Lady Heather's Tbolt oscillator auto-tune function
Here's a little info on Lady Heather's oscillator autotune function for the Thunderbolt GPSDO:
The autotune function tries to optimize the settings for the oscillator disciplining parameters, antenna signal mask angle, and the signal level amplitude mask beyond what the default setting (which are more for telecom timing applications) are.
To use the auto-tune function the receiver should be warmed up and stable. Manually set the antenna mask angle to a low value (say 0-5 degrees) with the FE keyboard command and set the signal level mask to a low value (1 AMU, 20-30 dBc) with the FL command. Clear the signal level log with the CS command. Let the receiver collect signal level data for several hours (overnight is good). The data collected is used to determine where your signal levels begin to drop vs the satellite elevation angles.
Since the unit should be locked and stable, the current DAC setting is where the oscillator is at 10.0000000 MHz and will be set in EEPROM as the initial DAC setting. The tbolt uses this value to speed up locking the oscillator when powering up.
Then issue the autotune command (&a). This will put the DAC into manual control mode and step the DAC control voltage 5 mV high and then low and measure how the oscillator frequency changes. This takes a few minutes to complete. This is used to determine the oscillator voltage gain. The calculated value seems to be quite accurate. Tbolts set up for external oscillators can be used to determine unknown 10 MHz oscillator EFC characteristics.
Next, Lady Heather sets the loop time constant to 500 seconds and the damping to 1.0 These value seem to be good conservative general purpose values for a typical unit and should not cause any loop stability issues.
Finally, Lady Heather sets the antenna mask angle setting to where the signal level starts to drop off rapidly and the signal level mask value to 30 dBc (or 1 AMU unit). After the auto-tune completes, the antenna mask angle setting might need to be manually set to a lower value... it tends to find that the signal level falls off at a higher angle than one might expect. You can check the signal level vs elevation plot with the SAE keyboard command (or ZE in the next release will show the plot zoomed to full screen). High antenna mask angles cause more satellite constellation changes which is not good. Low antenna mask angles subjects the receiver to multi-path effects which is also not good. You are damned if you do and damned if you don't...
Mark wrote:
Here's a little info on Lady Heather's oscillator autotune function for the Thunderbolt GPSDO
Thanks, Mark, that is very helpful.
Accordingly, for people interested in best frequency accuracy and
stability, I suggest (1) running autotune, then (2) manually setting
damping to around 10, and (3) setting the holdover recovery parameters
manually as I described in my last post (jam synch at 65-75nS and allow
a large frequency error in recovery mode). The value autotune sets for
loop time constant (500 seconds) is a good starting point for Tbolts
with the Trimble p/n 37265 OCXO. A particular Tbolt may respond to
further optimization by tweaking the TC to match the individual OCXO (by
trial and error -- Plot ADEV, adjust TC, plot ADEV, adjust TC, etc.),
but the improvement will most likely be subtle. You should also review
the elevation mask settings and adjust if necessary.
Since the unit should be locked and stable, the current DAC setting is where the oscillator is at 10.0000000 MHz and will be set in EEPROM as the initial DAC setting. The tbolt uses this value to speed up locking the oscillator when powering up.
Actually, for best results from a cold start, it is best to set the
initial DAC voltage to whatever voltage produces 10.000000000 MHz when
the oven is cold. Setting it to the voltage that produces 10.000000000
MHz when the oven is warm guarantees that it is set wrong when the oven
is cold, so on a cold start the loop immediately goes into saturation to
slew the DAC to the voltage that does produce 10.000000000 MHz. This
slows down locking by quite a bit. Setting the initial DAC voltage so
that the frequency is 10.000000000 MHz with a cold oven allows the loop
to slowly adjust the DAC as the oven warms up, rather than racing off at
full speed to meet the warming crystal. This speeds up locking from a
cold start very substantially.
It is not clear to me how one could automate this process -- I found the
correct DAC settings by trial and error. Ideally, this would be
determined every time the Tbolt does a cold start and the new value
would take into account any crystal drift since it was last set. But LH
has no way to tell that any given start is a cold start (AFAIK), and it
would want to determine the correct cold-oven DAC voltage very quickly
after power-up from a known cold start (say, within 10 seconds). It
would be great if Tbolts had an oven on/off command, but to my knowledge
they don't. LH could possibly give the user instructions ("OK, now
power down the Tbolt for at least 30 minutes. When the Tbolt is fully
cold, click the RESUME button below and then re-apply power to the
Tbolt...").
Best regards,
Charles
Hi
On Sep 11, 2016, at 3:35 AM, Charles Steinmetz csteinmetz@yandex.com wrote:
Mark wrote:
Here's a little info on Lady Heather's oscillator autotune function for the Thunderbolt GPSDO
Thanks, Mark, that is very helpful.
Accordingly, for people interested in best frequency accuracy and stability, I suggest (1) running autotune, then (2) manually setting damping to around 10, and (3) setting the holdover recovery parameters manually as I described in my last post (jam synch at 65-75nS and allow a large frequency error in recovery mode). The value autotune sets for loop time constant (500 seconds) is a good starting point for Tbolts with the Trimble p/n 37265 OCXO. A particular Tbolt may respond to further optimization by tweaking the TC to match the individual OCXO (by trial and error -- Plot ADEV, adjust TC, plot ADEV, adjust TC, etc.), but the improvement will most likely be subtle. You should also review the elevation mask settings and adjust if necessary.
Since the unit should be locked and stable, the current DAC setting is where the oscillator is at 10.0000000 MHz and will be set in EEPROM as the initial DAC setting. The tbolt uses this value to speed up locking the oscillator when powering up.
Actually, for best results from a cold start, it is best to set the initial DAC voltage to whatever voltage produces 10.000000000 MHz when the oven is cold.
…… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the target.
Setting it to the voltage that produces 10.000000000 MHz when the oven is warm guarantees that it is set wrong when the oven is cold, so on a cold start the loop immediately goes into saturation to slew the DAC to the voltage that does produce 10.000000000 MHz. This slows down locking by quite a bit. Setting the initial DAC voltage so that the frequency is 10.000000000 MHz with a cold oven allows the loop to slowly adjust the DAC as the oven warms up, rather than racing off at full speed to meet the warming crystal. This speeds up locking from a cold start very substantially.
It is not clear to me how one could automate this process -- I found the correct DAC settings by trial and error. Ideally, this would be determined every time the Tbolt does a cold start and the new value would take into account any crystal drift since it was last set. But LH has no way to tell that any given start is a cold start (AFAIK), and it would want to determine the correct cold-oven DAC voltage very quickly after power-up from a known cold start (say, within 10 seconds). It would be great if Tbolts had an oven on/off command, but to my knowledge they don't. LH could possibly give the user instructions ("OK, now power down the Tbolt for at least 30 minutes. When the Tbolt is fully cold, click the RESUME button below and then re-apply power to the Tbolt...”).
…. It’s a little worse than that :)
Temperature does indeed get into the act, what was the temperature when the DAC setting was recorded, what is it now, what is the frequency vs temperature on that assembly? One could do a temperature run (or series of runs) and save the data. There are OCXO’s done that way.
Exactly how long has the OCXO been off? There is a difference between 3 hours and 5 hours. Similarly there is a difference between one day and a week.
What temperature (and humidity and …) has the assembly been at while off? Temperature will impact the sealed OCXO. The parts on the board most certainly are sensitive to humidity.
Did something weird happen? Was the unit rotated? (2g tip does matter). Was it dropped? Was it shaken around? There are a number of weird things that could happen. Some OCXO’s get acceleration compensated to take care of this.
Is it the DAC at 5, 10, 15 or 60 minutes? The OCXO is moving pretty fast right after it turns on. For instance, the FE 405’s compensate for this in firmware.
That’s a short list, one could easily double it in length.
=====
The simple answer is to use software. Rather than having the TBolt in one mode all the time, change it’s parameters when it first turns on. Give it a shorter TC and get it stabilized. Then feed it the “real” TC and let it go from there. I have absolutely no idea how a TBolt responds when you do this. I do have a lot of data on how other vendors parts respond when this takes place. In general it works pretty well.
Bob
Best regards,
Charles
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I wrote:
Actually, for best results from a cold start, it is best to set the initial DAC voltage to whatever voltage produces 10.000000000 MHz when the oven is cold.
Bob replied:
…… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the target.
No, DAC for 10.000000000 MHz with a stone cold oven is best for cold
starts. This setting is still best even if the oven is partly warm,
because the loop is moving the DAC in the same direction it needs to go
-- it can "catch up" gracefully without racing in the wrong direction in
full saturation to meet the crystal as it warms up, then banging back
and forth from saturation in one direction to saturation in the other
direction before finally leveling out. It is the damped oscillatory
approach to the capture zone, and recovering from saturation (not just
once, but multiple times), that slows things down.
By starting the DAC to produce 10.000000000 MHz from the cold oven, all
that is avoided and the loop just tracks the warming crystal (or, if it
can't quite keep up, at least it is moving in the same direction and can
catch up gracefully without overshoot -- still much faster than starting
from the DAC voltage that produces 10.000000000 MHz with a fully warm oven).
Trust me, I tried all of this experimentally and know whereof I speak.
Best regards,
Charles
Hi
On Sep 11, 2016, at 11:04 AM, Charles Steinmetz csteinmetz@yandex.com wrote:
I wrote:
Actually, for best results from a cold start, it is best to set the initial DAC voltage to whatever voltage produces 10.000000000 MHz when the oven is cold.
Bob replied:
…… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the target.
No, DAC for 10.000000000 MHz with a stone cold oven is best for cold starts. This setting is still best even if the oven is partly warm, because the loop is moving the DAC in the same direction it needs to go -- it can "catch up" gracefully without racing in the wrong direction in full saturation to meet the crystal as it warms up, then banging back and forth from saturation in one direction to saturation in the other direction before finally leveling out. It is the damped oscillatory approach to the capture zone, and recovering from saturation (not just once, but multiple times), that slows things down.
By starting the DAC to produce 10.000000000 MHz from the cold oven, all that is avoided and the loop just tracks the warming crystal (or, if it can't quite keep up, at least it is moving in the same direction and can catch up gracefully without overshoot -- still much faster than starting from the DAC voltage that produces 10.000000000 MHz with a fully warm oven).
Trust me, I tried all of this experimentally and know whereof I speak.
To me, “stone cold oven” = OCXO has been off power for > 24 hours and the entire internal structure is at ambient. That might be 25C, it might be something else. You applied power about 2 seconds ago and you have an output. The oven is now beginning to warm up to some temperature in the 80C to 120C range.
It’s an SC cut crystal. When the oven is at 25C (or worse, even colder) the frequency is off by > 30 ppm. The tuning range of the DAC is maybe a couple of ppm. Effectively, with an un-heated oven, there is no DAC setting. The DAC is railed and you still are not on 10 MHz exactly. As the oven warms up, the frequency is changing at ppm’s / minute sort of rates. You are not going to be able to follow that with the TBolt loop. As a practical matter, you have to let the oven warm up a bit before there even is a DAC voltage. You have to let it warm up a bit past the "EFC in range” point to be able to begin the lock process. That is likely to be some number of minutes (5, 10, 15) past the point that you apply power to a cold OCXO.
My guess is that we have different interpretations of the term “stone cold oven”.
Bob
Best regards,
Charles
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