[time-nuts] Assistance needed to understand some V_OCXO stability concepts.

Thu Oct 1 21:10:17 UTC 2020

Hi

I do understand what you are talking about. The only solution I’ve ever seen to this
sort of thing involves a “buddy” who happens to bring in a cargo container every so 
often. You help him with this and that, you get a couple of cubic feet of space in the
container for free ….

Good luck !!

Bob

> On Oct 1, 2020, at 3:17 PM, Joe & Gisela Noci <jgnoci at gmail.com> wrote:
> 
> Hello Bob.
> Thank you for your comments. I fear this one may then have me beat!  I stay
> in a small town on the West Coast of Namibia, and access to the kind of
> equipment and facilities you mention is impossible!
> Even trying to purchase equipment via Ebay, etc is complicated - there are
> no direct cargo flights to Namibia from most of the rest of the world, all
> is via South Africa first, and thats a mess - cost are terrible - up to
> USD450.00 for 1kg from the USA...!
> 
> Most of my equipment followed me from a RF career past, in the bigger
> corporate world, which did not include the kind of equipment 'time nuts'
> seem to require!
> Well, my GPSDO makes a nice clock on the bench, and is probably more
> accurate than my wall clock!
> 
> Thanks again.
> Kind Regards
> Joe
> 
> 
> On Thu, Oct 1, 2020 at 5:58 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>>> ……..
>>> So, I am a little lost in understanding how to characterise this animal,
>>> and how to measure what. I get the feel its performance is actually very
>>> good, but I don't know why, nor how to defend that statement.
>>> 
>>> HELP!!
>>> 
>>> …...
>> 
>> 
>> Unfortunately the only accurate way to characterize a GPSDO is to compare
>> it to “something better”. There are no internal measurements that actually
>> let
>> you validate the design. There are simply to many things going on at once
>> in
>> a newly designed GPSDO.
>> 
>> That involves a couple of things:
>> 
>> 1) A device with enough resolution and stability to do the comparison.
>> 
>> 2) A reference with good enough ADEV / accuracy / stability / performance
>> to compare to
>> 
>> 3) Time and space to do the measurements
>> 
>> 4) A good understanding of your design / measurement goals
>> 
>> Since there are no (practical) single references out there for basement
>> use that
>> meet the needs of 2 above for all tau / all specs, you generally have
>> multiple
>> references to compare to.
>> 
>> The next layer to that onion is needing to test the references to work out
>> ( = select )
>> how good they are. It is not at all uncommon to test a few dozen (up to a
>> few
>> hundred …) OCXO’s to get one that is suitable as a “reference part”. They
>> also
>> may need to be re-tested as time goes on ( I unfortunately have empirical
>> data
>> to support this … :( )
>> 
>> One possible reference setup:
>> 
>> Use OCXO(s) for phase noise, short tau ADEV
>> 
>> Use Rb(s) for medium tau (100 to 10,000 second) ADEV
>> 
>> Use a “good” Cs for long term ( or find a GPSDO you feel you can trust …)
>> 
>> The device you use for comparison may be different for phase noise than for
>> multi week runs. You may want to set up for three corner hat evaluations.
>> Budget
>> quickly limits just how crazy you can go :)
>> 
>> A possible set of things to check:
>> 
>> 1) Phase noise, at least in the 1 Hz to 1K Hz region. This will tell you a
>> lot about
>> how much your control circuits are messing with the OCXO.
>> 
>> 2) ADEV in the 1 to 1000 second range. This will take 100x 1000 = 100,000
>> sec
>> (or about a day) per run. The data will tell you a lot about the damping
>> and noise
>> contribution of your control setup.
>> 
>> Both the tests above will make more sense if compared to the numbers on
