Help with Novatel OEM6 external oscillator
Hello Time-Nuts,
Hopefully, this doesn't get too long. What I desire to do is hook up
an external oscillator to a Novatel OEM6 series dual-band GPS
receiver.
Sounds easy. But, Novatel has four fixed (TCXO, OCXO, rubidium,
cesium) plus a USER specifications to model the external clock
performance. Each model specifies three coefficients of the power law
spectral density (h-2, h-1, h0). If you google 'novatel
externalclock' you can see the command. My question is, how do I
translate an oscillator ADEV plot into the three coefficients?
I'm not an oscillator noise guru by any means (in fact, I'm probably a
noise dummy). It took me a while to figure out that the number range
that Novatel is looking for is Allen Variance. So taking the square
root of their model numbers would be Allen Deviation (hope this is
correct). I believe the h-2 coefficient represents Random Walk FM
(Would this be slope=-2 on ADEV plot?), the h-1 would be Flicker FM
(slope =-1?), and the h0 coefficient would be White FM (slope =1 (or
is it 0)?).
If I look at oscillator ADEV plots I often don't see anything as steep
as -2 slope. Is this because it is below the lowest tau of the plot
and/or below the noise floor of the measurement system?
The real question is how to I translate a typical ADEV plot into the
three coefficients that the Novatel receiver wants to see. An
illustrative example would be most helpful. From looking at the
numbers in the Novatel documentation it appears that the h-2 number
represents short taus, h-1 intermediate taus, and h0 long taus.
For extra credit - Google ' Novatel clockmodel', and explain (simply)
what the covariance matrix is. Again, if an illustrative example
could be shown it would be most helpful.
That's all for now before my brain explodes. Thanks in advance for any help.
Regards,
Skip Withrow
Hi
Simple answer is:
-
(assuming you just want GPS) to try each of the settings and see what gives
you the best signal to noise. I’d bet they have very little impact if you have a good
oscillator. -
If you are building a GPSDO, the only useful setting is USER. There it’s a matter
of measuring the noise on the composite GPS + OCXO (or whatever) and tweaking
to get the best result. Their presets might or might not be reasonable starting points
for the type of oscillator you have.
I’ll let others write the 200 pages that it would take to go through the other stuff completely :)
Part of it gets into why we have more than just ADEV out there ….
Bob
On Jan 21, 2019, at 4:51 PM, Skip Withrow skip.withrow@gmail.com wrote:
Hello Time-Nuts,
Hopefully, this doesn't get too long. What I desire to do is hook up
an external oscillator to a Novatel OEM6 series dual-band GPS
receiver.
Sounds easy. But, Novatel has four fixed (TCXO, OCXO, rubidium,
cesium) plus a USER specifications to model the external clock
performance. Each model specifies three coefficients of the power law
spectral density (h-2, h-1, h0). If you google 'novatel
externalclock' you can see the command. My question is, how do I
translate an oscillator ADEV plot into the three coefficients?
I'm not an oscillator noise guru by any means (in fact, I'm probably a
noise dummy). It took me a while to figure out that the number range
that Novatel is looking for is Allen Variance. So taking the square
root of their model numbers would be Allen Deviation (hope this is
correct). I believe the h-2 coefficient represents Random Walk FM
(Would this be slope=-2 on ADEV plot?), the h-1 would be Flicker FM
(slope =-1?), and the h0 coefficient would be White FM (slope =1 (or
is it 0)?).
If I look at oscillator ADEV plots I often don't see anything as steep
as -2 slope. Is this because it is below the lowest tau of the plot
and/or below the noise floor of the measurement system?
The real question is how to I translate a typical ADEV plot into the
three coefficients that the Novatel receiver wants to see. An
illustrative example would be most helpful. From looking at the
numbers in the Novatel documentation it appears that the h-2 number
represents short taus, h-1 intermediate taus, and h0 long taus.
For extra credit - Google ' Novatel clockmodel', and explain (simply)
what the covariance matrix is. Again, if an illustrative example
could be shown it would be most helpful.
That's all for now before my brain explodes. Thanks in advance for any help.
Regards,
Skip Withrow
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Hi Skip,
On 2019-01-21 22:51, Skip Withrow wrote:
Hello Time-Nuts,
Hopefully, this doesn't get too long. What I desire to do is hook up
an external oscillator to a Novatel OEM6 series dual-band GPS
receiver.
