[time-nuts] Quick check of a GPS controller oscillator

Sat May 9 14:50:56 UTC 2020

Hi

You have a situation where you have only one standard. It has nothing
to compare to. It is very much like asking for the correct time from a single
watch. You can look at the various status outputs. You can look at how it
is set up. Past that, all you know is that it *thinks* it is doing ok. 

As mentioned earlier, there are a number of things you can look at and 
evaluate in the setup of the device. If they are all done properly, then it
is a good bet that the TBolt is good to < 1 ppb. 

To be *really* sure, you would need another reference or two. With one more
you can see if they agree. With two more you can let them “vote” to see which
one is most likely wrong. The sub-$100 Symmetricom or Trimble commercial
GPSDO’s on eBay are plenty good enough to do this. 

Another alternative would be a telecom Rb and a cheap GPS module. Use
the module pps and a ’scope to get the Rb on frequency. Let it run full time
and check it every few months. 

Past that, all anybody can do is make a guess. It could be a pretty accurate
guess, but none the less it is a guess. Mine is that your TBolt is doing fine and
it will give you < 1 Hz accuracy at 1 GHz. 

Bob

> On May 9, 2020, at 10:36 AM, Giuseppe Marullo <giuseppe at marullo.it> wrote:
> 
> Hi,
> could please anybody tell me if it is possible to confirm good operation of my Trimble?
> It is almost three days that is working (position hold mode, operation mode LOCKED, PLL locked see attach), want to know if it is a reliable source to be used to compare a 0.01pps oscillator.
> I see OSC < 1000ppt, does it mean it its accuracy is better than 1ppb or what?
> 
> Thanks.
> 
> Giuseppe Marullo
> IW2JWW - JN45RQ
> 
> 
> On 5/6/2020 9:23 PM, Giuseppe Marullo wrote:
>> Sorry, images where too big, had to repost.
>> 
>> Please use this link
>> 
>> https://www.iw2jww.it/trimble-and-progrock/
>> 
>> to download them, thanks.
>> 
>> On 5/6/2020 8:57 PM, Giuseppe Marullo wrote:
>>> Ok,
>>> 
>>> first of all many thanks for your  help. Yep, not exactly a time-nut piece of engineering, but I need more to be reassured about the Trimble accuracy.
>>> If the Trimble is good enough, everything will be easier.
>>> 
>>> @Alex:
>>> 
>>> >in Europe you should be able to get one DCF77 signal based 10MHz, which you could trust
>>> I am not planning to add a DCF77 synced 10MHz, I would rather add another Trimble or Rubidium.
>>> 
>>> The whole point is, given my time nuttery is limited, I could be okay with the Trimble provided it is "good enough" to consider it a suitable reference.
>>> 
>>> @Dana:
>>>> Is the ProgRock GPS disciplined or a Rb, or is it just a free-running
>>>> crystal oscillator?
>>> The ProgRock in itself is a simple Si5351A based programmable oscillator. It may have a OCXO option (that I built and using) and could use a GPS PPS to be calibrated(that I built and using, as the other pieces from QRP labs).
>>> The GPS does output NMEA sentences but the ProgRock just use the PPS signal. It is placed in a Clock kit enclosure and the clock does use/display the NMEA sentences but the ProgRock just use the PPS signal.
>>> 
>>>> And does LH say that the Trimble is locked OK?
>>> I have included a screenshot of LH(20200506LH.png) as per now.
>>> 
>>>> Unless the ProgRock is GPS disciplined (or is a Rb), it's a pretty safe bet
>>>> that it has simply grown old and the crystal has aged.  And this notion is much strengthened if LH gives
>>>> a positive indication that your Trimble is locked OK.
>>> Don't think it could be aging related, it should use the GPS to self calibrate anyway within minutes. It does/should use the GPS PPS data, saying it is disciplined may be a little optimistic.
>>> 
>>> @Bob:
>>>> Hi
>>>> 
>>>> With any GPSDO (not just the TBolt) you can do some basic checks:
>>>> (They are in no particular order)
>>> 
>>>> 1) Are there > 6 sats in view at all times, if not, move the antenna
>>> Yes they are. Antenna is Simmetricom missile head 28dB amplified (timing antenna) clear view of the sky, on a 2m pole on a balcony facing South.
>>> 
>>>> 2) Is the unit locked to at least 4 sats at all times, if not, move the antenna.
>>> Yes, see LH attachment
>>> 
>>>> 3) Has the unit completed its survey process? If not, wait for it to complete.
>>> Not at the moment I power cycled it this night. It is powered since this night (>12hours).
>>> Do I have to initiate it?
>>> 
>>>> 4) Is the EFC > 10% and < 90% of it’s range after an hour on? If not, repair the unit.
