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From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: BAW
Date: Sat, 25 Jan 2025 18:29:09 +0000
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On 22/01/2025 18:40, John R Walliker wrote:
> 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
> 
The piezoelectric resonator layer is aluminium nitride according
to a video on the TI website.
John