Author Topic: Lars DIY GPSDO with Arduino and 1ns resolution TIC  (Read 73777 times)

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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #500 on: July 19, 2020, 06:45:04 am »
@cdev,

I have found that boxing in the OCXO with dense insulation of 10-15 mm thick walls works really well separating the ambient temperature changes to that of the OCXO. Note that I also use the insulation between the PCB and the bottom of the OCXO. I extended the leads so I could do that. Furthermore, OCXO's with a thick/double casing like the Oscilloquartz, Bliley and Trimble are much, much better in this respect as well.

The OCXO temperature measured by the sensor inside the insulated box still changes somewhat with the ambient temperature, but the effects on the DAC are barely recognizable or so small that there is no need to use temperature compensation.

Below is my Oscilloquartz version.
« Last Edit: July 20, 2020, 06:42:42 am by Dbldutch »
 

Online bingo600

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #501 on: July 20, 2020, 04:59:34 am »
@Dbldutch

Any plans on publishing the Schematic + PCB ?

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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #502 on: July 20, 2020, 06:38:45 am »
@Bingo

Yes I will do so pretty soon. I'm on vacation right now while I'm still collecting data from my three OCXO's.
I will publish my findings on my own blog to not pollute this thread and when I'm done I will announce that here.
 

Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #503 on: July 31, 2020, 01:35:42 pm »
As I promised, here is the link to my own blog where I will describe my project and findings in more details, without "polluting" this blog.
I will be adding information as I go, since there is a lot to share and learn.

I would like to thank several of you for your generous support in picking me up when I stumbled or fell.

Here is the blog post: http://www.paulvdiyblogs.net/2020/07/a-high-precision-10mhz-gps-disciplined.html
 
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Offline cdev

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #504 on: July 31, 2020, 06:07:59 pm »
I've been saving foam boxes to build a thermally managed enclosure at some point. Some time ago Lief Asbrink demonstrated a method for keeping his miri sdr dongle thermally stable, he buried it in a bucket of fine sand. He found that worked really well. The only thermally stable place in my house is the basement, so I am thinking it might be interesting to see if putting a GPSDO there made for a more stable environment.

@cdev,

I have found that boxing in the OCXO with dense insulation of 10-15 mm thick walls works really well separating the ambient temperature changes to that of the OCXO. Note that I also use the insulation between the PCB and the bottom of the OCXO. I extended the leads so I could do that. Furthermore, OCXO's with a thick/double casing like the Oscilloquartz, Bliley and Trimble are much, much better in this respect as well.

The OCXO temperature measured by the sensor inside the insulated box still changes somewhat with the ambient temperature, but the effects on the DAC are barely recognizable or so small that there is no need to use temperature compensation.

Below is my Oscilloquartz version.


I am thinking about building Lars's gpsdo in part so I can a second point of reference, to evaluate my surplus telcom GPSDO, and also because. I have almost all the parts already, with the exception of the OCXO.  Because part of the reason I am interested in the project is "affordable science" I dont want to spend a lot on the OCXO.

Maybe $15-20 tops.  I plan on using a navspark mini for the GPS. Do you think the Bliley you are using is a good choice?

I think spending much more than that would defeat my purpose of showing how it can be done for cheap. The navspark doesnt make sawtooth correction or anything like that available but it does let you tell the kalman filter its stationary.  And it does keep a stable 1PPS even when it is deprived of most of the signal (one way to do that is put it in a antistatic bag. It is remarkably sensitive so works almost anywhere where there is any signal at all.
« Last Edit: July 31, 2020, 06:24:59 pm by cdev »
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #505 on: August 01, 2020, 08:02:48 am »
Hi cdev,

Quote
Do you think the Bliley you are using is a good choice?

The Bliley NV47M1008 I finally got, after receiving 2 failing ones, is the best of the 5 OCXO's that I currently have.
It has some strange jumps, but before and after the jumps it is amazingly stable with no temperature effects on the DAC at all.
I got mine for about $20-25
« Last Edit: August 01, 2020, 08:09:51 am by Dbldutch »
 

Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #506 on: August 01, 2020, 08:35:43 am »
Correction from post 108:
Quote
The OCXO temperature measured by the sensor inside the insulated box still changes somewhat with the ambient temperature, but the effects on the DAC are barely recognizable or so small that there is no need to use temperature compensation.

After running the three OCXO's for several weeks, including with higher outside temperatures (very warm weather), I noticed that both the Oscilloquartz and the Trimble had some small temperature effects on the DAC. My assumption is that this effect will be less again when the outside temperature drops back, making the delta difference larger. This is probably the reason why I didn't notice it earlier, with lower outside temperatures.

