Phase measurement of my GPSDO
Dear colleagues,
as you may remember I am still working on my own GPSDO project.
So far I have populated all the components on the PCBs I made, and I have
written some very basic software to test everything. Currently, I am
running following simple test:
I set my DAC to a constant value (5 Volts at the EFC input of the OCXO),
and then I send the phase measurement to the PC via RS-232 where I can log
it. More on that later.
Here is my schematic (not much has changed since last time):
https://hb9fsx.ch/files/gpsdo/gpsdo_schematic.pdf
The GPS phase detection happens on page 3. I use the TDC7200 as
interpolator.
Inside the STM32 microcontroller, I let a timer run with the 10MHz clock.
The timer overflows to 0 when it reaches the value 9999999.
Further, the 1PPS pulse from my GPS module triggers the interpolator, and
is delayed by max. 2 clock cycles. The delayed signal stops the
interpolator and is used as a capture event of the timer.
So, when the OCXO has the right frequency, the captured value would always
be the same. To determine the phase, I use the following code:
if(captured value >= 5000000)
phase = 10000000 - captured value;
else
phase = -captured value;
with the aid of this bit of code, my phase value is always perfectly in the
interval -5000000 to +5000000. My test today was to let the OCXO run with a
fixed voltage at its EFC input, while I recorded the resulting phase data.
My recorded data is here:
https://hb9fsx.ch/files/gpsdo/phase.txt
Now I made two plots (here: https://hb9fsx.ch/files/gpsdo/phase.png). The
upper is the raw phase data, and I also fitted a linear function to it
(i.e. if the OCXO has a constant frequency but has some offset from 10 MHz,
the phase would drift linear into one direction - at least I expect it to
do so).
The lower plot is what results when I subtract my linear fitted function
from the measured phase. I am wondering whether this makes some sense? One
can see some wiggles and ripple, especially between sampel 0 and 3000. Is
this the GPS jitter I see there?
Further, I also imported this phase data into Timelab and determined the
ADEV. Here is my plot:
https://hb9fsx.ch/files/gpsdo/timelab.png
This trace is kind of what I expect, but since I use a OSA/UCT 8663 double
oven OCXO, I would have expected it to be much lower. Something in the
1e-12 regime perhaps? is that realistic? As one can see at tau=800s, the
ADEV is around 5e-11, so is this really all I can expect from this very
sophisticated OCXO? (However I should maybe add that the OCXO is brand-new
and has never been used before, so maybe it has some initial aging?).
I have no plots about the temperature, but this is quite constant.
For the GPS module, I used an active outdoor antenna at the window ledge.
Not optimal, but better than an indoor desktop antenna, I guess.
Is this some plausible data I have here, or do I measure complete crap?
It would also be great if someone would look at my code. Maybe I am doing
something wrong when I read out the TDC7200?
The code is here https://hb9fsx.ch/files/gpsdo/code/ and the most
important function is perhaps get_tic().
I am looking forward to any good hints on this! :-)
best
Tobias
HB9FSX
Hi
On Mar 19, 2020, at 3:32 PM, Tobias Pluess tpluess@ieee.org wrote:
Dear colleagues,
as you may remember I am still working on my own GPSDO project.
So far I have populated all the components on the PCBs I made, and I have
written some very basic software to test everything. Currently, I am
running following simple test:
I set my DAC to a constant value (5 Volts at the EFC input of the OCXO),
and then I send the phase measurement to the PC via RS-232 where I can log
it. More on that later.
Here is my schematic (not much has changed since last time):
https://hb9fsx.ch/files/gpsdo/gpsdo_schematic.pdf
The GPS phase detection happens on page 3. I use the TDC7200 as
interpolator.
Inside the STM32 microcontroller, I let a timer run with the 10MHz clock.
The timer overflows to 0 when it reaches the value 9999999.
Further, the 1PPS pulse from my GPS module triggers the interpolator, and
is delayed by max. 2 clock cycles. The delayed signal stops the
interpolator and is used as a capture event of the timer.
So, when the OCXO has the right frequency, the captured value would always
be the same. To determine the phase, I use the following code:
if(captured value >= 5000000)
phase = 10000000 - captured value;
else
phase = -captured value;
with the aid of this bit of code, my phase value is always perfectly in the
interval -5000000 to +5000000. My test today was to let the OCXO run with a
fixed voltage at its EFC input, while I recorded the resulting phase data.
My recorded data is here:
https://hb9fsx.ch/files/gpsdo/phase.txt
Now I made two plots (here: https://hb9fsx.ch/files/gpsdo/phase.png). The
upper is the raw phase data, and I also fitted a linear function to it
(i.e. if the OCXO has a constant frequency but has some offset from 10 MHz,
the phase would drift linear into one direction - at least I expect it to
do so).
