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

Bob Camp kb8tq at n1k.org
Wed Dec 9 07:37:48 EST 2015


> 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. 

> 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 …. 


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