>> the “raw”
>> OCXO.
>> 
>> 3) ADEV done on runs of a few weeks each. This is the real test of the
>> accuracy /
>> performance of the device.
>> 
>> Yes this is a whole bunch of rabbit holes to go chasing down. Indeed the
>> bench setup
>> to do all this is pretty massive. Sorry about that …..
>> 
>> The “normal” outcome of testing the part is that you spot issues. You then
>> get into
>> a tweak / test / tweak / test loop. The normal design process is to spend
>>> 5X the
>> time in that loop compared to the time to get the device up and running.
>> That sounds
>> a bit nuts, it turns out to be just as true on your 20th GPSDO design as
>> on your first one ...
>> 
>> Bob
>> 
>> 
>> 
>>> 
>>> 
>>> On Thu, Oct 1, 2020 at 10:33 AM Joe & Gisela Noci <jgnoci at gmail.com>
>> wrote:
>>> 
>>>> Good Morning all.
>>>> I am new to the group, very green as to how the group/mailing, etc
>> works,
>>>> so if I mess up, please put me straight, gently...!
>>>> I do not have much experience or knowledge in this field, but have spent
>>>> the last year or so building a GPDSO and need some help in understanding
>>>> the results I get - they seem to good to be true, so I suspect it's not
>>>> true!
>>>> I have built the unit based on the Brooks Shera concept, with some
>> changes
>>>> in collaboration with Jeff, K6JCA.
>>>> To describe it would be best done by means of a block diagram ( can I
>> just
>>>> add a PDF as an attachment to this mail and will everyone then see
>> it?). In
>>>> essence - a UBLOX M7N GPS provides a 1PPS signal which controls a pulse
>>>> width detector, fed from the HP_00105-6013 OCXO ( pulled from a defunct
>>>> HP-5061A) . This pulse width is converted to a DC level, into a D/A. A
>> uP
>>>> then drives a 16bit DAC to control the Varicap drive to the OCXO.
>>>> I have measured the KV of the OCXO to be close to 0.03Hz/volt.
>>>> 
>>>> The DAC drive is derived for a set of software filters, first is a 32tap
>>>> FIR, for initial stabilization, then into IIR filters of increasing time
>>>> constants, 30sec, 100sec, 200sec, 400sec.
>>>> 
>>>> I log plots of the pulse width detector, DAC voltage, Oven temp, and
>> many
>>>> other parameters.
>>>> I do not have any fancy equipment to measure Adev, etc, nor any accurate
>>>> frequency counters...
>>>> NOTE - The OCXO is a 5MHz unit, but I double to 10MHz and the ref clock
>> to
>>>> the pulse width detector is 10MHz/10 = 1MHz. So the pulse counter has a
>> max
>>>> period of 1us.
>>>> 
>>>> I understand the  KV of the OCXO works out to 3ppt / millivolt (from the
>>>> 0.03 Hz/volt).
>>>> When I run the unit, it locks very well, and while running with IIR4
>>>> active ( 400sec) is observe the following:
>>>> The DAC output voltage curve shapes closely resembles the temp control
>>>> voltage to the oven - as I would expect.
>>>> Over a 24hour period the oven control voltage varied by 0.5 volts. The
>>>> total DAC voltage during this varies by 36millivolts ( Is that 0.108ppb
>> in
>>>> delta freq?)
>>>> Over a specific period of 3 hours ( afternoon), where the temp control
>>>> voltage varied by less than 200millivolts, the DAC output varied by
>>>> 10millivolts ( is that 30ppt ??)
>>>> 
>>>> Please forgive my questions - I lack the knowledge to know if I am
>> asking
>>>> the right questions!
>>>> 
>>>> I can post some plots if it will help folk explain what I should be
>> asking!
>>>> 
>>>> Thank You
>>>> Joe
>>>> V51JN, Swakopmund
>>>> Over a
>>>> 
>>>> 
>>> <JoNo GPSDO Block diagram.pdf><Sawtooth Correction_1.jpg><15hour
>> plot.jpg><Main Page.JPG><TOP_open.jpg><TOP_Insulated OCXO.jpg><Front_SETUP
>> Page.jpg>_______________________________________________
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