Discussion:
More Galileo satellites available.
(too old to reply)
Mark Sims
2018-08-05 03:41:54 UTC
Permalink
Three of the four Galileo satellites launched last December have been activated (passive hydrpren maser clocks) bringing the total number of usable sats to 17. The fourth one is still "under comiisioning" . Four more sats were launched last month... hopefully it won't take them 8 months to commission...
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paul swed
2018-08-05 16:04:25 UTC
Permalink
Mark
I have several questions.
Would we observe and be able to take advantage of the better clock. Or is
that lost in the various other transmission effects?
If better what would it take to have a reference built up leveraging that?
Perhaps a precision timing receiver. I stop there because that opens up
several ways to leverage the 1PPS or perhaps the code clock.
Regards
Paul
WB8TSL
Post by Mark Sims
Three of the four Galileo satellites launched last December have been
activated (passive hydrpren maser clocks) bringing the total number of
usable sats to 17. The fourth one is still "under comiisioning" . Four
more sats were launched last month... hopefully it won't take them 8 months
to commission...
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Bob kb8tq
2018-08-05 16:43:47 UTC
Permalink
Hi
Post by paul swed
Mark
I have several questions.
Would we observe and be able to take advantage of the better clock.
Right now … now so much
Post by paul swed
Or is
that lost in the various other transmission effects?
Modeling matters a lot, they are still working the kinks out of that part of it.
Post by paul swed
If better what would it take to have a reference built up leveraging that?
A multi-band receiver that picks up all three bands ( and is licensed to do so …
a whole other issue) probably is the first link in the chain.
Post by paul swed
Perhaps a precision timing receiver.
Right now, a “typical” multi band receiver is in the > $10K range. Some are
two to three times that.

Bob
Post by paul swed
I stop there because that opens up
several ways to leverage the 1PPS or perhaps the code clock.
Regards
Paul
WB8TSL
Post by Mark Sims
Three of the four Galileo satellites launched last December have been
activated (passive hydrpren maser clocks) bringing the total number of
usable sats to 17. The fourth one is still "under comiisioning" . Four
more sats were launched last month... hopefully it won't take them 8 months
to commission...
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paul swed
2018-08-05 17:40:48 UTC
Permalink
Bob you sure about that?
I could have sworn a multi-band receiver was on ebay for $32.
Made special in N Korea.
That said I guess I will stick with the always working simple GPS for now.
Post by Bob kb8tq
Hi
Post by paul swed
Mark
I have several questions.
Would we observe and be able to take advantage of the better clock.
Right now … now so much
Post by paul swed
Or is
that lost in the various other transmission effects?
Modeling matters a lot, they are still working the kinks out of that part of it.
Post by paul swed
If better what would it take to have a reference built up leveraging
that?
A multi-band receiver that picks up all three bands ( and is licensed to do so …
a whole other issue) probably is the first link in the chain.
Post by paul swed
Perhaps a precision timing receiver.
Right now, a “typical” multi band receiver is in the > $10K range. Some are
two to three times that.
Bob
Post by paul swed
I stop there because that opens up
several ways to leverage the 1PPS or perhaps the code clock.
Regards
Paul
WB8TSL
Post by Mark Sims
Three of the four Galileo satellites launched last December have been
activated (passive hydrpren maser clocks) bringing the total number of
usable sats to 17. The fourth one is still "under comiisioning" .
Four
Post by paul swed
Post by Mark Sims
more sats were launched last month... hopefully it won't take them 8
months
Post by paul swed
Post by Mark Sims
to commission...
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Post by paul swed
and follow the instructions there.
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and
Bob kb8tq
2018-08-05 18:28:08 UTC
Permalink
Hi

There are multi GNSS receivers out there. Some may do Galileo. That’s
different than a multi-band (L1 / L2 / L5) receiver that will do Galileo. Since
the band naming conventions change between systems, even working out
what’s what is a bit exciting.

