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 <mag...@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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > -- --Jim Harman
TIC data.xlsx
Description: MS-Excel 2007 spreadsheet
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