[time-nuts] sand9 TCMO

[Attila Kinali]

On Mon, 06 Jan 2014 06:24:18 -0800
Jim Lux <jimlux at earthlink.net> 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).

But advanced oscillator structures make it possible to have oscillators
which show very low temperature dependence (IIRC <5ppm over -30°C to +80°C)
without any electronic temperature compensation applied (yet).
(done by SiTime. Sorry, don't have any exact reference)

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.

				Attila Kinali

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