Galileo is a bit more “closed source” than the other systems. If you want to do
this or that, there’s a license fee. The more you want to do, the more it costs
to license your design. Like the other systems, they also have restrictions on
some signals ( = not for civilian use).

Bob
Post by paul swed
Bob you sure about that?
I could have sworn a multi-band receiver was on ebay for $32.
Made special in N Korea.
That said I guess I will stick with the always working simple GPS for now.
Post by Bob kb8tq
Hi
Post by paul swed
Mark
I have several questions.
Would we observe and be able to take advantage of the better clock.
Right now … now so much
Post by paul swed
Or is
that lost in the various other transmission effects?
Modeling matters a lot, they are still working the kinks out of that part of it.
Post by paul swed
If better what would it take to have a reference built up leveraging
that?
A multi-band receiver that picks up all three bands ( and is licensed to do so …
a whole other issue) probably is the first link in the chain.
Post by paul swed
Perhaps a precision timing receiver.
Right now, a “typical” multi band receiver is in the > $10K range. Some are
two to three times that.
Bob
Post by paul swed
I stop there because that opens up
several ways to leverage the 1PPS or perhaps the code clock.
Regards
Paul
WB8TSL
Post by Mark Sims
Three of the four Galileo satellites launched last December have been
activated (passive hydrpren maser clocks) bringing the total number of
usable sats to 17. The fourth one is still "under comiisioning" .
Four
Post by paul swed
Post by Mark Sims
more sats were launched last month... hopefully it won't take them 8
months
Post by paul swed
Post by Mark Sims
to commission...
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Post by paul swed
and follow the instructions there.
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paul swed
2018-08-05 19:09:02 UTC
Permalink
I remember various licensing issues in GNSS magazine.
Post by Bob kb8tq
Hi
There are multi GNSS receivers out there. Some may do Galileo. That’s
different than a multi-band (L1 / L2 / L5) receiver that will do Galileo. Since
the band naming conventions change between systems, even working out
what’s what is a bit exciting.
Galileo is a bit more “closed source” than the other systems. If you want to do
this or that, there’s a license fee. The more you want to do, the more it costs
to license your design. Like the other systems, they also have
restrictions on
some signals ( = not for civilian use).
Bob
Post by paul swed
Bob you sure about that?
I could have sworn a multi-band receiver was on ebay for $32.
Made special in N Korea.
That said I guess I will stick with the always working simple GPS for
now.
Post by paul swed
Post by Bob kb8tq
Hi
Post by paul swed
Mark
I have several questions.
Would we observe and be able to take advantage of the better clock.
Right now … now so much
Post by paul swed
Or is
that lost in the various other transmission effects?
Modeling matters a lot, they are still working the kinks out of that
part
Post by paul swed
Post by Bob kb8tq
of it.
Post by paul swed
If better what would it take to have a reference built up leveraging
that?
A multi-band receiver that picks up all three bands ( and is licensed to do so …
a whole other issue) probably is the first link in the chain.
Post by paul swed
Perhaps a precision timing receiver.
Right now, a “typical” multi band receiver is in the > $10K range. Some are
two to three times that.
Bob
Post by paul swed
I stop there because that opens up
several ways to leverage the 1PPS or perhaps the code clock.
Regards
Paul
WB8TSL
Post by Mark Sims
Three of the four Galileo satellites launched last December have been
activated (passive hydrpren maser clocks) bringing the total number of
usable sats to 17. The fourth one is still "under comiisioning" .
Four
Post by paul swed
Post by Mark Sims
more sats were launched last month... hopefully it won't take them 8
months
Post by paul swed
Post by Mark Sims
to commission...
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Post by paul swed
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Mark Sims
2018-08-05 19:41:05 UTC
Permalink
A little while back an update to Ublox Ucenter had a mention of a soon (?) to be released Ublox M9 receiver that supposedly does L1/L2/L5 data. Shortly afterward, the update was sanitized. If an M9 comes out that will be very interesting to play with.

