Re: BAW

On 22/01/2025 09:36, Jeroen Belleman wrote:

On 1/22/25 09:38, John R Walliker wrote:

On 22/01/2025 04:30, Bill Sloman wrote:

On 22/01/2025 10:52 am, Buzz McCool wrote:

On 1/21/2025 10:56 AM, john larkin wrote:

>
This has got to crush the XO business. It looks like it's a single
chip that's encapsulated like any other IC. XOs are complex and need
hermetic cans.

>
XOs used to use handmounted crystals. There's nothing all that complicated about them, but low volume manufacture tends to be a bit knife and fork.
>

Looking through this BAW app note
https://www.ti.com/lit/an/snaa362/snaa362.pdf, I couldn't tell BAW devices would work at high altitude or in a vacuum. I have to ask because the "acoustic" in the name makes me want to make sure that some sort of atmosphere isn't needed.

>
As far as I can see these are surface acoustic wave devices. Integrated circuits are pretty well encapsulated. This one probably dumps any heat it generates into the board it is mounted on, which might run a bit warmer in a vacuum, but that would be the only risk I can see.
>

>
It does look more like bulk acoustic waves - hence the name!  There
is no cavity inside, so they should be immune to the effects of
helium, unlike the  oscillators that use MEMS resonators where it
appears that helium can diffuse through the silicon to clog up
the vacuum cavity.
Phase noise performance looks excellent as is the temperature
stability.
John
>

 You can look up the patent:
<https://patents.google.com/patent/US20070285191A1/en>.
 Apparently the resonator is built up layer by layer, first
the multi-layer Bragg reflector, then the bottom resonator
electrode, the resonator material itself, and finally the
top electrode. The patent does not mention a top reflector,
so such a resonator would need a bit of free space above.
 

The data sheet does however:
TI’s BAW resonator technology uses piezoelectric transduction
to generate high-Q resonance at 2.5 GHz. The
resonator is defined by the quadrilateral area overlaid by
top and bottom electrodes. Alternating high- and
low-acoustic impedance layers form acoustic mirrors beneath
the resonant body to prevent acoustic energy
leakage into the substrate. Furthermore, these acoustic mirrors
are also placed on top of the resonator stack
to protect the device from contamination and minimize energy
leakage into the package materials. This unique
dual-Bragg acoustic resonator (DBAR) allows efficient excitation
without the need of costly vacuum cavities
around the resonator. As a result, TI’s BAW resonator is
immune to frequency drift caused by absorption of
surface contaminants and can be directly placed in a non-hermetic
plastic package with the oscillator IC in small
standard oscillator footprints
John

The patent text is voluntarily vague about the materials
used for the reflector and resonator layers. They mention
lots of examples, without clearly saying what they really
used. I'd expect that most examples don't work.
 Jeroen Belleman