[time-nuts] Lars GPSDO on EEVblog

Jim Harman j99harman at gmail.com
Sat Sep 8 20:14:12 EDT 2018


OK, I gathered some TIC data from my system, using the TIC circuit I posted
earlier. I opened the loop and forced the oscillator off-frequency with a
constant DAC setting so that the phase detector wraps, i.e. the accumulated
phase error is 1 usec, in a little less than 5 minutes. The range of the
ADC readings is about 90 to 1023.

The timing reference is the 1 pps from my Adafruit (GlobalTop) gps with a
pretty good antenna.. This is a navigation type GPS and has no sawtooth
correction. Of course it would be better if I had a timing GPS. The
attached spreadsheet has 6 cycles of this data, gathered over about 28
minutes.

The plot on the spreadsheet is just for the first cycle, rows 1 - 278.

The linear trendline on the plot shows a slope of -3.3878 counts/sec and
the R squared is .9998. So I guess the linearity is pretty good. You can
try other cycles to see how they compare.

On Sat, Sep 8, 2018 at 4:49 PM Magnus Danielson <magnus at rubidium.dyndns.org>
wrote:

> Hi Jim,
>
> On 09/08/2018 10:02 PM, Jim Harman wrote:
> > Magnus said,
> >
> > It would be interesting to test the linearity of the TIC separately for
> >
> > instance.
> >
> > I have done some testing of the TIC. It works quite well for the Arduino
> > Uno with its 1 V full scale ADC setting, but the exponential shape of the
> > RC charging from the 5 V is quite evident if you  use a processor like
> the
> > Micro's 32u4 with a 2.56 V sensitivity. This circuit is also sensitive to
> > any noise on the 5 V supply. The linearity is not too important for this
> > application as long as it does not affect the loop gain so much that the
> > loop becomes unstable.
>
> Agreed. It is expected, but it would be fun to see how well it would do.
>
> > However I have modified the circuit to use a simple 2 ma current source
> as
> > shown in the attached schematic, with significantly improved linearity.
> H/T
> > to Horowitz and Hill's The Art of Electronics for the idea of using an
> LED
> > as the voltage reference for the current source. The diode-connected
> 2N3904
> > has less leakage at a small forward voltage than any small-signal diodes
> I
> > tested, so the output voltage with a very narrow input pulse is close to
> > zero.
>
> A more serious current source helps.
>
> It comes as no big surprise that this has been a topic of interest to
> several designers, hence there exists a number of patents on it.
>
> Different approaches have been used. The HP5335A for instance uses a
> three transistor setup, where the current source, a resistor, is
> buffered behind the transistor pair that also switches it in and out.
> The HP5335A then acts as pulse extender with x200 and a TTL counter to
> complete the ADC process. Modern designs use a proper ADC instead but
> the basic problem remains the same.
>
> > I could run some tests on my version by setting the oscillator slightly
> > off-frequency and capturing the resulting sloped TIC output, and would be
> > happy to share the results if you are interested.
>
> Please do. This is one of the methods and probably the easiest to setup
> for most. Another approach is to use a programmable delay generator with
> enough resolution, but having one of those around isn't as common as a
> RF generator which can be detuned with sufficient resolution.
>
> As I spend the evening refactoring the lab-bench, the RF generators gets
> a more prominent placement as well as one of the delay generators. Hope
> to get the TICs stacked up nicely for ones.
>
> Cheers,
> Magnus
>
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-- 

--Jim Harman
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