https://rtklibexplorer.wordpress.com/2018/02/27/u-blox-announces-the-f9-platform-with-l1-l2-l5-frequencies-supported/

Basic Galileo is supposed to be more accurate than GPS... I've seen mentions of up to 50%. When CSRS-PPP gets Galileo support in a couple of weeks we will see...
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Richard (Rick) Karlquist
2018-08-06 05:12:56 UTC
Permalink
I need to measure ADEV on a source that has spurious
sine wave frequency modulation on it. I am looking for a
formula that would tell me ADEV, vs FM deviation and
carrier frequency. I'm not sure if modulation frequency
or tau matter. I am hoping to determine how much
I need to clean up the signal order to get down to a particular
ADEV measurement threshold.

I tried a literature search but didn't turn up anything
I could use.

I remember when I worked on the output section of the 5071A
we specified 100 kHz and 1 MHz spurs to be down -80 dBc.
These didn't seem to affect the ADEV of the 10 MHz outputs,
but I can't prove why this should be the case. Does the
f-sub-h measurement bandwidth come into play?

Thanks.

Rick Karlquist N6RK

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Poul-Henning Kamp
2018-08-06 05:24:29 UTC
Permalink
--------
Post by Richard (Rick) Karlquist
I need to measure ADEV on a source that has spurious
sine wave frequency modulation on it.
The biggest factor is the ratio between the tau's
you do allan for, and the modulation frequency.

If modulation divides *cleanly* into tau and preferably
with a big factor, then you can ignore it.

Otherwise things get really messy and quite intractable.

Picking your taus as multiple of the modulation is usually the best strategy.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
***@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Magnus Danielson
2018-08-06 06:07:59 UTC
Permalink
Hi Rick,
Post by Richard (Rick) Karlquist
I need to measure ADEV on a source that has spurious
sine wave frequency modulation on it.  I am looking for a
formula that would tell me ADEV, vs FM deviation and
carrier frequency. I'm not sure if modulation frequency
or tau matter.  I am hoping to determine how much
I need to clean up the signal order to get down to a particular
ADEV measurement threshold.
I tried a literature search but didn't turn up anything
I could use.
I remember when I worked on the output section of the 5071A
we specified 100 kHz and 1 MHz spurs to be down -80 dBc.
These didn't seem to affect the ADEV of the 10 MHz outputs,
but I can't prove why this should be the case.  Does the
f-sub-h measurement bandwidth come into play?
Thanks.
What does exists is a formula for how a single sine spur would produce
ADEV. A FM deviation with low enough modulation index creates two
side-bands of opposite sign but same amplitude. The modulation index,
which is just another aspect of frequency deviation, is direct steering
the amplitude of these sidebands through Bessel polynomial, but for low
deviations this is linear. The modulation frequency will care, and it
will depend on tau. The actual sample rate of the ADEV will interact
with the sideband "spurs" and add onto each other.

The additional ADEV comes on top of any other ADEV response, so it is
clearly a factor on ADEV polution.

Without doing the math, expect the double amplitude to that of the
single sideband sine of the same amplitude itself.

Cheers,
Magnus

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Poul-Henning Kamp
2018-08-06 06:22:07 UTC
Permalink
--------
Post by Magnus Danielson
What does exists is a formula for how a single sine spur would produce
ADEV. A FM deviation with low enough modulation index creates two
side-bands of opposite sign but same amplitude.
I find the easiest way to wrap my head around this is to think
about measuring Adev by timing zero-crossings.

If we had a perfect zero-crossing detector, perfect AM would not
matter, because perfect AM does not move the zero-crossings.

But in theory at least one electron, and in practice many, must run
the opposite direction before we can detect the zero-crossing, so
even perfect AM matters in practice. (Interesting detail: Measuring
at high impedance may be smarter than at 50 Ohm impedance)

For FM or PM there is no loophole: They move the zero-crossings and that's that.