I used the advised calculation described by Lars to determine the tempRef and tempCoeff values. For my Trimble that turned out to be a tempRef of 521 (daily avg OCXO temp of 52.17 degrees C), and a tempCoef value of 400. I still noticed some effects on the DAC, so the next day I increased that value first to 450, and a day later to 500.

The Oscilloquartz tempRef was set to 515 (daily avg 51.52 degrees C) and a calculated tempCoef value of 175. That did it, the DAC curve is no longer following the daily OCXO temp changes.

The Bliley still has absolutely no temperature effects on the DAC whatsoever. Not even a single DAC count, and that with a gain of 902. Splendid!

 

Online thinkfat

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #507 on: August 01, 2020, 08:48:42 am »
The Oscilloquartz tempRef was set to 515 (daily avg 51.52 degrees C) and a calculated tempCoef value of 175. That did it, the DAC curve is no longer following the daily OCXO temp changes.

That's a bit odd. The DAC temperature compensation should not stop the DAC from following the temperature changes, rather it should stop the TIC (and thus, the frequency) deviating in the presence of temperature changes. If the DAC is no longer correcting temperature effects, the frequency would change, this should be visible in the TIC values and cause the control loop to compensate (potentially driving it into unlock).

Maybe the DAC value Lars' firmware is reporting has the temperature compensation already subtracted?
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Offline cdev

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #508 on: August 01, 2020, 01:38:17 pm »
Thank you for the reminder to check it thoroughly when it arrives!

Are there any other cheap OCXOs you would recommend that are comparable to the Bliley, in your opinion?

Hi cdev,

Quote
Do you think the Bliley you are using is a good choice?

The Bliley NV47M1008 I finally got, after receiving 2 failing ones, is the best of the 5 OCXO's that I currently have.
It has some strange jumps, but before and after the jumps it is amazingly stable with no temperature effects on the DAC at all.
I got mine for about $20-25
« Last Edit: August 01, 2020, 02:31:32 pm by cdev »
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #509 on: August 02, 2020, 08:19:54 am »
@thinkfat : Yes that is the case, I verified that in the code. The DAC calculation takes the temp compensation into account. This means that the DAC curve no longer follows the OCXO temperature curve while still compensating the TIC differences. I'll put some example graphs in my blog soon.

@cdev: The Bliley comes in several versions, with sine wave and square wave out. They can be obtained rather inexpensively. I have used this seller a lot. https://www.ebay.com/itm/1-Bliley-NVG47A1282-10MHz-5V-Sine-Wave-OCXO-Crystal-Oscillator/332416522967?hash=item4d65919ad7:g:~ecAAOSwPDRfGQUS Both the Trimble and especially the Oscilloquartz are popular and therefore have higher prices. I have not tried other "heavy"or thick cased OCXO's. My CTI and IsoTemp OCXO's both have very thin casings and are therefore very sensitive to ambient temperatures, even when isolated, which is why I don't use them at this moment. My setup and environment (put everything in a sand box in the basement?) has to be changed considerably for me to use them.
 

Offline cdev

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #510 on: August 03, 2020, 10:50:45 pm »
Bliley is the brand that my True Position GPSDO uses (its a NVG47A1282) , and as far as I know the True Position's accuracy is very good, so it will be interesting to see how the two compare if they do.

The True Position is telecom surplus GPSDO that was used in a TDOA system for locating 911 phone callers physical location from the time the signals arrived. They typically lived in either dispatcher setings, or often, at cell phone sites, typically cabinets on telephone poles that were exposed to a wide variety of weather and still maintained a high level of precision, good enough for TDOA.

@thinkfat : Yes that is the case, I verified that in the code. The DAC calculation takes the temp compensation into account. This means that the DAC curve no longer follows the OCXO temperature curve while still compensating the TIC differences. I'll put some example graphs in my blog soon.

