Measuring gpsdo vs itself
If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks,
Doc
KX0O
Bill,
This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.999999900 s measurement.
I'm not sure I'd call this a "reference independent" system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter.
/tvb
----- Original Message -----
From: "Bill Dailey" docdailey@gmail.com
To: "Time Nuts" time-nuts@febo.com
Sent: Sunday, November 04, 2012 7:19 PM
Subject: [time-nuts] Measuring gpsdo vs itself
If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks,
Doc
KX0O
I guess what I am saying is if I discipline the counter with 10MHz and then measure the same 10MHz. Just making sure we are on the same page.
Doc
Sent from my iPad
On Nov 4, 2012, at 10:22 PM, "Tom Van Baak" tvb@LeapSecond.com wrote:
Bill,
This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.999999900 s measurement.
I'm not sure I'd call this a "reference independent" system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter.
/tvb
----- Original Message -----
From: "Bill Dailey" docdailey@gmail.com
To: "Time Nuts" time-nuts@febo.com
Sent: Sunday, November 04, 2012 7:19 PM
Subject: [time-nuts] Measuring gpsdo vs itself
If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks,
Doc
KX0O
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Hi Tom,
I think you missed the point. He is trying to measure the noise floor of the counter itself. So what he wants to know is if using the same signal for the time base and input, would that cancel out the signals contribution to the noise measurement.
Bill....WB6BNQ
Tom Van Baak wrote:
Bill,
This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.999999900 s measurement.
I'm not sure I'd call this a "reference independent" system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter.
/tvb
----- Original Message -----
From: "Bill Dailey" docdailey@gmail.com
To: "Time Nuts" time-nuts@febo.com
Sent: Sunday, November 04, 2012 7:19 PM
Subject: [time-nuts] Measuring gpsdo vs itself
If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks,
Doc
KX0O
time-nuts mailing list -- time-nuts@febo.com
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Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor.
I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference.
This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale.
/tvb
----- Original Message -----
From: "Bill Dailey" docdailey@gmail.com
To: "Tom Van Baak" tvb@leapsecond.com; "Discussion of precise time and frequency measurement" time-nuts@febo.com
Sent: Sunday, November 04, 2012 8:33 PM
Subject: Re: [time-nuts] Measuring gpsdo vs itself
I guess what I am saying is if I discipline the counter with 10MHz and then measure the same 10MHz. Just making sure we are on the same page.
Doc
Sent from my iPad
On Nov 4, 2012, at 10:22 PM, "Tom Van Baak" tvb@LeapSecond.com wrote:
Bill,
This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.999999900 s measurement.
I'm not sure I'd call this a "reference independent" system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter.
/tvb
----- Original Message -----
From: "Bill Dailey" docdailey@gmail.com
To: "Time Nuts" time-nuts@febo.com
Sent: Sunday, November 04, 2012 7:19 PM
Subject: [time-nuts] Measuring gpsdo vs itself
If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system. Thanks,
Doc
KX0O
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
On 11/05/2012 06:30 AM, Tom Van Baak wrote:
Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor.
I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference.
This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale.
It's a little more complex than interpolator non-linearities alone. You
also need to include cross-talk between the signals. This cross-talk is
usually higher between A and B inputs than from reference, but never the
less.
You would need to sweep the trigger input delays to illustrate these
non-linearities. From a single measurement you can get both a better or
worse number compared to the average which is what you would expect to
see for free-running signals.
So, you can get a rough idea about the baseline, but it is not a
sufficient method.
See the SR620 manual for a plot of non-linearities.
Cheers,
Magnus
Thanks guys,
Like usual more complicated than I thought. I was hoping that this would
cancel any stability issues common to both the reference and the signal
thus giving me best case ability. I seem to be getting numbers too good to
be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly
linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there
was some kind of gotcha.
Doc
On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson <magnus@rubidium.dyndns.org
wrote:
On 11/05/2012 06:30 AM, Tom Van Baak wrote:
Ah, I see what you mean now. Yes, that setup can give you a rough
estimate of the counter's noise floor.
I can't give you specific numbers but one danger with this sort of test
is that the input and the timebase are artificially locked together (i.e.
fixed phase relationship) through the common reference. Your measurements
may thus show artificially less noise than a real-life case of independent
input(s) and reference.
This can happen if your sub-ns counter is based on interpolators. Because
the input and the timebase are locked in phase, the counter lands near the
same point of the interpolator scale on every single measurement, rather
than experiencing the noise (and non-linearity) of the entire scale.