Depending on the modulation signal, there may be moments where the
zero-crossing is "where it should be", for instance if the modulation
is sine or triangular, but not if it is a signed square wave.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
***@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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jimlux
2018-08-06 13:17:20 UTC
Permalink
Post by Poul-Henning Kamp
--------
Post by Magnus Danielson
What does exists is a formula for how a single sine spur would produce
ADEV. A FM deviation with low enough modulation index creates two
side-bands of opposite sign but same amplitude.
I find the easiest way to wrap my head around this is to think
about measuring Adev by timing zero-crossings.
<snip>
Post by Poul-Henning Kamp
Depending on the modulation signal, there may be moments where the
zero-crossing is "where it should be", for instance if the modulation
is sine or triangular, but not if it is a signed square wave.
What about doing some sort of fit to the measurement data before
calculating the ADEV? Similar to removing a linear ramp.

basically you'd solve for the three sine parameters (f, phase,
amp/deviation), then remove that from the data, then run the ADEV
calculation.


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Dana Whitlow
2018-08-06 14:18:42 UTC
Permalink
One might also do some trials based on comparing ADEV results between a
clean carrier signal
and ones corrupted with varying degrees of FM (for example), to get a feel
for the problem. If
nothing else, one ought to be able to get some feel for the sensitivities
involved.

Dana K8YUM
Post by jimlux
Post by Poul-Henning Kamp
--------
What does exists is a formula for how a single sine spur would produce
Post by Magnus Danielson
ADEV. A FM deviation with low enough modulation index creates two
side-bands of opposite sign but same amplitude.
I find the easiest way to wrap my head around this is to think
about measuring Adev by timing zero-crossings.
<snip>
Post by Poul-Henning Kamp
Depending on the modulation signal, there may be moments where the
zero-crossing is "where it should be", for instance if the modulation
is sine or triangular, but not if it is a signed square wave.
What about doing some sort of fit to the measurement data before
calculating the ADEV? Similar to removing a linear ramp.
basically you'd solve for the three sine parameters (f, phase,
amp/deviation), then remove that from the data, then run the ADEV
calculation.
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Poul-Henning Kamp
2018-08-06 14:25:50 UTC
Permalink
--------
Post by jimlux
What about doing some sort of fit to the measurement data before
calculating the ADEV? Similar to removing a linear ramp.
basically you'd solve for the three sine parameters (f, phase,
amp/deviation), then remove that from the data, then run the ADEV
calculation.
That is incredibly shaky ground from a scientific point of view.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
***@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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jimlux
2018-08-06 14:54:30 UTC
Permalink
Post by Poul-Henning Kamp
--------
Post by jimlux
What about doing some sort of fit to the measurement data before
calculating the ADEV? Similar to removing a linear ramp.
basically you'd solve for the three sine parameters (f, phase,
amp/deviation), then remove that from the data, then run the ADEV
calculation.
That is incredibly shaky ground from a scientific point of view.
If you know the *source* of the bias you're trying to remove (i.e. you
know it's a sinusoidal frequency modulation), I don't know that it's any
different than removing long term drift.

As a practical example of where something like this is done, consider
measuring the Allan Deviation of a coherent deep space transponder that
is orbiting a moon or planet (i.e. there's a Doppler on both uplink and
downlink).

Coherent transponders produce an output that is precisely a rational
fraction of the input frequency (e.g. for X-band 880/749 for a 7.15 GHz
input, 8.45 GHz out) and the frequency/phase of the received signal is
compared with the frequency/phase of the transmitted signal to measure
range and range rate to the spacecraft (as well as the propagation
properties of the path).

We'll test them with fixed frequency (put a spectrally pure tone in and
measure the ADEV of the , and back in the day when the "turnaround" was
done with all analog circuitry, you could be reasonably sure that the
whole thing would have comparable performance in a Doppler shifted
environment.