@cdev: The Bliley comes in several versions, with sine wave and square wave out. They can be obtained rather inexpensively. I have used this seller a lot. https://www.ebay.com/itm/1-Bliley-NVG47A1282-10MHz-5V-Sine-Wave-OCXO-Crystal-Oscillator/332416522967?hash=item4d65919ad7:g:~ecAAOSwPDRfGQUS Both the Trimble and especially the Oscilloquartz are popular and therefore have higher prices. I have not tried other "heavy"or thick cased OCXO's. My CTI and IsoTemp OCXO's both have very thin casings and are therefore very sensitive to ambient temperatures, even when isolated, which is why I don't use them at this moment. My setup and environment (put everything in a sand box in the basement?) has to be changed considerably for me to use them.
« Last Edit: August 04, 2020, 02:00:28 pm by cdev »
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #511 on: August 04, 2020, 01:40:26 pm »
After setting the temperature compensation and running with it for a few days, I noticed a strange side effect.
Here are the graphs for the Trimble, with the temperature compensation in effect:

[attach=1]

Note the Ambient and OCXO oven temperature graphs on the bottom first. When the temperature drops a bit around 30.000 seconds, there is a sudden steep rise in the oven temperature, until about 64.000 seconds when there is a sudden and steep drop. This is unrelated to the room temperature! These transitions can also be observed in the ns and DAC graphs. So despite the fact that the DAC graph is much flatter due to the compensation, there are some pretty serious side effects. The Oscilloquartz has exactly the same behavior.

As a result, I decided to disable the temperature compensation again. These two OCXO's don't really need it anyway.
 

Online thinkfat

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #512 on: August 04, 2020, 04:46:43 pm »
I don't quite understand - are you saying the temperature compensation has an effect on the OCXO temperature? Or that there is a dip in phase error and DAC when the temperature changes?
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #513 on: August 05, 2020, 09:08:45 am »
thinkfat, it is indeed very strange but after spending more time on it, I may have an explanation  :-//

Here are some observations to see if we can shed some more light on this.

Normally, and without temp compensation, all three GPSDO's virtually show the same daily ambient and OCXO temperature curves.
It's very hard to put my finger on it due to the fuzzy graphs, but I have to assume that the OCXO temperatures follow the ambient temperatures. They transition at the same time, but since the delta excursion for the ambient is a little bit larger than that of the OCXO, that must be the lead. That makes sense and is to be expected.

Below are the Bliley temperature curves during the same period as the previously shown curves of the Trimble with the temp comp.
The Trimble should have been the same. However, just when the temperature compensation was activated several days ago, the daily curves changed as I show in the Trimble example graphs above. These are very strange changes I cannot explain yet. I also looked in more detail to the Oscilloquartz, because it behaved different after the compensation as well, but it is not the same as the Trimble.

Also below is a zoomed-in portion of the Trimble graphs above.
Note that the transition in the OCXO temperature starts at point 338, followed by the gradual transition in the ambient temp that starts at 342 and the dip in the DAC that starts at point 347. These graphs seem to indicate that the OCXO temperature is the lead and that seems to indicate that it's a Trimble issue.

The big question still is if it's related to the activation of the temp compensation (unlikely because how?) or is it a strange coincidence that the Trimble itself suddenly behaves weird?
I don't know yet, it will take a few days of further observation to see if it happens again.

I'm also going to add more temp sensors to figure out where the ambient temp changes are related to. I doubt if it is driven by the room temperature alone because the ambient variations inside the enclosure are larger than those of the room temperature. I'll also put a sensor on top of the Arduino CPU.



« Last Edit: August 05, 2020, 09:11:08 am by Dbldutch »
 

Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #514 on: August 05, 2020, 11:38:41 am »
Initially, I thought the Oscilloquartz had the same problem as the Trimble, because it also showed this strange jump in the temperatures after I initiated the temp compensation. However, on careful examination, it appears that this jump is driven by the ambient temperature. It starts 2 seconds before the OCXO temp follows suit. I see no evidence that the ambient temperature is driven by the room temperature, so what is causing this???

Very weird coincidence!

As I mentioned, I need to add more temperature measurements to learn more.
 

Online thinkfat

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #515 on: August 05, 2020, 11:44:31 am »
You mentioned there's an isolating box around the devices, right? So, it must be something happening in that box, a temperature jump of such magnitude can hardly originate from outside.
I have seen such jumps with my own GPSDOs, but they're just on my workbench, without any protection against air drafts and completely exposed to ambient.
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #516 on: August 05, 2020, 11:57:44 am »
@thinkfat,
It's a pretty serious box made from 10-15mm hi-density foam. Look at the picture at post #500.
There are no leaks in the box.

The logical conclusion is that it must come from the OCXO itself.
 

Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #517 on: August 05, 2020, 01:24:47 pm »
What just dawned on me...

Could it be that the internal OCXO oven controller switches to a higher temperature range when the delta with the ambient temperature becomes too small?
That would make sense, if it is really that smart and the frequency is not changing because of this...

