[time-nuts] The 5MHz Sweet Spot

[Magnus Danielson]

Hi Warren,

On 11/03/2013 08:55 PM, WarrenS wrote:
>
> Said Jackson posted:
>> Crystal jumps are the biggest menace facing users of
>> crystals/oscillators today.
>
> Are you including both phase jumps and frequency jumps together?
> Is one more or likely to happen than the other?
> Is it mostly a jump that effects just the phase or freq, or is there
> everything in-between, jumps that effects both phase and freq at the
> same instant in time also just as likely?

A jump in frequency will cause the phase to go parabolic as it
integrates the frequency difference from the previous rate. Eventually
the PLL will react and hunt it it. Sluggish PLL will let the jump cause
a larger phase deviation than a quick PLL, but that is expected.

> We all know each effects the other, but that is only over time,
> instantaneously and over short time spans phase and freq jumps are
> separate things and maybe from different causes.
> A true phase jump causes only a one cycle freq error and a true freq
> offset jump does not cause an instantaneous phase jump.

That type of phase-jump sounds like a support-oscillator having issues,
possibly with not sufficient suppression of other oscillating modes  of
the crystal.

With oven crystals, there is the effect of the oven as it gets a
heat-bump och chill-bump that it takes time for the oven control to
react to it. During that time, the thermal sensitivity of the crystal
(complex issue as it is) will cause the crystal to run higher or lower
than intended, and when the oven balance up again the frequency will be
about where it was before, but as you integrate the frequency deviation,
you have a phase-step in either direction. This phase-step is then being
tracked in by the PLL if you have one. I know of only two oscillator
products (from the same vendor) that have been trying to address this
issue. It's an important aspect, as today we work with stable phase
rather than stable frequency, yet many of our stabilization techniques
is aimed at stable frequency, which is a step in the right direction. To
make things worse, we want stable phase in challenging environments
where particularly size is an issue.

>
> If the main causes of random freq jumps and random phase jumps are
> from different things, then with a high speed, high resolution detector,
> I wonder if knowing which event has really occurred, that then some
> correction compensation could be applied that does not effect the other.

Good idea. Keep monitoring the events and their characteristics and you
might find out.

I would say they have different origin. Also notice how a PLL or other
control-loop can change the characteristic of a behavior, so you need to
understand what processes does to the shape in order to draw conclusion
of phase, frequency, drift step, ramp or whatever.

> An Oversimplified example;
> A Phase lock loop does not care what the instantaneous freq is, and a
> true Freq Lock loop does not care what the phase difference is.
> With a DDS, one can change the freq without causing a phase step or it
> can cause a phase step, without causing a freq offset.
> With two variables (instantaneous phase and freq offset control) and
> two unknowns (instantaneous jumps in either), couldn't one apply a
> correction to the right place for any random step error that occurred?
> It would depend if the errors are caused by true independent random
> fast jumps or just slowly drifting interacting changes.

For a control-loop, you could naturally monitor the shape of the phase
detector deviation and draw conclusion about what just happen to your
oscillator (or input). You need to compensate or consider what the loop
does to the shape. Recording of both phase and frequency to record the
state of the loop  will aid significantly.

Cheers,
Magnus