[time-nuts] LTE-Lite module
saidjack at aol.com
Tue Oct 21 12:04:17 EDT 2014
This behavior is called hysteresis and it is related to vendors, and related to the chips used (or varactor diode) inside the tcxo/ocxo. It is so subtle that most vendors are not even aware that their oscillator is doing it. Some vendors have product lines that do it and others that don't. We have spent a lot of energy and time locating vendors and products that don't do it, but we still test for it. You can only see it when you discipline the crystal and can measure phase drift over 10's of minutes as the frequency shifts will typically be below the noise floor and masked by thermal stability of the tcxo.
For example if a crystal has 50 parts per trillion hysteresis (5E-011) this means the phase will drift back and forth at up to 0.05ns per second which means the equivalent of less than 50ns every 16 minutes or so. Depending on how fast the loop goes back and forth around this 50ppb dead zone the crystal could phase drift back and forth some 10's of nanoseconds. That makes a big difference in ADEV and standard deviation. The solution: identify vendors and products that don't do it.. This is part of the art.
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> On Oct 21, 2014, at 0:12, Hal Murray <hmurray at megapathdsl.net> wrote:
> time-nuts at febo.com said:
>> The problem is that the ocxo maintains its frequency even though the EFC
>> control voltage is changing. Thus phase error is accruing making the efc
>> larger and larger due to the P term.
>> Then at some point the crystal 'snaps' and jumps in frequency, overshooting
>> the desired frequency and causing the P term to start pushing in the
>> opposite direction repeating the cycle.
> Does anybody understand the mechanism behind that behavior?
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