Can somebody maybe confirm this?
 

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #518 on: August 05, 2020, 02:02:44 pm »
Lots of the OCXO's are not made to be wrapped in extra thick foam (isolation) , they can't "Dump their heat when needed.
A cardboard box (or just inside a box)  to isolate from "draft" is recommended , but take care when using thick isolation.

/Bingo
 
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Online thinkfat

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #519 on: August 05, 2020, 04:55:04 pm »
Are these double oven types? Maybe something triggers the secondary heater, an ambient temperature drop perhaps, and then the isolation is in the way of dumping the excess heat
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #520 on: August 05, 2020, 08:00:22 pm »
@Bingo600,
Thanks for the heads-up. I started with the insulation project when I had effects I could not explain. Maybe now that everything is working as expected, I should revisit the insulation part. Before I do however, I want to learn more about the temperature sources and development inside my enclosures. There are still a few weird things going on.

@thinkfat,
The Trimble seems to be a double oven type, the Oscilloquartz apparently is not, but both have these issues.

I'll be back with more data...
 

Offline Johnny B Good

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #521 on: August 06, 2020, 01:02:08 am »
 I believe you may be overthinking this 'problem' (mainly on account you seem to be monitoring irrelevant data such as the OCXO's case (?) temperature - it's not entirely clear what's being monitored here - readings far too low to be actual oven temperatures and the "ambient" temperature (is this actual room temperature or that of the air within the enclosure?).

===========================================================================================
[EDIT 2020-08-07]

 My bad! :-[ :palm: That data is only irrelevant in the case of a James Miller styled GPSDO design such as I've used in my MKs I and II builds. I apologise for belittling what is actually a vital sensor component in a micro-controller based design. Monitoring the OCXO's case temperature provides a 'catch all' temperature monitoring point to compensate for all sources of temperature variation.

[END_EDIT]
===========================================================================================

 The oven control (going by the way my own CQE OCXOs respond to even the briefest of interruptions to their 12v supply) is done using PWM of a constant current supply to the heater (actually a power transistor, not a Ni-Chrome wire heating element) controlled by a simple minded microcontroller algorithm maintaining a thermistor at a fixed resistance value that corresponds to a precise target temperature (to better than +/- 0.5 deg C), usually around the 65 to 85 deg C mark.

 I say "simple minded microcontroller algorithm" on account of the way it seems to assume every 'reboot event' can only mean it must be starting from stone cold so turns the heater up to "eleven" whilst it runs through the rest of its reboot initialisation sequence before doing the digital equivalent of exclaiming "Oh, shit! I've overcooked the XO!" and shutting off the heater current which then results in the inevitable under/overshoot wobble you'd normally only expect to witness during an actual cold startup event. :palm: :palm: :palm:

 Of course, it may only be my CQE OCXOs that use such a daft reboot algorithm that tests the depth of the water with both feet and plunges straight into its "High speed warmup routine" before testing whether such an action is actually called for. Again :palm: :palm: :palm:

 What I'm suggesting is, if possible, log the OCXO's supply rail voltage (and any other supply rails for good measure) to see whether there are any glitch events taking place to trigger these temperature anomalies (which rather look to my mind, more like software/hardware glitches than actual temperature events).

 Single ovened OCXOs are influenced by changes of temperature to their immediate surroundings when chasing down parts per trillion stability levels.  Double ovened OCXOs are influenced to a lesser extent but still not entirely immune to this effect which is why a modest amount of insulation around a single ovened OCXO can help by reducing internal thermal gradients between the XO, the heater transistor and the sensing thermistor despite the manufacturer's best efforts to keep all three tightly coupled to each other to minimise the effect of these temperature gradients.

 Exactly how much insulation can be considered "modest" rather depends on the expected range of operating temperatures within the enclosure. Given enough insulation, even the modest 145mW dissipation from the XO and its LDO (CQE example) may be enough to raise the temperature above the set point in a particularly warm running enclosure on a hot summer's day outside of an air conditioned laboratory environment (a more likely scenario for a typical DIY GPSDO).

 To my mind, the only sane way to deal with such temperature variations is to use a double ovened OCXO since trying to compensate for them requires the response to match the actual thermodynamics involved else you risk making the situation worse.

