[time-nuts] PLL performance?

David Scott Coburn scotttt at optonline.net
Tue Mar 21 21:10:19 EDT 2017


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 <scotttt at 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 11┬Ás 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



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