On 12/9/15 4:37 AM, Bob Camp wrote:
Hi
On Dec 8, 2015, at 11:20 PM, Jim Lux <jim...@earthlink.net> wrote:
On 12/8/15 3:31 PM, Bob Camp wrote:
Hi
Let’s see:
EFC uses reference out of the OCXO.
EFC comes on the OCXO at no added cost.
16 bit DAC costs ~$2 to $5
Total cost for EFC setup $2 to $5. Net result is a system with
spurs that are how ever far down you wish them to be. (It’s all
about grounding in this case).
Bob
If the OCXO has steering, the Q of the resonator has to be lower than if the
OCXO wasn't steerable.
If the OCXO has an oscillator attached to the crystal, it has a lower Q than
the crystal it’s self…..
The contribution of a “normal” (relatively narrow band) tuning circuit is
actually quite small.
for very high performance oscillators, I'm not sure about that. Here,
I'm thinking about things like USOs where the crystal is in a double
vacuum bottle with multiple heat shields, etc.
There's been several proposals from JHU/APL where a good oscillator is
teamed with a high performance DDS so you don't have to get a crystal at
the *exact* frequency you need. A great idea in my opinion
(historically, the crystal frequency is tied to the channel allocation
for your spacecraft, and non-adjustable frequency makes using spare
oscillators from one mission for another one hard)
As good an idea as this is, it seems that (very risk averse) folks seem
to stick with the "make lots of oscillators and pick the closest one to
the desired frequency after initial aging".
The recent GRAIL mission that measured the moon's gravity used two USOs,
one on each spacecraft, with the frequencies slightly different (so
they're used as both Tx source, and LO for Rx for the signal from the
other spacecraft).
A high quality DDS USO would have made this easier in many ways (you
could cherry pick from the dozen or so oscillators for aging and phase
noise properties, rather than also frequency)
So conceivably (if such things were available) you could get a non-steerable
OCXO with better (very) close in noise.
Except when you actually wire up that circuit that’s not the outcome.
And then move the frequency with the DDS. It's fairly straightforward to make
a DDS circuit that pushes the spurs and such away from the carrier (at the
expense of higher noise farther out).
Which gets you into a variety of spur and noise issues if you want those spurs
to be below the noise floor of a good OCXO. Getting them into the -130 to -150
db down range is far from trivial even
with the spreading stuff.
But hey, that's brand new and exotic.
And it pushes the spurs out to where the noise floor should be -170 or -180 …
hmmm ….
But there are applications where far out noise isn't as important, for
instance, in a deep space transponder used for ranging. The transponder
is basically a phase locked loop with a very narrow loop bandwidth (a
few Hz). And the receiver on the ground is also very narrow band, so
noise that's say, 10 kHz away, isn't a big deal, compared to noise
within a few Hz, which is.
(ADEV of 4E-16 at tau of 1000 seconds is a typical state of the art
requirement)
Bob
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