 I've been following this thread with some interest since I may find myself having to use a microcontrolled solution to improve on my current hardware PLL controlled GPSDO project where the only major issue I have is due to the unpredictable nature of the ionosphere's effect on the propagation delay of the satellite signals which manifests itself as a variation of 30 to 40ns in the case of the M8N based MK I unit and around 10ns for the M8T based MK II unit I'm currently building. Sadly, even the timing receivers when configured to a fixed location aren't immune to these imperfectly corrected for perturbations in the electron density of the ionosphere :(

 I'm now only awaiting the imminent delivery of panel mount 3.5mm stereo jack sockets to finalise my MK II build. I'm using one to provide a connection to the buffered EFC voltage and a stable 2v dc offset to let me monitor the voltage difference between them on the 1000mV scale of my cheap Mestek DM91A 9999 counts DMM in leu of spending over a hundred quid on a cheap 4 1/2 digit bench meter. I can now monitor variations in EFC voltage in 100uV increments with this cost effective solution - an order of magnitude improvement over using the DM91A just on its own.

 Obviously, with no microcontroller nor DAC, I have no simple automated way to log data to generate fancy MDEV or ADEV plots, just observations from 'scope traces comparing both GPSDOs against each other and my stand in for a Rubidium reference, the OCXO in my FY6600, trimmed to within 5ppt by exploiting the 2G tipover effect to fine tune it against diurnal temperature variations and the almost imperceptible daily ageing effect.

 It's not an ideal solution but it comes close enough to reveal the slow wobbles of phase shifting for minutes to tens of minutes variations in propagation delay from the changing ionospheric conditions. Unfortunately, the once superb stability of this free running OCXO seems to have gone all to pot this past week or so and it looks like I need to revamp the calibration trimmer circuitry (better 22 turn trim pot and more padding to limit the frequency trim range and quite possibly the use of a 5v CMOS RRO opamp to reduce slider contact resistance volt drop effects).

 OTOH, I might just build my own double ovened OCXO using one of my ample stock of 10MHz CQE OCXOs to create a stable enough independent reference to compare the behaviour of my MKs I and II GPSDOs against. I'd like to take the basic hardware PLL based GPSDO to its limit before I start experimenting with microcontroller based designs since it will provide a useful benchmark to test such a new design against.

 In the meantime, I wish you all the best with your own Lars based GPSDO design. I think you just need to step back and reconsider other possible causes for your current anomalous results. I know just how easy it is to get stuck down a dead end road where nothing seems to make sense any more. :-//

JBG
« Last Edit: August 07, 2020, 10:42:31 pm by Johnny B Good »
 
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Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #522 on: August 06, 2020, 08:13:49 am »
This is mostly for others following this blog, they may get confused by the following statement from JBG, the rest is mostly irrelevant.
Quote
I believe you may be overthinking this 'problem' (mainly on account you seem to be monitoring irrelevant data such as the OCXO's case (?) temperature

When you don't have the luxury of an atomic clock to your disposal to measure (compare) the accuracy and stability of your DIY GPSDO, you can use TimeLab to characterize your tool. Lars has several examples in his documents, go read them if this is new to you.

The way to use Timelab to characterize your GPSDO is to collect the DAC values and import them into the tool, while converting them back to a frequency representation. However, this only works when the DAC values have a 1:1 relation to the differences measured against the 1PPS coming from the GPS, measured by the TIC system in nanoseconds and the OCXO through the DAC settings.

When the DAC values are influenced by things like temperature, this will skew and distort the TIC relationship and hence the TimeLab representation is of less value. It no longer shows the true disciplined OCXO performance and stability.

The OCXO oven temperature can be monitored by Lars' program to get a handle on the temperature based deviations on the DAC, and his firmware can compensate the DAC settings by taking the OCXO temperatures into account.
The idea behind this is to get a DAC curve that is as much as possible devoid from temperature based deviations.

Below is an example of one of my GPSDO's, with the Trimble OCXO. The graphs show the DAC values, the OCXO temperature, the ambient temperature within the enclosure and the ns value coming from the TIC over a 24 hr period. The second graph shows the room temperature (in red) in my office. All temperatures show a dip of several degrees Celsius, and you can clearly see that the DAC follows this curve, which will compromise a TimeLab measurement. Note that the ns graph does not follow this.

This shows that it is very important to monitor the OCXO temperature and also find the source of deviations in order to properly compensate the DAC.

 
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Offline Johnny B Good

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #523 on: August 06, 2020, 09:50:34 pm »
 Apologies for using the phrase, " irrelevant data". Upon reflection, knowing how the seemingly small variations of room temperature can effect the OCXO in my cheap FG, monitoring the OCXO's case temperature seems about the best choice of monitoring point since changes of temperature here, once the oven has reached thermal equilibrium, will have the greatest impact on frequency stability. Not only that but it also includes not just the room temperature contribution but all other contributions from the rest of the internal components.

