[time-nuts] Help understanding an ADEV

[Bob Camp]

Hi

The plot pretty much confirms what you already know - there is a noise spectrum and it’s not what you want to have. In a closed loop system, it’s all going to add up. The big unknown is - do you get noise out of the DAC trying to correct the OCXO’s noise or just noise out of the DAC? Is the DAC noise from the DAC it’s self or from the loop?

One source of noise is the LSB on the DAC. If you have a 1 ppm EFC and a 1x10^-12 OCXO and a 1 LSB change at 1 second (yes that’s a lot …) - That’s a 1x10^6 delta, you must have a ~20 bit DAC. If you have 1x10^-8 out of your GPS,  your loop (or filter) must have a 10,000:1 attenuation at 1 Hz. That’s 80 db. If you have a simple loop, you get 20 db / decade. That would put your tau below 0.0001 Hz.

Yes those are fast and simple numbers. The real stuff has a lot more if’s and’s and but’s in it.  

Simple way to check things:

1) Do an ADEV on the OCXO + EFC and make sure it looks right outside the loop.
2) Do an ADEV on your GPS and figure out where it’s at. 
3) Consider checking the noise in the frequency domain, it’s easier to understand.

So much fun.

Bob

On Sep 10, 2014, at 11:06 PM, Bob Stewart <bob at evoria.net> wrote:

> After spending a lot of effort trying to get some useful tuning figures for the PID in my GPSDO engine, I decided to capture the DAC output with my 3456A.  And, of course I've made an ADEV plot.  If I understand this correctly, it means that there is mostly justnoise out to 2 tau, and an area from about 4-5 tau is mostly noise, as well.  Could someone tell me if I have this right?  It matches what I'm seeing on the delta plot, and would explain why we couldn't get anything approaching reasonable stability from the OCXO, since the noise is larger than the increments being made to the DAC.
> 
> 
> I've ordered the PWM version of the PIC, and hopefully, since it's the motor control version (as opposed to the audio version) it will have much better noise performance.
> 
> The red scatter is the EFC measured at the OCXO in tens of microvolts, and the blue line is the ADEV.  I'm using a short shielded twisted-pair with mini-clipsas the probe.  Hopefully I've got things scaled properly and have run the test properly.
> 
> 
> http://evoria.net/AE6RV/TIC/DAC.wander.png
> 
> Bob - AE6RV
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