[time-nuts] PLL performance?

David Scott Coburn scotttt at optonline.net
Tue Mar 21 21:47:06 EDT 2017

On Tuesday, March 21, 2017 12:22:46 PM EDT Tom Van Baak wrote:
> Hi Scott,
> That's a nice project.
> Combining quartz and pendulum like that is essentially how a GPSDO works. In
> your case, instead of a 10 MHz oscillator you have a 1 MHz oscillator and
> instead of 1PPS you have 1/2 PPS. Whether you use an analog loop or a
> digital loop there are dozens of examples on the web and hundreds of
> time-nuts postings that cover this territory.

I've looked for some, but did not find much.  I guess I am not using the right search terms?

(Lots of PLL information on the web.  Was having trouble finding others who had done this syncing of two 0.5 Hz, or similar, signals.)

> Note that you have a choice of using the hp 107a quartz oscillator to
> discipline the pendulum, or use the pendulum to discipline the quartz. Each
> project has a certain charm.

This is part of a larger long-term project to build a precision dual pendulum set.  The HP 107A will probably be involved in the instrumentation for monitoring the pendulums.  (This is the subject of another posting later on.)
> Your decision to use a 32-bit time stamping counter (TSC) is good. Pendulum
> clocks tend to be less accurate than quartz oscillators and so they tend to
> "wrap" more often. A TSC avoids the zero boundary, sign, and sample rate
> issues that can plague a traditional start/stop, aka time interval counter
> (TIC).

The TSC works very well.

> A standard deviation of ~50 counts out to 10 million counts over a day
> represents a consistency or stability of 50 / 1e7 or 5e-6 or 5 ppm at "tau"
> 1 day. I would guess the shape of your Gaussian merely reflects the loop
> parameters you have chosen, and not so much the quality of the 1 MHz quartz
> or the 0.5 Hz pendulum. For example, tighten the loop and I bet your
> histogram will narrow.
> I'm not sure of your terminology -- at one point you mention 1 MHz, then
> mention 10,000,000, then mention 10e6, which some people might read as 10^6
> as in 1,000,000 or 1e6 and others may read as 10x10^6, as in 10^7 or
> 10,000,000 or 1e7.

I guess this was a bit confusing.  The 1 MHz is generated by the VCO.  The 10,000,000 comes from counting the 5 MHz signal for 2 seconds.  The 10e6 was intended to represent 10,000,000, but I see that would have been more clear as 10^7.

> Either way, this level of performance for a hp 107 oscillator or for a
> pendulum clock seems right. I don't think there's any problem with your
> setup. Pendulum clocks can easily get to ppm levels; some even get to ppb
> levels.

I was imagining that, to a first order, the stability of the HP 107A would not show up in the histogram to a noticeable degree over the course of the day.  (If it drifts by 10^-12 during the day this would be only about 1/2 of a single 5 MHz cycle?)

I was assuming that for this purpose the HP 107A was 'perfect' and all of the jitter shown on the histogram is from the instability of the VCO.
> One suggestion is for you to make several runs against an independent
> reference: 1) hp 107A only, 2) pendulum only, 3) hp 107A and pendulum with
> PLL. When you see these ADEV plots you will get a hint of how the PLL
> should be tuned. Here's a classic example:

I don't have the pendulums yet!  :)

I did run the setup with the PLL out of the loop.  That is, just running the reference 0.5 Hz into the TSC.  This gives me an endless stream of mildly uninteresting 10^7 readings......  (However, it does show that my TSC triggering and latching circuits are working correctly.)

> http://www.thinksrs.com/assets/instr/PRS10/PRS10diag2LG.gif
> Some additional GPSDO, pendulum/PLL, pendulum ADEV links:
> http://leapsecond.com/pages/gpsdo/
> http://leapsecond.com/hsn2006/

I hope to be generating plots like this some day.

> /tvb



> ----- Original Message -----
> From: "David Scott Coburn" <scotttt at optonline.net>
> To: <time-nuts at febo.com>
> Sent: Monday, March 20, 2017 6:07 PM
> Subject: [time-nuts] PLL performance?


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