It's a little more complex than interpolator non-linearities alone. You
also need to include cross-talk between the signals. This cross-talk is
usually higher between A and B inputs than from reference, but never the
less.
You would need to sweep the trigger input delays to illustrate these
non-linearities. From a single measurement you can get both a better or
worse number compared to the average which is what you would expect to see
for free-running signals.
So, you can get a rough idea about the baseline, but it is not a
sufficient method.
See the SR620 manual for a plot of non-linearities.
Cheers,
Magnus
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--
Doc
Bill Dailey
KXØO
Try this setup: feed the GPSDO into A and B inputs but not to the
reference. That is, use the counter internal reference to time the
difference so that you have an uncorrelated source that can span all the
interpolator's nonlinearities.
On Mon, Nov 5, 2012 at 11:42 AM, Bill Dailey docdailey@gmail.com wrote:
Thanks guys,
Like usual more complicated than I thought. I was hoping that this would
cancel any stability issues common to both the reference and the signal
thus giving me best case ability. I seem to be getting numbers too good to
be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly
linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there
was some kind of gotcha.
Doc
On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson <
magnus@rubidium.dyndns.org
wrote:
On 11/05/2012 06:30 AM, Tom Van Baak wrote:
Ah, I see what you mean now. Yes, that setup can give you a rough
estimate of the counter's noise floor.
I can't give you specific numbers but one danger with this sort of test
is that the input and the timebase are artificially locked together
(i.e.
fixed phase relationship) through the common reference. Your
measurements
may thus show artificially less noise than a real-life case of
independent
input(s) and reference.
This can happen if your sub-ns counter is based on interpolators.
Because
the input and the timebase are locked in phase, the counter lands near
the
same point of the interpolator scale on every single measurement, rather
than experiencing the noise (and non-linearity) of the entire scale.
It's a little more complex than interpolator non-linearities alone. You
also need to include cross-talk between the signals. This cross-talk is
usually higher between A and B inputs than from reference, but never the
less.
You would need to sweep the trigger input delays to illustrate these
non-linearities. From a single measurement you can get both a better or
worse number compared to the average which is what you would expect to
see
for free-running signals.
So, you can get a rough idea about the baseline, but it is not a
sufficient method.
See the SR620 manual for a plot of non-linearities.
Cheers,
Magnus
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mailman/listinfo/time-nuts<
and follow the instructions there.
--
Doc
Bill Dailey
KXØO
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Hi
As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something.
Bob
On Nov 5, 2012, at 5:42 AM, Bill Dailey docdailey@gmail.com wrote:
Thanks guys,
Like usual more complicated than I thought. I was hoping that this would
cancel any stability issues common to both the reference and the signal
thus giving me best case ability. I seem to be getting numbers too good to
be true so there must be a hitch. I get an ADEV 5x10-13 at 1 s mostly
linear to 7x10-16 at 10,000 s with a small hump at 20s-80s. Figured there
was some kind of gotcha.
Doc
On Mon, Nov 5, 2012 at 2:26 AM, Magnus Danielson <magnus@rubidium.dyndns.org
wrote:
On 11/05/2012 06:30 AM, Tom Van Baak wrote:
Ah, I see what you mean now. Yes, that setup can give you a rough
estimate of the counter's noise floor.
I can't give you specific numbers but one danger with this sort of test
is that the input and the timebase are artificially locked together (i.e.
fixed phase relationship) through the common reference. Your measurements
may thus show artificially less noise than a real-life case of independent
input(s) and reference.
This can happen if your sub-ns counter is based on interpolators. Because
the input and the timebase are locked in phase, the counter lands near the
same point of the interpolator scale on every single measurement, rather
than experiencing the noise (and non-linearity) of the entire scale.
It's a little more complex than interpolator non-linearities alone. You
also need to include cross-talk between the signals. This cross-talk is
usually higher between A and B inputs than from reference, but never the
less.
You would need to sweep the trigger input delays to illustrate these
non-linearities. From a single measurement you can get both a better or
worse number compared to the average which is what you would expect to see
for free-running signals.
So, you can get a rough idea about the baseline, but it is not a
sufficient method.
See the SR620 manual for a plot of non-linearities.
Cheers,
Magnus
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and follow the instructions there.
--
Doc
Bill Dailey
KXØO
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Hi Bob,
On 11/05/2012 01:30 PM, Bob Camp wrote:
Hi
As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something.
I agree that this is a good self-test strategy, it usually gives a good
clue if something is really bad or not, but as I pointed out, it doesn't
give a fair idea of the measurement noise floor.
Cheers,
Magnus