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

[Bob kb8tq]

Hi

Sure, phase lock vs frequency lock is a fun twist to this. Does it matter
in the general case? 

Regardless of phase lock vs frequency lock, the error on the device should
be below  the 1 ppb  limit mentioned in the post. That’s plenty good enough 
to resolve the 0.25 ppm (ppm not ppb) errors the OP is seeing. 

Most of the frequency lock devices are down in the <0.05 ppb range for error.
In terms of a ham radio or other practical application, that sort of offset simply
does not matter. In terms of a frequency measurement, they may actually be 
more accurate than a phase lock device. 

Sure, this is Time Nuts and it’s all about finding those little errors. Most of the 
world is after a device that is ok at the ~ 1 ppb level. Most telecom GPSDO’s 
were built to a *frequency* spec around 10 ppb (which they vastly exceed in
order to hit a time based holdover spec ….). 

Does this rule out a “I designed it myself” GPSDO with problems? Of course not.
The list I gave is by no means adequate for checking out a design from scratch
device. It is a reasonable list to figure out if a stand alone GPSDO you happen to 
have lying there is working right or not. 

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. 

Bob

> On May 6, 2020, at 8:49 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> Bob, your list isn't bad, but it completely misses the point whether the
> lock is phase-lock (certainly what anyone in telecom trying to lock a wide
> network would expect) or frequency-lock (like some BG7BTL's and probably
> some other Chinese clone cobbled-together units).
> 
> A large chunk of hobbyist units out there - and I think this includes the
> Progrock that the OP has - are frequency locked, not phase locked. And they
> usually show systemic frequency offsets due to gating in the frequency
> counter.
> 
> Another way of asking the question to the OP, is he a phase-nut or a
> frequency-nut? :-)
> 
> Tim N3QE
> 
> On Wed, May 6, 2020 at 8:22 AM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> 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
>> 2) Is the unit locked to at least 4 sats at all times, if not, move the
>> antenna.
>> 3) Has the unit completed its survey process? If not, wait for it to
>> complete.
>> 4) Is the EFC > 10% and < 90% of it’s range after an hour on? If not,
>> repair the unit.
>> 5) Has the unit been on for at least 3 days? If not wait.
>> 6) Does the output look ok on a ’scope? ( it’s volts not milivolts …) if
>> not repair the unit
>> 7) Does the unit show “locked” or something similar?
>> 8) Is the EFC consistent ( = stable) and not wandering around? If not
>> repair the unit.
>> 9) Do the sat’s show SNR’s above 40 db? If not, replace the antenna /
>> coax.
>> 10) Does it read same / same /same compared to your counter’s internal
>> timebase?
>> If not, check the counter / repair the GPSDO.
>> 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 ….).
>> 
>> ======
>> 
>> 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?  :)
>> 
>> Bob
>> 
>>> On May 5, 2020, at 8:16 PM, Giuseppe Marullo <giuseppe at marullo.it>
>> wrote:
>>> 
>>> Hi,
>>> 
>>> I have a ProgRock oscillator that generates 10MHz and 125MHz signals.
>> Checking with my Trimble the ProgRock 10MHz signal is consistently below
>> 2.5Hz compared to the Trimble and also the 125MHz is below about 24Hz.
>>> 
>>> My setup:
>>> 
>>> - Simmetricon time antenna (26dB gain) in clear view
>>> - about 10m of RG8X cable
>>> - Trimble GPS (chinese reboxed)
>>> - Racal Dana 1998 frequency counter with Trimble as time base
>>> 
>>> DUT
>>> - Progrock (https://www.qrp-labs.com/progrock.html) with
>>> - ovenized oscillator (https://www.qrp-labs.com/ocxokit.html)
>>> - GPSQ board (https://www.qrp-labs.com/qlg1.html) outside on the
>> balcony, first floor facing south (almost clear view except North of
>> course). connection is made using rs232 cable (about 6m)
>>> 
>>> I tested for harmonics on my FT857D and Argo, every odd harmonic the
>> error increase. The radio has high stability but it is not exact for sure.
>> I could test harmonics with my SA but Trimble would be my only reference.
>>> 
>>> The question is: how could I be 100% sure that the Trimble frequency is
>> absolutely trustworthy? I have LH and it says oscillator is within
>> 300-1000ppt.
>>> Is it enough to conclude the frequency is okay and the ProgRock is wrong?
>>> 
>>> TIA
>>> 
>>> Giuseppe Marullo
>>> IW2JWW - JN45RQ
>>> 
>>> 
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