[time-nuts] GPSDO control system

Bob Camp lists at rtty.us
Sun Mar 23 09:02:15 EDT 2014


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 .. ..


On Mar 23, 2014, at 5:06 AM, Bill Hawkins <bill at iaxs.net> wrote:

> An idea is struggling to take shape in my fevered brain.  I'd like to
> check some foundation assumptions.
> 1. The difficulty with disciplining a local oscillator to a GPS signal
> is due to variations in the received GPS signal and the LO.
> 2. The variations occur slowly, as crystal aging, and quickly - perhaps
> sawtooth or crystal crack propagation - and maybe something in between.
> 3. The gain of the system, in degrees of phase angle at 10 MHz (or
> higher) per microvolt of control signal, is fairly constant in a
> controlled environment.
> 4. The power supply for the device providing the control signal cannot
> be regulated to the accuracy required of the system, and so is a source
> of variance. (Does anyone put the voltage reference device in the oven
> with the crystal?)
> 5. The principle source of environmental variation is temperature.
> Humidity and barometric pressure are not significant. This may not be
> true of the received GPS signal due to atmospheric variations.
> 6. A digital computational device is available to calculate the control
> signal from various measurements and previous values.
> 7. There are no supernatural forces at work, such as the experimenter
> mentally influencing the results. :-)
> That's a start . . .
> Thanks for any replies.
> Bill Hawkins
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