[time-nuts] quick and very dirty phase comparator

[Ulrich Bangert]

Bruce,

thank you for correcting me. Here I have clearly fooled myself. However
your posting originated some new ideas: With the GCD becoming THAT low
an analogue phase lock to a 10 MHz reference will not be easy. But if we
stop to think about phase locked VCXOs we need not bother anymore about
odd exotic xtal frequencies at all that may generate us a GCD of 10.
Instead we are free to choose for example 10000010 Hz for the
controller's frequency. Which brings us back to a construction of a good
offset generator. 

Until now I have believed that a good (low phase noise, high stability)
offset generator would involve

a) a number of single sideband mixers (as described in
www.horology.jpl.nasa.gov/papers/fssa.pdf)

or

b) the well known offset synthesizer circuitry as described by Rick
Karlquist.

I am sure that both ideas work excellent, although I am unsure whether
a) can generate an 10 Hz offset. However, both methods involve circuitry
that I would not call exactly "quick and dirty" and their use would
overstress the try to make something really simple. On a new internet
search for "offset generator" I came over this one:

www.diva-portal.org/diva/getDocument?urn_nbn_se_liu_diva-1838-1__fulltex
t.pdf 

What do you think about that topology? Let the "IF in" be the needed
offset and the "offset OSC" be our 10 MHz reference. Would that not make
an really easy way to generate an precise offset with the wanted
features?

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Bruce Griffiths
> Gesendet: Dienstag, 3. Juni 2008 01:29
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] quick and very dirty phase comparator
> 
> 
> Ulrich Bangert wrote:
> > Kasper,
> >
> > I am impressed a lot by the simplicity of your ideas. Added 
> what Bruce 
> > has said to it I think the idea can even be improved by
> >
> > a) using a 12.288 MHz source for the micro.
> >
> > and
> >
> > b) using one (or two)external d-flip-flop(s)
> >
> > The GCD of 10000000 Hz and 12288000 Hz is 16000 instead of 3200 as 
> > with 10000000 Hz and 11059200 Hz (or the also used 14745600 
> Hz) which 
> > results in a repetition rate of 19660.8 clock cycles for the 
> > coincidence between the two clocks. This gives you 5 times the 
> > resolution. And instead of heavily sampling port inputs allow the 
> > external flip-flop to generate the capture signal for one of the 16 
> > bit timers/counters using the FULL resolution of EVERY clock slope. 
> > With all AVRs that feature 2 16-bit counters it will be possible to 
> > time stamp at least 2 sources against the locked 12.288 MHz.
> >
> > With this arrangement your micro will expect a capture interrupt 
> > roughly every 1.6 ms which is kind of armchair condition for an AVR 
> > runing at 12.288 MHz.
> >
> > I am not sure whether you are really interested in a update rate of 
> > 300 mikroseconds. I am interested in stability on a second 
> to second 
> > base. So why not use 500 of these 1.6 ms apart time stamps 
> to compute 
> > a linear fit from as suggested in your counter paper. Should be no 
> > problem to update all the sums needed for that online and 
> use the last 
> > 100 ms to perform everything else including communication.
> >
> > Best regards
> > Ulrich Bangert   
> >
> >   
> Ulrich
> 
> Actually you need the largest common denominator to be 
> smaller not larger. In your case the duration of 768 cycles 
> at 12.288MHz is equal to the 
> duration of 625 periods at 10MHz.
> Thus the time interval between coincidences is only 62.5us which is 
> somewhat shorter than the corresponding time interval of 312.5us with 
> 10MHz and 11.0592MHz clocks.
> 
> A frequency like 17.73447 MHz (1734470=1229x37x13x3 x10 ) is a better 
> choice as the duration of 1773447 cycles at 17.34470MHz is 
> equal to the 
> duration (0.1sec) of 1000000 cycles at 10MHz.
> The largest common denominator of 17734470 and 10000000 is 
> 10. However the resultant 56 fs resolution will exceed that 
> of all but the 
> very fastest flipflops.
> In this case it is critical that the 17.73447 MHz crystal oscillator 
> have very low phase noise/jitter and be phase locked to one 
> of the 10MHz 
> sources.
> Also the 10MHz sources being compared should not differ from 
> each other 
> by more than 0.1Hz or so.
> 
> Bruce
> 
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