Chris,

Thanks for sharing the plot.

What instrument are you using to make the measurements? And what frequency reference is being used? Is this a free-running PRS10, or is it locked to GPSDO with some time constant?

Is that same GPSDO being used as a reference for your measurements? The plateau looks very much like GPSDO behavior and the slide into e-14 and e-15 territory looks like you're measuring a PLL.

/tvb

----- Original Message -----
From: "Chris Burford" CBURFORD1@austin.rr.com
To: time-nuts@lists.febo.com
Sent: Sunday, October 14, 2018 7:07 PM
Subject: [time-nuts] PRS10 MDEV

On Sun, October 14, 2018 9:07 pm, Chris Burford wrote:

Frequency stability measurements must always compare one device against
another.
I do not recall that you mentioned what is providing the reference
frequency for your measurement.  Is it possible that your reference signal
has some odd behavior that falls in that time range?  It reminds me of the
behavior of a PLL crossing over between the noise behavior of the local
oscillator and the noise of the reference signal.  Are you by chance using
a GPS disciplined oscillator for reference, which would have effectively a
PLL which crosses over noise behavior from the local oscillator at short
time scales to the GPS receiver behavior at long time scales?
For that matter a rubidium standard would have an OCXO providing the
output frequency, but it would be locked to the rubidium transitions at
long time scales, so even with a perfect reference signal you may still
see PLL crossover behavior in the output of a rubidium standard.

--
Chris Caudle

Hi Chris,

On 10/15/18 4:07 AM, Chris Burford wrote:

Actually, that is where you start to see real stability data.

Look to the left of the plot, all 4 traces have very close traces from
tau 1 s to about tau of 100 s. This is becuase of your measurement
instrument noise, which falls with 1/tau^1.5 with MDEV and 1/tau with
ADEV. This is just the measurement resolution obscuring your real device
performance. It is only as this becomes lower than the actual DUT noise
that you see the actual DUT noise. The cause of the plataue then becomes
somewhat obscured as you don't see the full picture.

Another aspect is that the traces is on the short side for confidence
intervals of the measurements have gone down, as the truncated series in
itself can cause variations of the estimated value around the actual value.

One effect that can create similar behavior is a systematic signal
either as a side-band or as a modulation such as a sine that cause a
additive effect on top of the noise performance. This type of systematic
disturbance may be an effect in the measurement setup or in the device,
so care must be take to locate and eliminate it as it does not represent
the systematic noise of the source.

There is a few other effects that can cause similar behavior.

It is recommended to monitor the phase deviation data plot. Whenever I
see deviations like that I ask for the phase plot, which typically also
get the frequency compenated out and only look at the residues of that
prediction. It may also be convenient to look at the FFT of the data.

ADEV and MDEV is a poor way to estimate stability that has systematic or
semi-systematic behaviours, it's only intended for truely random noise
of the noise-slope types. Other noise-types should be identified by
other means and compensated out prior to ADEV/MDEV processing.

While that is a good reading, it doesn't go in depth onto how to
interpret the plots properly. Some have been written by David Howe as
found in NIST T&F publication list.

Cheers,
Magnus