>>> I don't know what EFC is. If it is the DAC voltage, seems so (looking at its values it never went over the range but I may have not seen it happening)
>>> 
>>>> 5) Has the unit been on for at least 3 days? If not wait.
>>> Not yet, waiting...
>>> 
>>>> 6) Does the output look ok on a ’scope? ( it’s volts not milivolts …) if not repair the unit
>>> See attach(20200506_DSO.JPG), it is a vanilla scope nevertheless it seems good to me
>>> 
>>>> 7) Does the unit show “locked” or something similar? 
>>> Yes, it seems so. Operation mode: LOCKED / PLL: LOCK
>>> 
>>>> 8) Is the EFC consistent ( = stable) and not wandering around? If not repair the unit. 
>>> Can't say. If EFC is the DAC, well sort of but not exactly stable. Keep in mind I am opening and closing the window now, so there is a change in temperature.
>>> 
>>>> 9) Do the sat’s show SNR’s above 40 db? If not, replace the antenna / coax.
>>> They seem fine, according to the screenshot (C/N values, right?). Cable is 10m RG-8X, Yes it could be better but not shorter. I have some meters of Heliax (.5" or 1+5/8" AKA coke can size) not that time nut yet. They could act as a mast for the antenna, LOL.
>>> 
>>>> 10) Does it read same / same /same compared to your counter’s internal time base? 
>>>  If not, check the counter / repair the GPSDO.
>>> I would say yes. Counter does have high stability option (04E: Ultra High Frequency Ovened Oscillator <5x10 E-10, not 04R rubidium), so last digits may vary. I have included a photo of what is displayed(20200506_FC.jpg). 10.00000007 almost all the time ending with a 7 once I saw it briefly flickering to an 8.
>>> I added a second photo with overflowing the FC would squeeze out another digit and it is pretty stable at   [1]0.000000069MHz (20200506_FC_OV.jpg).
>>> 
>>>> 11) Does the reported ( survey ) location make sense on Google Maps? If not restart
>>>> the survey / repair the unit. If the unit passes all those checks, it’s a really good bet that you can trust the output. It likely is good to < 1 ppb, almost regardless of who made the device. Not every check
>>>> will be possible on every GPSDO. The checks assume you have a ’scope and a counter.
>>>> They also assume you can talk to the status port on the device. Some readings ( EFC range, SNR, may take a bit of research to put into rational units ….).
>>> At the moment it is in position holdover. Previous time was able to complete the survey.
>>> Current Reported position is correct within 5m according to Google Earth.
>>> 
>>>> ======
>>>> 
>>>> If all of that sounds like a lot of work (or not complete enough), buy a second ( or third or fourth ….) GPSDO. Next get a couple Rb’s …. maybe a Cs or three …. have you looked into Masers?
>>> I was thinking to play with Aluminum, seems the coolest thing nowadays :)
>>> 
>>> @Tim:
>>>> Another way of asking the question to the OP, is he a phase-nut or a
>>>> frequency-nut?
>>> I fear phase-nut is EVEN more costly, so I will go for frequency-nut at this time :)
>>> 
>>> @Bob(take two):
>>>> Since the ProgRock is not a GPS disciplined unit unless you add this or that to it, the best bet is that this one is simply the tuned version. Would the list apply
>>>> to a gizmo like that? Who knows. A lot depends on just how it’s cobbled together. 
>>> What do you mean by tuned version?
>>> Anyway...I am more questioning the Trimble than the ProgRock. It is clear they are not in the same league, just think that by default ProgRock adjust its frequency only when its 27MHz XTAL is off by 5 or more Hz. The only way to increase the control is to ask the MCU to calibrate every second, so you should see almost every second a frequency change!
>>> Not exactly a smooth transition (this is the current setting). sadly there is no feedback about corrections, error, lock or whatsover.
>>> 
>>> but two questions arises:
>>> 1) is MY specific Trimble accurate enough(I hope Trimble qualify for this ml)? If the answer is a "solid" yes, then the ProgRock is not accurate enough for its claimed performance.
>>> 2) Why the thing stays always about 2.5Hz below the Trimble? Shouldn't they move in relation to each other? It seems like a coding error, I am tempted to insert 10.000002MHz in the ProgRock to see if it will settle to 9.9999995MHz.
>>> 
>>> I could try to feed the PPS output of the Trimble to the ProgRock and see if accuracy improves (may not work out of the box, pulse may need to be stretched).
>>> 
>>> Giusepe Marullo
>>> CISSP - GCFA - CSSA
>>> 
>>> 
>> 
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