The lower plot is what results when I subtract my linear fitted function
from the measured phase. I am wondering whether this makes some sense? One
can see some wiggles and ripple, especially between sampel 0 and 3000. Is
this the GPS jitter I see there?
Further, I also imported this phase data into Timelab and determined the
ADEV. Here is my plot:
https://hb9fsx.ch/files/gpsdo/timelab.png
This trace is kind of what I expect, but since I use a OSA/UCT 8663 double
oven OCXO, I would have expected it to be much lower. Something in the
1e-12 regime perhaps? is that realistic? As one can see at tau=800s, the
ADEV is around 5e-11, so is this really all I can expect from this very
sophisticated OCXO? (However I should maybe add that the OCXO is brand-new
and has never been used before, so maybe it has some initial aging?).
I have no plots about the temperature, but this is quite constant.
For the GPS module, I used an active outdoor antenna at the window ledge.
Not optimal, but better than an indoor desktop antenna, I guess.
That all looks like the raw / uncorrected for sawtooth output of a GPS module.
Bob
Is this some plausible data I have here, or do I measure complete crap?
It would also be great if someone would look at my code. Maybe I am doing
something wrong when I read out the TDC7200?
The code is here https://hb9fsx.ch/files/gpsdo/code/ and the most
important function is perhaps get_tic().
I am looking forward to any good hints on this! :-)
best
Tobias
HB9FSX
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On Donnerstag, 19. März 2020 20:32:30 CET Tobias Pluess wrote:
My recorded data is here:
https://hb9fsx.ch/files/gpsdo/phase.txt
Now I made two plots (here: https://hb9fsx.ch/files/gpsdo/phase.png). The
upper is the raw phase data, and I also fitted a linear function to it
(i.e. if the OCXO has a constant frequency but has some offset from 10 MHz,
the phase would drift linear into one direction - at least I expect it to
do so).
The lower plot is what results when I subtract my linear fitted function
from the measured phase. I am wondering whether this makes some sense? One
can see some wiggles and ripple, especially between sampel 0 and 3000. Is
this the GPS jitter I see there?
I'd say so, yes. What type of receiver did you use? You mentioned a Ublox NEO-
M8T, did you trigger a self-survey when you started the measurement?
It looks like the receiver entered position-hold around 3100 seconds into the
data file. At least that would explain why the wiggles became much less
pronounced. Also, you're evidently not correcting the PPS quantization error
yet.
Further, I also imported this phase data into Timelab and determined the
ADEV. Here is my plot:
https://hb9fsx.ch/files/gpsdo/timelab.png
This trace is kind of what I expect, but since I use a OSA/UCT 8663 double
oven OCXO, I would have expected it to be much lower. Something in the
1e-12 regime perhaps? is that realistic? As one can see at tau=800s, the
ADEV is around 5e-11, so is this really all I can expect from this very
sophisticated OCXO? (However I should maybe add that the OCXO is brand-new
and has never been used before, so maybe it has some initial aging?).
I have no plots about the temperature, but this is quite constant.
For the GPS module, I used an active outdoor antenna at the window ledge.
Not optimal, but better than an indoor desktop antenna, I guess.
Is this some plausible data I have here, or do I measure complete crap?
I'd say it's plausible. I got similar results with my Rb-GPSDO when doing the
same test.
https://www.eevblog.com/forum/projects/diy-gpsdo-project-w-stm32-tdc7200/
msg2860766/#msg2860766
You cannot expect to see the OCXO for small tau if your reference is the GPS.
The GPS 1PPS jitter and the quantization errors will dominate the ADEV graph
for small tau. Once you correct the quantization error (you need to parse the
UBX-TP5 message for that) your 1s ADEV will be in the 1e-9 range.
BR,
Matthias
On Donnerstag, 19. März 2020 22:17:22 CET Matthias Welwarsky wrote:
You cannot expect to see the OCXO for small tau if your reference is the
GPS. The GPS 1PPS jitter and the quantization errors will dominate the ADEV
graph for small tau. Once you correct the quantization error (you need to
parse the UBX-TP5 message for that) your 1s ADEV will be in the 1e-9 range.
That would be UBX-TIM-TP, not UBX-CFG-TP5. My bad.
BR,
Matthias
On Thu, 19 Mar 2020 20:32:30 +0100
Tobias Pluess tpluess@ieee.org wrote:
The GPS phase detection happens on page 3. I use the TDC7200 as
interpolator.
Inside the STM32 microcontroller, I let a timer run with the 10MHz clock.
The timer overflows to 0 when it reaches the value 9999999.
Further, the 1PPS pulse from my GPS module triggers the interpolator, and
is delayed by max. 2 clock cycles. The delayed signal stops the
interpolator and is used as a capture event of the timer.
