[time-nuts] GPSDO and oscillator steering - EFC vs DDS schemes?

[Bob Camp]

Hi

> On Dec 9, 2015, at 9:34 AM, Jim Lux <jimlux at earthlink.net> wrote:
> 
> On 12/9/15 4:37 AM, Bob Camp wrote:
>> Hi
>> 
>>> On Dec 8, 2015, at 11:20 PM, Jim Lux <jimlux at 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.

Regardless of the oven technology, the Q range on the crystal is pretty much fixed by the blank diameter and the blank design. The USO crystals are no 
higher or lower Q than those used in other oscillators. The first order impact of the tuning circuit is adding a bit of resistance in series with the crystal. The
oscillator circuit does this to a much greater extent. The active components in the oscillator stage have 1/F noise issues to a much greater extent than a narrow 
bandwidth tuning circuit. Can you crank the tuning range up far enough that a noisy enough reference becomes a problem? Sure you can. Can you tune the oscillator off far enough with a mechanical 
capacitor that it gets into trouble - yup. 

What works to your advantage with a fancy oscillator is that it has a very low aging rate. You also can afford to hand select parts. The *required* tuning range for an OCXO typically 
scales with it’s performance level. A low cost TCXO may indeed need a +/-10 ppm range to tune for 20 years. A high performance OCXO may be equally “happy” with a range 
below +/- 0.05 ppm.  

> 
> 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)

…. and has been since the 1970’s when I first started talking with JPL people about this :)….

====

All that said, the real question is — can you change the fabrication of the crystal in ways that improve it’s stability by
tuning a long way with DDS rather than a short way with reactance (select parts plus varicap(s)). 

Bob

> 
> 
> 
> 
>> 
>> Bob
>> 
>>> 
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