[time-nuts] Bye-Bye Crystals
j99harman at gmail.com
Tue Mar 14 16:58:57 EDT 2017
For other common crystal frequencies, let's not forget
3.579545 MHz and 4x that - NTSC TV color burst
and others listed here...
On Tue, Mar 14, 2017 at 3:39 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Tue, 14 Mar 2017 13:39:02 +0100
> Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> > Some claims that MEMS will kill crystals. It will surely eat a good
> > market share, but I think there is applications where MEMS is not mature
> > enough compared to crystals.
> MEMS is quite mature, it's just that it is playing a different game.
> While with quartz (and other piezoelectric crystals) we know how
> to design a crystal to frequency, things aren't so simple for MEMS.
> Simply scaling the design doesn't work apparently.
> What they instead do is to use the MEMS oscillator as a reference
> for a PLL locked VCO. As the whole thing is going to be a few mm^2
> of silicon anyways, reserving some µm^2 for the PLL and VCO don't
> cost much. And it gives the ability to "tune" the oscillator
> for the frequency needed after production (the same technique is used
> with "programmable" crystal oscillators). Of course, having a PLL,
> mostly a fractional-N PLL, causes a lot of spurs in the output,
> which can cause problems, depending on the application.
> The big promise of MEMS oscillators was, that they'd be cheaper (due to
> integration in silicon) and used less power. As far as I am aware,
> neither promise could be upheld. MEMS need a quite different production
> process than normal digital electronics, hence it's usually more economic
> to have the oscillator on a different die than the digital chip. As for
> power consumption, the low power MEMS are about at the same level as the
> low power 32kHz crystal oscillators (and also in the same frequency).
> One place where MEMS are exceedingly good is temperature characteristics.
> Silabs demonstrated an oscillator, which, prior to any compensation,
> exhibited only <5ppm shift over the full temperature range.
> As for the demise of single quartz crystal units, I think that is not
> going to happen any soon. It is rather that the economics shift. Most
> of the single crystals are used as reference oscillators for digital
> and analog/RF chips. Ie most these chips have an internal oscillator
> that uses an external crystal to drive their internall VCO+PLL.
> As the crystal frequency is dictated by the frequencies these chips
> have to generate, there is a kind of standardization going on due to
> the limited number of protocols that need special frequencies. Two very
> common frequencies are 12MHz, for USB, and 25MHz, for Ethernet.
> 16MHz is base for CAN, some Wifi chipsets and USB as well. Then there
> are a couple of frequencies that are related to GSM, UMTS and the various
> other telephone standards. There are maybe a handfull of these frequencies,
> which "everyone" needs (ie are used in many high volume products). These
> the crystals we will be able around for the forseeable future. There are
> other frequencies that are less used, which you will still get, but need
> to pay more or are made to order. Frequencies for protocols that are
> not used much anymore, or can be easily generated from another frequency
> that is more common, are bound to die out (as has happend with all those
> UART crystals, which are only used in legacy systems or for historical
> For specialized applications, where the crystal is not directly interfaced
> to a chip that provides the oscillator, it is more convenient for the
> designer to just use a complete oscillator than to design his own
> with all the problems that it involves. Getting such a device reliable to
> work in production volumes is nothing an average engineer without prior
> experience in can just pull off. Heck, I design my stuff to use oscialltors
> instead of crystals, because that's one thing less I have to care about.
> But even with these oscillators, there is only a limited number of
> that are easy to get. Those are again the standard frequencies from above,
> and a couple of round numbers (like multiples of 10MHz)
> Attila Kinali
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