These days, the turnaround is implemented in digital logic, and in
particular, the receive carrier tracking loop is done digitally, and one
wants to make sure that it tracks "adequately" in the face of Doppler.
So one builds a test set that produces a well characterized frequency
modulated input (representative of the Doppler profile) and you make
your measurements, remove the Doppler profile (with the appropriate
phase ratio) and make your ADEV measurement.

[This, of course, means that you have to generate a waveform with good
ADEV for testing, a technical challenge in its own right]

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Tom Van Baak
2018-08-06 20:56:09 UTC
Permalink
Post by jimlux
If you know the *source* of the bias you're trying to remove (i.e. you
know it's a sinusoidal frequency modulation), I don't know that it's any
different than removing long term drift.
The TimeLab 'n' command (apply notch filter to phase records) is specifically for this purpose. JohnM added it when he ran into an H-maser which suffered from some sort of consistent periodic modulation. It spoiled the ADEV plots, but it did so in a deterministic manner. In a case like this you either debug the root cause of the h/w problem and make the repair, or just "repair it" in s/w. Like you say, it's the same concept (and danger) as removing linear drift or other deterministic / model-able effects.

To explore this for yourself, you can use Stable32 to generate synthetic phase/frequency data with your choice of noise and modulation. Then use TimeLab to view the raw data and to apply the notch filter.

I've written up some quick examples here: www.leapsecond.com/pages/adev-fm/

The plots dramatically show what effect slow FM can have on an ADEV plot. It also shows how well the TimeLab notch filter works. If you don't have time to look at that page, I've attached one plot to whet your appetite.

/tvb
Richard (Rick) Karlquist
2018-08-06 22:37:04 UTC
Permalink
The discussion has gotten off track.
I probably didn't make myself clear.

When I talked about cleaning up the signal, I meant
exactly that, cleaning up the signal using a filter.
As opposed to cleaning up the measurement after the
fact. I just need to know how well I need to filter
the signal to meet a certain ADEV measurement level.

The FM I have is not "slow". The rate
is in the MHz range. Can Stable 32 simulate that?

Again, this is analogous to the 5071A having 1 MHz
spurs of about -90 dBc, yet the ADEV is not much
different than an open loop 10811. How high would
the 1 MHz spurs have to be to affect ADEV?

Thanks.

Rick
Post by Tom Van Baak
Post by jimlux
If you know the *source* of the bias you're trying to remove (i.e. you
know it's a sinusoidal frequency modulation), I don't know that it's any
different than removing long term drift.
The TimeLab 'n' command (apply notch filter to phase records) is specifically for this purpose. JohnM added it when he ran into an H-maser which suffered from some sort of consistent periodic modulation. It spoiled the ADEV plots, but it did so in a deterministic manner. In a case like this you either debug the root cause of the h/w problem and make the repair, or just "repair it" in s/w. Like you say, it's the same concept (and danger) as removing linear drift or other deterministic / model-able effects.
To explore this for yourself, you can use Stable32 to generate synthetic phase/frequency data with your choice of noise and modulation. Then use TimeLab to view the raw data and to apply the notch filter.
I've written up some quick examples here: www.leapsecond.com/pages/adev-fm/
The plots dramatically show what effect slow FM can have on an ADEV plot. It also shows how well the TimeLab notch filter works. If you don't have time to look at that page, I've attached one plot to whet your appetite.
/tvb
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Bruce Griffiths
2018-08-06 23:03:47 UTC
Permalink
Surely all that's required is a simplistic worst case analysis.
Just assume that the value of Tau is always the worst possible so the full effect of the modulation is always seen.

