On Tuesday, March 21, 2017 12:26:59 PM EDT Attila Kinali wrote: > Moin, > > On Mon, 20 Mar 2017 21:07:03 -0400 > > David Scott Coburn <scot...@optonline.net> wrote: > > 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.) > > [...] > > > 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. > > The VCO in the 4046 is an odd mixture between a relaxation and an > delay line oscillator. It's stability is not that good (at least > not by modern standards). As such, your phase comparator frequency > of just 0.5Hz is too low for the 4046 to show its peak performance, > as it is basically free running for 2s before a slight correction > is applied. Usually the frequencies used for a 4046 are in the range > of 1kHz to 100kHz. > > Alternatively, you can dissable the internal VCO (inhibit pin) and > use an external oscillator that is more stable. The VCXOs by Abracon > (ASVTX-*) are readily available and cheap enough. If you use a 20MHz > oscillator (e.g. ASVTX-09-20) divide the output first by 2 (using a > D-flipflop) and then by 10 (e.g. using 74LV161) > until you are at 1Hz, then use second D-flipflops to get to 0.5Hz. > This gives you 1MHz inbetween.
I'll have a look at the ASVTX units. Thanks! > > The standard deviation for the data is about 55 counts. > > This means that your jitter (at 0.5Hz) is 11µs RMS. > > > 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.) > > Yes, It looks very much Gauss shaped, it is very likely that this is > indeed a Gauss process, but to be sure (in a statistical sense) you > would need to do something like a qq-plot or similar to check, > whether it's actually a Gaus distribution (there are others that > look very similar). But for all practical purposes, that does not > really matter. > > Please be aware that a noise process can be Gaussian and not be white. > E.g. 1/f-noise has a Gaussian distribution as well. > > > 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? > > What information are you looking for? I was curious if anyone else has tried to do a PLL with two 0.5 Hz signals. Does not seem like a particularly popular pastime! I did find a few articles which used a digital PLL to lock a OCXO to GPS at 1 Hz. > > If so, do you know what kind of performance they got? > > Your performance seem's ok, but limited by the VCO of the 4046. It is OK, and better than I was expecting. It is probably good enough for its intended purpose, but this being the time-nuts channel I will look into a better VCO. > As your PLL frequency is very very low, you will face a lot of > difficulties due to leakage and other non-idealities of the > various components. I would recommend to use a digital PLL > implemented in a uC (PIC, AVR32, ARM Cortex-M0/M3) instead. > If you clock the uC from the 1MHz signal and use the > capture/compare (aka timer) unit of the uC will give you > a resolution in the region of 10-40ns, which should be good > enough considering the noise/stability of the pendulum. > With that you can easily implement a PLL with a loop time constant > of several seconds without the fear of running into problems from > the analog parts. I was interested in doing this as an analog PLL, for the challenge. I've done my time on microprocessors and microcontrollers! > > > Attila Kinali 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.