On 06/01/14 20:13, Attila Kinali wrote:
On Mon, 06 Jan 2014 06:24:18 -0800
Jim Lux <[email protected]> wrote:
MEMS might be good for certain tasks, but for closer in noise I've only
seen some progress recently, but not measured it myself. Close-in noise
seems to have been pretty bad for all MEMS so far.
I think that's probably related to the physically small size. It's hard
to get a high Q in something that's smaller than a gnat's eyelash.
It's not only the Q (although i have not seen any Q values yet), but
also that (almost?) all of those MEMS oscillators have a fixed frequency
oscillator structure and use a fractional-N PLL together with a standard
CMOS VCO. The spurs of the PLL are clearly visible if you go down to 1kHz
(i haven't seen any spectrums going further down than 1kHz for MEMS
oscillators).
Well, you want to make sure you do not have multiple resonant modes
oscillating at the same time. The Q of each of those modes will
naturally be of interest. In quartz oscillators you kill of nearby modes
to avoid oscillating in modes with unwanted behaviour, such as high TC
dependence. You can modify the oscillator circuit of 10811A in order to
have it oscillate in a more temperature dependent mode, as it helps to
identify the stability of the oven. MEMS is just another acoustical
resonator, but implemented in silicon rather than silicon dioxide.
Q values of 75000 mentioned in this little article, which is early in
the MEMS oscillator industrialization:
http://www.memsjournal.com/2006/02/timing_with_mem.html
This is a good read:
http://www.ifcs-eftf2011.org/sites/ifcs-eftf2011.org/files/editor-files/Slides_Piazza.pdf
Notice that on page 46 there is a phase-noise plot with close-in noise
down to 10 Hz shown.
The big promise of MEMS oscillators of having very low power consumption
is not fullfiled yet. About half a year ago i checked all the 32kHz
MEMS oscillators i could find and got numbers for power consumption that
were about the same as an MSP430 would use, with its 32kHz crystal
oscillator running.
I think their main advantage is size rather than power-consumption.
Turns out that low power-consumption comes at a performance reduction,
that have never happen before :)
I've noticed that there now seems to be a split, depending on what needs
there is.
For good performance, I still want a good crystal, for many auxillary
oscillators MEMS can be useful, or locked to a good crystal.
Cheers,
Magnus
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