If the worst case phase modulation is say phi

then the worst case ADEV (Tau) will be proportional to phi*T/Tau
where T is the nominal signal period.

i.e. ADEV(Tau) < constant*phi*T/Tau

Bruce
Post by Richard (Rick) Karlquist
The discussion has gotten off track.
I probably didn't make myself clear.
When I talked about cleaning up the signal, I meant
exactly that, cleaning up the signal using a filter.
As opposed to cleaning up the measurement after the
fact. I just need to know how well I need to filter
the signal to meet a certain ADEV measurement level.
The FM I have is not "slow". The rate
is in the MHz range. Can Stable 32 simulate that?
Again, this is analogous to the 5071A having 1 MHz
spurs of about -90 dBc, yet the ADEV is not much
different than an open loop 10811. How high would
the 1 MHz spurs have to be to affect ADEV?
Thanks.
Rick
Post by Tom Van Baak
Post by jimlux
If you know the *source* of the bias you're trying to remove (i.e. you
know it's a sinusoidal frequency modulation), I don't know that it's any
different than removing long term drift.
The TimeLab 'n' command (apply notch filter to phase records) is specifically for this purpose. JohnM added it when he ran into an H-maser which suffered from some sort of consistent periodic modulation. It spoiled the ADEV plots, but it did so in a deterministic manner. In a case like this you either debug the root cause of the h/w problem and make the repair, or just "repair it" in s/w. Like you say, it's the same concept (and danger) as removing linear drift or other deterministic / model-able effects.
To explore this for yourself, you can use Stable32 to generate synthetic phase/frequency data with your choice of noise and modulation. Then use TimeLab to view the raw data and to apply the notch filter.
I've written up some quick examples here: www.leapsecond.com/pages/adev-fm/
The plots dramatically show what effect slow FM can have on an ADEV plot. It also shows how well the TimeLab notch filter works. If you don't have time to look at that page, I've attached one plot to whet your appetite.
/tvb
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John Miles
2018-08-07 01:39:01 UTC
Permalink
Post by Richard (Rick) Karlquist
Again, this is analogous to the 5071A having 1 MHz
spurs of about -90 dBc, yet the ADEV is not much
different than an open loop 10811. How high would
the 1 MHz spurs have to be to affect ADEV?
With ADEV, you have to consider the measurement bandwidth before you can ask
questions like this, much less answer them. :) 1 MHz spurs are far outside
the restricted measurement bandwidth that would be desirable when measuring
a high-performance source like a 5071A. I suppose there might be some
influence if you use a DC-to-daylight sampling process such as a counter,
but the instrument noise floor will probably obscure any interferers in the
-90 dBc range.

With a TimePod or similar device, a general rule of thumb is that a spur a
few Hz away from a 10 MHz carrier at -120 dBc will cause ripple in the 1E-11
to 1E-12 range. This coincides with typical levels of coupling between
nearby RG-58 cables -- see page 38 of
http://www.miles.io/TimePod_5330A_user_manual.pdf for instance. That
particular situation wouldn't have shown up on a traditional counter-based
measurement, but others might.

For instance, in your 1 MHz example, I doubt you'd see any effect at exactly
1 MHz, but if your spurs are at 1.000001 MHz and your counter is capable of
1-ns single-shot resolution, I can imagine that there will be some
corruption. The exact numbers will depend on so many factors that it's
almost easier to set up a test and observe the effects firsthand.

-- john, KE5FX
Miles Design LLC



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Richard (Rick) Karlquist
2018-08-07 02:05:02 UTC
Permalink
I am thinking that I should simply make a very simple notch filter
that suppresses the spurious line by 10 or 20 dB. I can then do
an A/B comparison of the measured ADEV with and without the filter.
If there is no change, I'm done. Otherwise, I add filtering in
steps until the ADEV reaches an asymptote.