Sounds easy. But, Novatel has four fixed (TCXO, OCXO, rubidium,
cesium) plus a USER specifications to model the external clock
performance. Each model specifies three coefficients of the power law
spectral density (h-2, h-1, h0). If you google 'novatel
externalclock' you can see the command. My question is, how do I
translate an oscillator ADEV plot into the three coefficients?
First of all, an excellent question!
I'm not an oscillator noise guru by any means (in fact, I'm probably a
noise dummy). It took me a while to figure out that the number range
that Novatel is looking for is Allen Variance. So taking the square
root of their model numbers would be Allen Deviation (hope this is
correct).
Actually, these numbers is really power-levels for the various noise-types.
You can however use Allan Deviation or Allan Variance to figure these
out, so you got that part right.
I believe the h-2 coefficient represents Random Walk FM
(Would this be slope=-2 on ADEV plot?), the h-1 would be Flicker FM
(slope =-1?), and the h0 coefficient would be White FM (slope =1 (or
is it 0)?).
If you look at this part of the Allan Deviation Wikipedia page, I
prepared a handy table for you:
https://en.wikipedia.org/wiki/Allan_variance#Power-law_noise
The White Frequency Modulation (WFM) is the h0 value and has the -1/2
slope in the ADEV, i.e. 1/sqrt(tau).
The Flicker Frequency Modulation (FFM) is the h-1 value and has the flat
slope in the ADEV.
The Random Walk Frequency Modulation (RWFM) is the h-2 value and has the
+1/2 slope in the ADEV, i.e. sqrt(tau).
As you see there is a scaling factor from each h-value to the ADEV
beyond the tau-one, and this is important.
So, if you estimate the line-slope, you need to compensate for tau and
scale factor to estimate the power-level of that noise-type. You do this
by dividing your ADEV reading with the scale-factor for that noise-type,
and for the tau-value of the ADEV reading. Naturally you can do that on
the AVAR values.
If I look at oscillator ADEV plots I often don't see anything as steep
as -2 slope. Is this because it is below the lowest tau of the plot
and/or below the noise floor of the measurement system?
For the shortest tau you see the -1 slope of ADEV which is either the
instruments resolution or the white phase modulation (WPM) noise. This
represents the h2 level. The flicker phase (FPM) is the h1 level, but in
ADEV we can't tell them apart in any practical manor, for that we need
to go to MDEV which is what Dave Allan wants us to use, for that
specific reason.
The real question is how to I translate a typical ADEV plot into the
three coefficients that the Novatel receiver wants to see. An
illustrative example would be most helpful. From looking at the
numbers in the Novatel documentation it appears that the h-2 number
represents short taus, h-1 intermediate taus, and h0 long taus.
The other way around. h-2 is the long-tau vs. h-1 and h0 which is
shorter taus.
I'd love to show it more elaborative, but emailing of the night-train
has it's limits.
For extra credit - Google ' Novatel clockmodel', and explain (simply)
what the covariance matrix is. Again, if an illustrative example
could be shown it would be most helpful.
That's all for now before my brain explodes. Thanks in advance for any help.
We can't have you do that. I hope we can help you to figure this out.
Maybe I can do better on my way back tomorrow.
Cheers,
Magnus
On Sat, 26 Jan 2019 00:32:46 +0100
Magnus Danielson magnus@rubidium.se wrote:
Sounds easy. But, Novatel has four fixed (TCXO, OCXO, rubidium,
cesium) plus a USER specifications to model the external clock
performance. Each model specifies three coefficients of the power law
spectral density (h-2, h-1, h0). If you google 'novatel
externalclock' you can see the command. My question is, how do I
translate an oscillator ADEV plot into the three coefficients?
First of all, an excellent question!
I'm not an oscillator noise guru by any means (in fact, I'm probably a
noise dummy). It took me a while to figure out that the number range
that Novatel is looking for is Allen Variance. So taking the square
root of their model numbers would be Allen Deviation (hope this is
correct).
Actually, these numbers is really power-levels for the various noise-types.
[... big explenation snipped ...]
Enrico Rubiola's Noise chart comes handy in these kind of calulations:
http://rubiola.org/pdf-static/Enrico's-chart-EFTS.pdf
Attila Kinali
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the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neal Stephenson