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

[Bob kb8tq]

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>_______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.