[time-nuts] PLL performance?

[Scott Stobbe]

Neat Project. I don't know if it will come up for you but optical or hall
rotary encoders are notorious for jitter. While a generic IC comparator may
have an open loop-gain of 100 dB, creating the mechanical equivalent is not
so easy. Hall/optical have a softer switch on/off curve. Depending what you
choose to instrument your pendulum may also introduce more jitter. The
20logN dosen't help either, 1 millideg at 0.5 Hz is 5.5 cycles at 1 MHz.

On Mon, Mar 20, 2017 at 9:07 PM, David Scott Coburn <scotttt at optonline.net>
wrote:

> Hi All,
>
> I have built and tested a PLL circuit that will be used to generate a 1
> MHz signal locked to a 0.5 HZ signal from a pendulum.  (Details available
> upon request.)
>
> The circuit is a classic 4046 generating the 1 MHz signal which is fed
> into a 2e6 digital divider which outputs 0.5 Hz which is fed back to the
> 4046 phase comparator (PC).
>
> I take a 1 MHz signal from an HP 107A run through another 2e6 divider to
> generate a reference 0.5 Hz signal for the other 4046 PC input.
>
> I tested this by feeding the 0.5 Hz output of the PLL into a "time-stamp
> counter" board which I built to go into an HP 3582A Data Acquisition unit.
> The TSC uses the 5 MHz signal from the HP 107A to feed a free-running
> 32-bit binary counter.  The 0.5 Hz input latches the count value (on the
> rising edge of the signal), which is then logged.
>
> See the attached diagram.  The PLL under test is in the red box.  (Not
> sure what the policy is here for attachments?)
>
> If all was perfect I would get a string of values of 10,000,000 counts
> each, one every 2 seconds.
>
> Over the course of one day the average reading is, in fact, 10e6, so the
> PLL looks to be working over "long" time scales.
>
> The attached histogram plot shows the actual data for the 0.5 Hz signal,
> showing the distribution of deviations from 10e6 counts.  This is almost a
> full day of data, about 40,000 readings.
>
> The standard deviation for the data is about 55 counts.
>
> The plot looks to my eye to be a nice Gaussian shape, so I assume that the
> deviations are caused mainly by (white?) noise.  There does not look to be
> much other structure in the shape of the data.  (Comments welcome.)
>
> Sorry for the long introduction, there are some questions coming!
>
> I have looked for information on the web about others who may have done
> this kind of PLL, but did not find much.
>
> Does anyone know of any articles related to this?
>
> If so, do you know what kind of performance they got?
>
> What kind of statement could I make about the 'stability' of this
> circuit?  Simplistically: a 'stability' of ~50 counts in 10e6 is ~5e-7?
>
> By the way, this performance is WAY WAY beyond what I was expecting....
>
> Cheers,
>
> Scott
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