[time-nuts] Help understanding an ADEV

Bob Stewart bob at evoria.net
Thu Sep 11 15:01:22 EDT 2014

Here are a couple of reference plots.  The only thing really important is the voltage trace in red.  It's in 10s of microvolts.

The first one is just measuring the EFC at the OCXO which is only being driven by the VRef output of the OCXO.  I think it clears the 3456A of any wrongdoing.


The second one tests the noise from the op-amp by supplying its input with 3.3V from the PIC's VDD.  The PIC's DAC output is disconnected for this plot.  There's a few more tens of microvolts of noise than in the one above, but that's to be expected, I think.  It's certainly nowhere near the noise on the other two plots driven by the DAC.



 From: Bob Stewart <bob at evoria.net>
To: Time Nuts <time-nuts at febo.com> 
Sent: Wednesday, September 10, 2014 10:06 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.


Bob - AE6RV
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