[time-nuts] Bye-Bye Crystals
attila at kinali.ch
Tue Mar 14 15:39:30 EDT 2017
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 are
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 reasons).
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 oscillator
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 frequencies
that are easy to get. Those are again the standard frequencies from above,
and a couple of round numbers (like multiples of 10MHz)
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