[time-nuts] Man with too many clocks.

Nick Sayer nsayer at kfu.com
Thu Nov 3 16:50:31 EDT 2016

I’m going to try and describe my thoughts, but it may not come out as “right” as some others here can do. Still…

One problem you’re going to run into if you go down the road of attempting to PLL one thing to another is that you have to find a balance between phase control and frequency stability.

You’re going to always be reacting to the phase drift of your disciplined device. Your “knob” for doing so is to adjust (probably in steps) the frequency. If your PLL is very “twitchy,” then you’re going to move that knob very quickly and firmly, resulting in very tight phase control, but a frequency that, at least over a short term, will jump around a bit.

By contrast, if you are very reluctant to move the “knob,” then you’re going to move it so slowly that by the time you have a meaningful effect on the phase, the phase will have drifted quite a bit. That said, your movement of the frequency knob will be so slow that the frequency stability would be much better, at least over the short term.

In essence, this is choosing the PLL time constant. How you do so depends on the behavior of your device as well as the stability you desire from the output.

> On Nov 3, 2016, at 5:20 AM, Peter Reilley <preilley_454 at comcast.net> wrote:
> I am the proverbial man with too many clocks and I don't know what time it is.
> To correct this situation I have decided to calibrate everything.
> I have a HP 5370B, a HP 6370A, and a HP 5328A all with the TCXO option.   I also
> have some TCXO modules.   I figured that I would calibrate them against my Trimble
> Resolution T GPS receiver.
> I put the 1 PPS signal in one channel of my scope and one of the 10 MHz TCXO
> signals in the other channel and look at the phase relationship. The TCXO's are
> already close enough that I should not be out by more than a fraction of a waveform.
> I understand that I have to deal with the 1 PPS without sawtooth correction.
> I expected to see the 10 MHz signal bounce around but not move more than 1/2
> of a wave length.   Instead I see the 10 MHz waveform appear steady for a few seconds
> then jump a significant portion of the wave.   The jump is too much to be confident
> that I have not slipped one cycle.
> Can I do what I am trying to do or am I missing something?
> Pete.
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