[time-nuts] Help understanding an ADEV

[Tom Van Baak]

Hi Bob S,

Since you're using DAC voltage as an input, did you calculate ADEV using the "y" equation (single difference) instead of the usual "x" equation (double difference)?

Looking at the red dots, there is significant measurement quantization (1 unit, or 10 uV) and a massive amount of noise (close to 50 units, half a millivolt). Is this due to your voltmeter, your measurement setup, or is the DAC really that terrible?

In your case, an ADEV plot made from a phase or frequency measurement should match an ADEV plot made from a DAC measurement. However, the fidelity of the plot is highly dependent on the resolution of the instrument being used. Typically you choose between a phase meter, frequency counter, or voltmeter by which one gives you the best resolution. Otherwise your ADEV plot is just showing the limitations of your instrument, not the GPSDO that you're trying to measure.

/tvb

----- Original Message ----- 
From: "Bob Stewart" <bob at evoria.net>
To: "Time Nuts" <time-nuts at febo.com>
Sent: Wednesday, September 10, 2014 8:06 PM
Subject: [time-nuts] Help understanding an ADEV


> 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