Hello to the group quite late to the discussion. Pretty interesting. But I assume the drive for the pendulum is some impulse. Not the old mechanical clock with a spring for power. Regards Paul WB8TSL
On Wed, Mar 22, 2017 at 7:48 PM, David Scott Coburn <scot...@optonline.net> wrote: > 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.) > > Cheers, > > Scott > > (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 < > scot...@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.) > > <snip> > > > > Cheers, > > > > > > Scott > > > _______________________________________________ > > > time-nuts mailing list -- time-nuts@febo.com > > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > > > mailman/listinfo/time-nuts > > > and follow the instructions there. > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the > > instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.