in this case the crystal, also
the length of the synchronizing pulse -- the energy of the pulse is
dependon it's length -- will influence the phase of the synchronized
oscillator.
there was a very good paper written on injection locking: R. Adler, “A
study of locking phenomena in oscillators,”Proc. IEEE, vol. 61, no. 10,
pp. 1380–1385, Oct. 1973
73
KL6UHN
Alex
On 3/21/2016 6:00 AM, Attila Kinali wrote:
On Sun, 20 Mar 2016 18:26:16 -0000
"Martyn Smith" martyn@ptsyst.com wrote:
I have a real time clock calendar chip that requires a 32.768 kHz crystal.
I want to feed it with 10 MHz signal instead, so it is synchronised to my
main 10 MHz in a frequency standard I am designing.
Currently, all that has been discussed were digital solutions.
But what about using an analog approach?
If you have a 32kHz crystal oscillator, you can injection lock it
to the 10MHz signal, by dividing the 10MHz down to 128Hz, then use this
to form short (as in a couple of ns) pulses, which you then couple
to the crystal using a small (a couple of pF) capacitor.
Given that the crytsal has an accuracy of better than 100ppm, then
even a very weak coupling at 128Hz should be enough to keep it locked.
Upper bound on the jitter is 1/128Hz*100ppm=781ps (very simplified
calculation, but it should be definitly less than 1-2ns)
Attila Kinali
On Mon, 21 Mar 2016 11:31:23 -0700
Alex Pummer alex@pcscons.com wrote:
On 3/21/2016 6:00 AM, Attila Kinali wrote:
Given that the crytsal has an accuracy of better than 100ppm, then
even a very weak coupling at 128Hz should be enough to keep it locked.
Upper bound on the jitter is 1/128Hz*100ppm=781ps (very simplified
calculation, but it should be definitly less than 1-2ns)
there was a very good paper written on injection locking: R. Adler, “A
study of locking phenomena in oscillators,”Proc. IEEE, vol. 61, no. 10,
pp. 1380–1385, Oct. 1973
Actually, if you want to calculate the jitter of an injection locked
oscillator, then the publications from Kurokawa from the 70s and 80s
are more approriate than Alders paper form '45 (the '73 version is
just an unmodified reprint of the original paper).
The value i gave with <2ns is a worst case bound on the jitter,
under the assumption that the oscialltor Q is low and it will
imediatly switch back to its original frequency once the phase
shift induced by the pulse is over. In reality the phase jump
should be much less as the Q acts as an integrator and thus
averages the phase jumps out. As modern 32kHz crystals have a Q in
the 10k-100k range, you can assume that the "averaging" time of
the crystal will be in the 0.3-3s range. I didn't orignally
include this complication in the mail, because I cannot reproduce
the formulas from the top of my head and the parameters of these
formulas are not always easy to extract. Hence the above worst
case bound for the jitter, which is easy to see and good enough
for this kind of application.
Attila Kinali
--
Reading can seriously damage your ignorance.
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