[time-nuts] LTE-Lite module
saidjack at aol.com
Tue Oct 21 07:02:58 UTC 2014
I mentioned yesterday about integrator windup, this problem is similar but happens even without any I term present:
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.
Very similar to integrator windup, but not quite the same.
Main problem is the crystal is not following the steering input.
Most TCXOs and cheap ocxos have this problem, and there is no way to do anything about it since in the worst case the crystal simply refuses to run at proper frequency and thus the frequency will be approximated by cycling below and above the target frequency. Mind you we are talking about effects on the tens of parts per trillion levels. Enough to jump 10s' of ns back and forth over many minutes.
Sent From iPhone
> On Oct 20, 2014, at 23:41, "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
> In message <766D6811-F733-4AB2-8574-24E4606E45FA at aol.com>, Said Jackson via tim
> e-nuts writes:
>> Thats exactly right Bob.
>> By the time your ocxo jumps to catch up to the efc voltage, you
>> have oversteered, then the process starts in reverse and the ocxo
>> jumps in the opposite direction.
> This is a well known PI effect called "windup".
> The cause is a phase offset of opposite sign of the frequency offset.
> The fix is simple:
> Start running with only the P term, and engage the I term only after
> 1. The input phase offset changes sign
> 2a. The input phase offset levels off
> 2b. Some calibrated amount of time has passed.
> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG | TCP/IP since RFC 956
> FreeBSD committer | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
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