[time-nuts] The 5MHz Sweet Spot

Jim Lux jimlux at earthlink.net
Sun Nov 3 00:21:03 EDT 2013


On 11/2/13 7:40 PM, Magnus Danielson wrote:
> On 11/03/2013 02:45 AM, Bob Camp wrote:
>> Hi
>>
>> I believe that you are talking to two very different groups, one who actually design the crystals and the other who use the products that are designed. One is talking about what they can buy, the other is talking about what could / could not be done and why.
> This is an important point. There is in fact a few different twists to this:
>
> 1) What is the best you could buy off the shelf
> 2) What is the best you can buy off the shelf
> 3) What is the best that could be built with the available tools
> 4) What is the best that could be built with sky-is-the-limit budget
> 5) What is the best that could be built, as physical size becomes smaller
>


AS someone who gets involved in buying #4s, and also interested in #5s, 
this is very true.

The appeal of the trapped mercury ion clock (aka DSAC) is that it gives 
orders of magnitude better performance than a state of the art USO with 
the rock in a dewar, with lower power and comparable 1 liter size.

When it comes to shrinking, I think there would be substantial interest 
in a "very small" oscillator with USO kinds of phase noise/ADEV, in say, 
a 50 cc/50 gram package (size of the standard 2x2" OCXO).  Not everyone 
will want to invest in DSAC


Bear in mind that "substantial interest" in the scientific space probe 
biz is a few units per year, max, with enormous lead times.  One reason 
we keep buying USOs that aren't a heck of a lot different than the ones 
of 20 years ago is that they're a known quantity.

I find those MEMS silicon ring resonators in the 3x3mm package really 
interesting, just becase they're so darn tiny.  Figure out a way to get 
really good ADEV performance in the 10-100 second sorts of tau, even if 
the frequency drifts and ages over days and months, and that's an 
interesting part for doing deep space navigation on very small 
satellites.  There's a huge problem as folks want to send cube-sats past 
GEO with "how do you know where it is" (GPS doesn't work when you get to 
lunar distances), so you need to do traditional deep space ranging of 
one sort or another.

When the entire spacecraft is a liter or two, you can't burn another 
liter on a USO, and even 100cc is a big chunk of volume.


> The last one is kind of relevant. Today packages shrink fast, and it's
> very handy and all... but what about the performance we get. We can hug
> our 5th overtone ovens all we want, but motivating their power and size
> doesn't always cut it. It's like comparing with 5 inch blanks in Bob's
> earlier post, it's more like 0,55 inch...
>
> Cheers,
> Magnus
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