[time-nuts] The 5MHz Sweet Spot

[Magnus Danielson]

Hi Jim,

On 11/03/2013 05:21 AM, Jim Lux wrote:
> 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.
Myself I try to fight people doing smallest possible, but not always
thinking about performance, so #5 is important, while I would like
somewhat more #2 normally. For my hobby #1 and #2 is much more like it.
>
> 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
2x2" size is a way too large oscillator in telecom these days. DIL14 is
large and 7x5 mm is on the large side, but could be crammed in. Some
better oscillators is a little in between DIL14 and 7x5, but that's OK
since you use one per board. Things are really crammed up, and finding a
thermally quiet spot on a board is ehm... challenging, as there is
really no way to go.

CSAC may be nice, but there is an overbelief in it, just because it is
caesium. Saw that in a presentation not to long ago. But yes, there is
good use for it.
> 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.
Large volume. Whoppa... (not)
> 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.
I find that performance have been improving with each generation.
Close-in phase-noise have been terrible and is often not even shown. I
consider them useful for free-running clocks which just drives logic or
when locked up to a reference. "MEMS" by itself is a low frequency
oscillator and a CMOS oscillator which is locked to the low frequency
MEMS oscillator. Last one I saw in that regard had cleaned up their act
in several regards and started to have pretty respectable phase-noise
plots... for being a MEMS. The one sales-rep that do listen to me now
comes better prepared with phase-noise and ADEV plots as I have requested.
>   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.
Ground-based GPS-like transmitters could be a nice option to consider.
Would also provide a frequency reference for steering/compensation of
that oscillator. Either that avoids deep space ranging, or it is only
needed for verification.
> 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.
I thought the important part of the mission was to keep stable phase and
frequency? :D

Cheers,
Magnus