[time-nuts] PLL performance?

David Scott Coburn scotttt at optonline.net
Wed Mar 22 19:48:57 EDT 2017

I will not be using an off-the-shelf optical interrupter  type sensor for this.

I have designed a custom IR LED -> IR photodiode unit which will have a flag that blocks half of the IR signal when the pendulum is stopped, and the motion of the pendulum will modulate this 50% signal from about 25% to 75%.  The output will be an analog 'sine' wave.  This sine wave goes into some custom analog electronics which separates the DC and AC parts of the signal, does some amplification, and then into a precision zero-crossing detector (ZCD) circuit.  I have assembled these circuits and have been testing them at zero DC (ie, no DC offset or sine input).  The next step is to connect the LED/photodiode circuit and characterize the circuit with just the DC signal (as it would be if the pendulum was not moving).  This stage of the testing is looking for noise and long-term stability issues.  I have not yet injected a reference sine signal to characterize the circuit's AC performance.  On my todo list!  :)

I hoped to characterize the jitter of the ZCD circuit with a good low-jitter reference AC signal as an input, but it is not trivial to generate such a signal!  (Short of spending lots of $$$ on a *good* signal generator.)  I have an HP3325A which I intend to use for this but the 0.5 Hz output has quite a lot of jitter (not unexpected considering the way it is generated).  But, I guess this is not so much of an issue, since I just need to see if the ZCD circuit makes the jitter worse.

The PLL circuit will not be used to characterize the performance of the pendulum, it will just be used to drive the display.  The output of the ZCD circuit will be fed directly into a 'time-stamp counter' circuit to monitor the pendulum performance.  To some degree it will be good to have a nice low-jitter signal from the pendulum, but I am more interested in the longer-term performance , where the jitter (hopefully!) is all averaged out.

(The DC part of the signal will be monitored for changes in the LED output (to monitor its stability) and there is a precision rectifier circuit to monitor the amplitude of the AC part of the signal (which is proportional to the amplitude of the pendulum motion).  And, there is a precision voltage reference for driving all of these circuits.)



(Maybe this answer was more than you bargained for!)

On Wednesday, March 22, 2017 12:19:09 PM EDT Scott Stobbe wrote:
> 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.)


> > Cheers,
> > 
> > Scott
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