On 23/03/14 14:02, Bob Camp wrote:
Hi
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?
Cheers,
Magnus
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