 From my own narrow viewpoint of a basic hardware PLL based GPSDO, it just seemed to be an inevitable variable that would be cancelled out in the disciplining process anyway. However, what had struck me about the graphed data was the suggestion that these anomalies looked more like hardware or software bugs in the logging process.

 These last graphs of the DAC values are very suggestive of variations due to ionospheric effects superimposed upon a longer term temperature trend (trying to interpret their meaning tends to leave me with a bit of a headache :-// ).

 Although the contrast between the Lars design being discussed here and my Gyro inspired version of the James Miller "GPS Stabilised 10 MHz Oscillator", described here : -http://www.leapsecond.com/pages/gpsdo/  as:-

"This is about the simplest design possible for a GPSDO; with no DAC, no Vref, no time interval counter, no microprocessor. Just an XOR gate acting as a phase comparator against the 10 kPPS output of a Jupiter GPS engine. In spite of this simplicity, the performance is amazingly good"

couldn't be any greater, I thought my own observations, such as they are, might offer some small insight into this problem of inexplicable temperature excursions, even if only by way of 'light relief'.

 As you pointed out, in the absence of such data to feed into mathlab, the only other way left open to gather meaningful statistics is to employ an atomic class standard such as a Rubidium oscillator. I'd love to possess one but long gone are those happy days when it had been possible to get hold of used units for just 100  to 200 quid. Since I'm not ready to justify the hundreds of pounds expenditure on one, I'm trying to make do with second best (a very finely trimmed OCXO).

 My recent past experience with the OCXO installed in my cheap FG suggests I could DIY a double ovened OCXO using one of the spare 10MHz CQE single ovened OCXOs for this project which could perform better than what I'd been able to achieve with my cheap FG setup and perhaps suffice as a cheap stand in for the ideal of a Rubidium oscillator reference.

===========================================================================================
[EDIT 2020-08-07]

 After embarking on yet another Holy Grail search for an affordable used Rubidium oscillator (10MHz), I finally located an LPRO-101 here:-

 https://www.ebay.co.uk/itm/Datum-EFRATOM-LPRO-101-Rubidium-Frequency-Standard-10MHz-with-PCB-0-95mHz-test/184374703310?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20131231084308%26meid%3Daddf23c2f69044fb9c1c50fa1541f942%26pid%3D100010%26rk%3D1%26rkt%3D12%26sd%3D181632863973%26itm%3D184374703310%26pmt%3D0%26noa%3D1%26pg%3D2047675%26algv%3DDefaultOrganic&_trksid=p2047675.c100010.m2109

 :wtf:  That's taking the piss! Here's a nice petite TinyURL  https://tinyurl.com/y23upfw5

 I've just "pulled the trigger" and for just under 200 quid delivered and customs charges pre-paid, I'll hopefully have a used but calibrated LPRO-101 in two to five weeks time. Sod the expense! It's only money after all (and I can't take it with me when I finally pop my clogs).

 I've also downloaded every relevant document on the LPRO-101 from KO4BB's very useful web site like a "Good Boy Scout".  :)

===========================================================================================

 I'll save the data gathering operation for when I'm ready to try my hand with the Lars or a similarly based design after I've completed my MK II basic GPSDO to give me that added element of interest and motivation that I now require to embark on ever more complex projects these days.

JBG
« Last Edit: August 07, 2020, 10:44:23 pm by Johnny B Good »
 

Offline Dbldutch

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Re: Lars DIY GPSDO with Arduino and 1ns resolution TIC
« Reply #524 on: August 16, 2020, 07:02:20 am »
After I took the insulation away from the OCXO's and ran the measurements again for several days, I have decided to put them back on.

Even with the DAC temperature compensation activated, the dependency of my GPSDO systems are too sensitive to the room temperature changes. So much so that I'm loosing lock constantly, while I ran for months without problems when they were boxed in. I only took the insulation away because it seemed that the OCXO oven temperature regulation was misbehaving when the room temperatures were in the order of 30 degrees C, during the current heatwave. My take on that is that maybe the delta temperatures between the ambient and the desired oven temperatures where becoming too small, and the oven jumped to a higher temperature setting.

I think I can live with that effect during a heat wave, rather then having them so sensitive to the room temperatures.

The alternative could be to put my metal enclosed GPSDO system wrapped in insulation inside another plastic enclosure to shield it better from "sudden" room temperature changes.

Has anybody collected experience using that approach?
 


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