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

[EWKehren at aol.com]

Some of the suggestions offered here are incorporated in the FE  
205/405/505. We have done extensive work using a 405 with very good results. Tom  did 
some tests on the 406 and we traced issues back to the temperature  control. 
We disabled it and continue to have an excellent GPSDO. Tests are on  hold 
do to my move and the realization that we need to improve our measuring  
capabilities, an ongoing process nearing completion.
This concept allows FE to use a wide range of OCXO's with no selection or  
tuning resulting in lower XTAL cost but also lower phase and AV.
Bert Kehren
 
 
 
In a message dated 12/9/2015 10:00:19 A.M. Eastern Standard Time,  
jimlux at earthlink.net writes:

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.

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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