[time-nuts] SITime oscillators

[Attila Kinali]

On Tue, 6 Aug 2013 18:54:41 +0200 (CEST)
Marek Peca <marek at duch.cz> wrote:

> However, there was a good introductory presentation of SiTime founder 
> Aaron Partridge two weeks ago at IFCS-EFTF conference, describing lots of 
> their MEMS' internals. Basically, the fine frequency tuning and 
> temperature independence is all done using frac-N PLL, implemented 
> completely within ordinary non-MEMS CMOS chip. So, the MEMS osc output is 
> not directly led out. The MEMS is there, as far as I got it, to maintain 
> long-term stability.

Yes, and that pretty much sums it up as well:
The MEMS oscillators are for applications where you need good long term
stability, can live with some temperature dependence (although i must
say the no-tempco parts from SiTime are pretty impressive*) and don't
care much about phase noise.

The biggest effect on the phase noise comes from the frac-N
synthesiser that is present in all parts. It generates a lot of
spikes in the spectrum that can be pretty bad for radio applications.
That is probably also the main reason why the phase noise plots
stop at 1kHz: they don't want to show those messy parts.

I guess the main applications for those MEMS parts are low footprint,
low power uC devices that need a precise frequency but care little
about superior phase noise. But then, current quartz oscillators
still beat them at power consumption.

One main advantage of those MEMS is that you can get odd frequencies
pretty quick, as it just means to reprogramm the PLL and you're done.
But then, Epson (and for sure others as well) had similar devices based
on quartz for ages.

Oh, and i would take the long term aging numbers with a grain of salt.
The company has been around for roughly 10 years and according to
wikipedia released their first production oscillator in 2006.


			Attila Kinali

* They managed to build a MEMS oscillator with IIRC <30ppm temperature
dependence, prior to temperature compensation, and over the whole
temperature range.

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