Rick
Post by John Miles
Post by Richard (Rick) Karlquist
Again, this is analogous to the 5071A having 1 MHz
spurs of about -90 dBc, yet the ADEV is not much
different than an open loop 10811. How high would
the 1 MHz spurs have to be to affect ADEV?
With ADEV, you have to consider the measurement bandwidth before you can ask
questions like this, much less answer them. :) 1 MHz spurs are far outside
the restricted measurement bandwidth that would be desirable when measuring
a high-performance source like a 5071A. I suppose there might be some
influence if you use a DC-to-daylight sampling process such as a counter,
but the instrument noise floor will probably obscure any interferers in the
-90 dBc range.
With a TimePod or similar device, a general rule of thumb is that a spur a
few Hz away from a 10 MHz carrier at -120 dBc will cause ripple in the 1E-11
to 1E-12 range. This coincides with typical levels of coupling between
nearby RG-58 cables -- see page 38 of
http://www.miles.io/TimePod_5330A_user_manual.pdf for instance. That
particular situation wouldn't have shown up on a traditional counter-based
measurement, but others might.
For instance, in your 1 MHz example, I doubt you'd see any effect at exactly
1 MHz, but if your spurs are at 1.000001 MHz and your counter is capable of
1-ns single-shot resolution, I can imagine that there will be some
corruption. The exact numbers will depend on so many factors that it's
almost easier to set up a test and observe the effects firsthand.
-- john, KE5FX
Miles Design LLC
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Magnus Danielson
2018-08-07 06:09:21 UTC
Permalink
Hi,
Post by Poul-Henning Kamp
--------
Post by Magnus Danielson
What does exists is a formula for how a single sine spur would produce
ADEV. A FM deviation with low enough modulation index creates two
side-bands of opposite sign but same amplitude.
I find the easiest way to wrap my head around this is to think
about measuring Adev by timing zero-crossings.
If we had a perfect zero-crossing detector, perfect AM would not
matter, because perfect AM does not move the zero-crossings.
But in theory at least one electron, and in practice many, must run
the opposite direction before we can detect the zero-crossing, so
even perfect AM matters in practice. (Interesting detail: Measuring
at high impedance may be smarter than at 50 Ohm impedance)
For FM or PM there is no loophole: They move the zero-crossings and that's that.
Depending on the modulation signal, there may be moments where the
zero-crossing is "where it should be", for instance if the modulation
is sine or triangular, but not if it is a signed square wave.
One type of FM detector is to run it through a limiter, which kills most
of the AM, just as you do for frequency measurements, so yes. The moment
the transitions is where they should be is the through zero moment.

So yes, we expect to see the FM creep into ADEV just fine.

Cheers,
Magnus

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Henk Peek
2018-08-07 10:24:15 UTC
Permalink
Post by Richard (Rick) Karlquist
I need to measure ADEV on a source that has spurious
sine wave frequency modulation on it. I am looking for a
formula that would tell me ADEV, vs FM deviation and
carrier frequency. I'm not sure if modulation frequency
or tau matter. I am hoping to determine how much
I need to clean up the signal order to get down to a particular
ADEV measurement threshold.
Use an ADEV sample period of the FM modulation frequency period
or sub-harmonic period. This samples the frequency modulated signal
at a constant phase of the modulation and generate notches at the
FM modulation side-bands of the ADEV samples.
Result: the ADEV samples don't contain the FM modulation.

Henk Peek


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Ralph Devoe
2018-08-07 13:12:48 UTC
Permalink
There's a simple way to put an upper bound on the ADEV due to a spur,
by treating the spur as random noise. Assume that the carrier is 1 volt at
10 MHz and the spur is 80 db down. Then the spur has 10(-4) volt amplitude.
The slope of the carrier is 1/(2 pi f) or 16 ns/volt. Then 10(-4) volts of
noise can shift the zero-crossing by 1.6 ps, which amounts to an ADEV of
1.6 x 10(-12) at a tau of 1 sec. This drops linearly with tau.
The effect can be smaller than this, of course, since it can average
away, like tvb's plot show.

Ralph DeVoe
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