[time-nuts] GPSDO and oscillator steering - EFC vs DDS schemes?
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:
>> 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).
> 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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