Empathy List Archives

time-nuts@lists.febo.com

Discussion of precise time and frequency measurement

frequency stabilty question

Tom Van Baak

Tue, Aug 16, 2011 5:10 PM

This brings in another subtle but significant issue.

We talk about the ADEV being done as the standard deviation of the frequency
differences, but often that's not what's done. Even with zero dead time,
there's another bit of magic in there. Drift is removed before the samples
are used.

Bob, look at the Allan deviation formula again. A constant phase
offset is effectively removed (because of the 1st difference); a
constant frequency offset is also removed (because of the 2nd
difference). But frequency drift is not removed with ADEV. Are
you thinking of Hadamard deviation instead? With HDEV linear
frequency drift is also removed (because of the 3rd difference).

Oddly there are multiple approaches to drift removal. It comes as no
surprise that the more aggressive the drift removal, the better looking the
result. If you are looking at ADEV, it's always worth asking if (and how)
the drift was removed.

Agreed. A good example of this is the selection of an oscillator
for a GPSDO. Strictly speaking a drift-removed ADEV or HDEV
is a better metric to use when selecting an OCXO in this case.

/tvb

> Hi > > This brings in another subtle but significant issue. > > We talk about the ADEV being done as the standard deviation of the frequency > differences, but often that's not what's done. Even with zero dead time, > there's another bit of magic in there. Drift is removed before the samples > are used. Bob, look at the Allan deviation formula again. A constant phase offset is effectively removed (because of the 1st difference); a constant frequency offset is also removed (because of the 2nd difference). But frequency drift is not removed with ADEV. Are you thinking of Hadamard deviation instead? With HDEV linear frequency drift is also removed (because of the 3rd difference). > Oddly there are multiple approaches to drift removal. It comes as no > surprise that the more aggressive the drift removal, the better looking the > result. If you are looking at ADEV, it's always worth asking if (and how) > the drift was removed. Agreed. A good example of this is the selection of an oscillator for a GPSDO. Strictly speaking a drift-removed ADEV or HDEV is a better metric to use when selecting an OCXO in this case. /tvb

Bob Camp

Wed, Aug 17, 2011 3:58 PM

Constant phase and frequency offsets are indeed removed up front.
There are cases where processing is done to remove frequency drift (aging)
and even "drift of frequency drift (decreasing rate aging)". They still
label the charts ADEV so it's not obvious what they have done.

Bob

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Tom Van Baak
Sent: Tuesday, August 16, 2011 1:11 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] frequency stabilty question

This brings in another subtle but significant issue.

We talk about the ADEV being done as the standard deviation of the

frequency

differences, but often that's not what's done. Even with zero dead time,
there's another bit of magic in there. Drift is removed before the samples
are used.

Oddly there are multiple approaches to drift removal. It comes as no
surprise that the more aggressive the drift removal, the better looking

the

result. If you are looking at ADEV, it's always worth asking if (and how)
the drift was removed.

Agreed. A good example of this is the selection of an oscillator
for a GPSDO. Strictly speaking a drift-removed ADEV or HDEV
is a better metric to use when selecting an OCXO in this case.

/tvb

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Hi Constant phase and frequency offsets are indeed removed up front. There are cases where processing is done to remove frequency drift (aging) and even "drift of frequency drift (decreasing rate aging)". They still label the charts ADEV so it's not obvious what they have done. Bob -----Original Message----- From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On Behalf Of Tom Van Baak Sent: Tuesday, August 16, 2011 1:11 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] frequency stabilty question > Hi > > This brings in another subtle but significant issue. > > We talk about the ADEV being done as the standard deviation of the frequency > differences, but often that's not what's done. Even with zero dead time, > there's another bit of magic in there. Drift is removed before the samples > are used. Bob, look at the Allan deviation formula again. A constant phase offset is effectively removed (because of the 1st difference); a constant frequency offset is also removed (because of the 2nd difference). But frequency drift is not removed with ADEV. Are you thinking of Hadamard deviation instead? With HDEV linear frequency drift is also removed (because of the 3rd difference). > Oddly there are multiple approaches to drift removal. It comes as no > surprise that the more aggressive the drift removal, the better looking the > result. If you are looking at ADEV, it's always worth asking if (and how) > the drift was removed. Agreed. A good example of this is the selection of an oscillator for a GPSDO. Strictly speaking a drift-removed ADEV or HDEV is a better metric to use when selecting an OCXO in this case. /tvb _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.