[time-nuts] GPSDO control system
lists at rtty.us
Sun Mar 23 10:31:56 EDT 2014
One thing that should be noted. Some of this stuff has zero impact at long tau. Some of it only has an impact at long tau. It might be easier to split a model into long and short versions to deal with this stuff.
Getting the data is the next step. You can find app notes on some voltage references that show things like hysteresis, warmup and retrace. Getting the same data on DAC’s or resistors is a bit tough. If you use ceramic caps, you can find non-linear model data on them (sort of). Even with the data, turning it into something that fits in a model isn’t easy at all. When you find that your <2 ppm reference has >40 ppm of hysteresis, including that in the model might move up your list a bit….
On Mar 23, 2014, at 10:02 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> On 23/03/14 14:02, Bob Camp wrote:
>> The real answer is always “that depends”.
>> 1) How much does the sensitivity of your OCXO change with a change in EFC? 1.4:1, 2:1, 4:1 …. (slope sensitivity not % linearity)
>> 2) How quiet is your DAC compared to your OCXO?
>> 3) How quiet is your reference compared to your OCXO?
>> 4) How much do the DAC, reference, op-amps, resistors, capacitors, … drift with time?
>> 5) How much test time is enough? (hours, days, weeks ,…..)
>> 6) How good is the survey on your GPS this time?
>> 7) How much does your room temperature impact your OCXO when you do this or that?
>> 8) Is your room temperature representative of the real world? (is mine like yours?)
>> 9) Do you intend this gizmo to work over a temperature range? Did you test that range?
>> 10) Are you trying for best frequency or best time? Is your definition of time “GPS local time”?
>> 11) Are there voltage drops on your real board? Do they change with anything? (or everything ?)
>> 12) Does your controller generate spurs inside the control loop and modulate the output with them when tuned to an offset of x.xxxxxxxxxxxxxxxxxx Hz?
>> 13) How do things respond to load changes or supply voltage changes?
>> 14) Are the parts (OCXO, reference, dac, op amps …) responses to temperature, load, supply, tip, tune, linear / immediate or do they have artifacts that extend out over longer time periods?
>> This is by no means a complete list. A lot of common GPS issues are notably absent. However, I’ve seen designs fail or fall short for problems related to every item on that list. Can you put this all in a model - sure. Did you put all this in the model .. ..
> 15) As the AC shifts voltage, how does that change the power consumption in the PSU and hence change the heating in or near your box?
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