So, when the OCXO has the right frequency, the captured value would always
be the same. To determine the phase, I use the following code:
if(captured value >= 5000000)
phase = 10000000 - captured value;
else
phase = -captured value;
with the aid of this bit of code, my phase value is always perfectly in the
interval -5000000 to +5000000. My test today was to let the OCXO run with a
fixed voltage at its EFC input, while I recorded the resulting phase data.
My recorded data is here:
I guess the units here are cycles of 10MHz/100ns?
BTW: keep in mind that you might run into metastability effects.
Depending on what the exact phase relation between your 10MHz
signal and the internal clock of the STM32 is, you might or might
not get a +/-1 cycle uncertainty due to metastability.
Now I made two plots (here: https://hb9fsx.ch/files/gpsdo/phase.png). The
upper is the raw phase data, and I also fitted a linear function to it
(i.e. if the OCXO has a constant frequency but has some offset from 10 MHz,
the phase would drift linear into one direction - at least I expect it to
do so).
The lower plot is what results when I subtract my linear fitted function
from the measured phase. I am wondering whether this makes some sense? One
can see some wiggles and ripple, especially between sampel 0 and 3000. Is
this the GPS jitter I see there?
Yes, it makes sense. I would have done a quadratic fit, as your OCXO
is just starting up and thus will have a "large" linear frequency drift,
which results in a quadratic phase drift.
The wiggles in the beginning could be uncorrected saw-tooth as Bob mentioned.
That would aproximately fit the quantization that the u-blox modules have
from their 24MHz clock. The change in the wiggles could be hanging bridges,
but it's hard to see.
With your antenna position you will also get quite high jitter on the PPS
due to not many satellites being visible. Make sure you are running on
a well surveyed position hold to mitigate this. Or get your antenna outside.
See also the measurements that Said Jackson did a few years ago:
http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2012-December/055731.html
http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2012-December/055774.html
BTW: there were some quite interesting discussions about GPSDOs in
late 2012 and early 2013. Might be worth a read.
Further, I also imported this phase data into Timelab and determined the
ADEV. Here is my plot:
https://hb9fsx.ch/files/gpsdo/timelab.png
This trace is kind of what I expect, but since I use a OSA/UCT 8663 double
oven OCXO, I would have expected it to be much lower. Something in the
1e-12 regime perhaps? is that realistic?
You are measuring your OCXO against GPS. At short tau, GPS is going
to dominate your ADEV. See http://www.leapsecond.com/pages/m12-adev/
for reference.
BTW: You can tell the difference in MDEV. At taus >1s the OCXO
will be clearly in the flicker phase regime, while GPS will still
be in the white phase regime up to 1000-10ks.
The Timing Appnote of u-blox GPS.G6-X-11007 might give you additional
information on the expected behaviour of the u-blox modules:
https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_(GPS.G6-X-11007).pdf
To see the actual performance of the system, you need to measure it
against a stable reference. If you have I would recommend using
a combination of 2 or more references: one stable, free running
OCXO (e.g. OSA 8607 or FE-405), an Rb standard (LRPO or PRS-10).
This way you have something that is more stable than your GPSDO
at (almost) all taus. Of course, to be sure that what you are
measuring is actually the performance of the GPSDO and not your
reference, you need to have at least 2 of each type and then
do an n-cornered hat. And yes, beyond one day, this will not
help you as the instability of the Rb standard will be the
limiting factor. That's when take out your Cs beam standard
and start measuring for real ;-)
And yes, characterizing your GPSDO is the biggest problem
of building one. Soon you will be building lots of
measurment equipment to characterize your GPSDO, get more
gadgets and gear until your garage is full of atomic clocks :-)
As one can see at tau=800s, the
ADEV is around 5e-11, so is this really all I can expect from this very
sophisticated OCXO? (However I should maybe add that the OCXO is brand-new
and has never been used before, so maybe it has some initial aging?).
Have you had a look at John Vig's Oscillator tutorial? There
are a few versions of it on Didiers site:
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=02_GPS_Timing/John_Vig_Tutorials_on_Crystal_Oscillators
It explains how crystal oscillators start up and how their frequency
changes over time. BTW: the newer ones focus slightly more on atomic
clocks, which makes it worthwhile to have a look at the older ones
for this kind of thing.
I have no plots about the temperature, but this is quite constant.
For the GPS module, I used an active outdoor antenna at the window ledge.
Not optimal, but better than an indoor desktop antenna, I guess.
Good sky view is paramount for a good GPSDO performance. Whether
you see only half the sky or have a full view already makes a big
difference. When I got my institute to disregard the fire/lightning
protection rules and let me put an antenna on the roof instead of
just out of my window (looking straight south), I saw almost an
order of magnitude improvement in adev at short tau.
Attila Kinali
--
Science is made up of so many things that appear obvious
after they are explained. -- Pardot Kynes