[time-nuts] SITime oscillators

[Bob Camp]

Hi

Some of the MEMS parts have resonators with multiple peaks in them. It's a function of how they design the resonator it's self. They can have "interesting" phase noise plots even without the fractional-N PLL. 

There is a world of applications out there that simply do not care about phase noise below some magic limit. It can be 12 KHz, it may higher than that. Some of those applications use a *lot* of oscillators ….

Bob

On Aug 6, 2013, at 4:41 PM, Attila Kinali <attila at kinali.ch> wrote:

> 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.
> 
> -- 
> 1.) Write everything down.
> 2.) Reduce to the essential.
> 3.) Stop and question.
> 		-- The Habits of Highly Boring People, Chris Sauve
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