Rather then trying to model the capacitor why not build a reliable clock and sample the clock before and after the signal you are trying to measure. In other words you calibrate using a (say) 1 uSec pulse. That would cover the case of passive parts aging.
On Sat, Feb 15, 2014 at 3:13 PM, Lars Walenius <lars.walen...@hotmail.com>wrote: > > > > > > > >>Bob Stewart wrote: > >> Tom tried to steer me to the PICTIC recently, and I sort of brushed him > off, because, quite frankly I didn't understand. Now that I've really > looked at it, it's a much better idea than using a dsPIC33 and > brute-forcing it. But, I don't really need everything the PICTIC offers so > I started doing surgery, and this is what I've come up with. > >> > >> The nsVolts would feed one of the 10-bit ADC channels of the 18F2220 on > the VE2ZAZ board, and the 1PPS signal does the obvious. I had no idea what > to use for the RC, so I used smaller and smaller values till LTSpiceIV > showed a large range over 360 degrees of phase. I realize that's probably > a bad idea, but I have no point of reference, nor do I probably need the > accuracy that would otherwise imply. > >> > >> The 1PPS input passes through the enabled tri-state buffers U2A and U2C > to charge the cap until the Q output from the D-flop is sent high from the > 10MHz signal and disables U2C. When the 1PPS goes low, the cap is > discharged and the D-FLop is reset. In practice, the chips would be 74AC > types. I could only find LTSpiceIV models for 74HCT chips. LTSpice says > it's workable, but in practice, I don't know. It might be finicky or > unstable. Any comments would be welcome. > >> > >> http://www.evoria.net/AE6RV/TIC/TIC.png > >> > >> Bob - AE6RV > > > > > Bruce Griffiths wrote: > >You should also include the effect of the A/D converter sampling > capacitance and saampling switch series resistace in the model. Since > the RC time constant of the sampling switch and associated sampling > capacitor can be 1us or more (temparature and Vcc dependent) the voltage > waveform at the sampling capacitor differs significantly from that > predicted by your simple modeel. > Aside from the nonlinearity due to the non constant charging current the > principle limitation on the resolution is due to the variable interrupt > latency for ADCs where the conversion is triggered by software. > This problem can be avoided if the ADC conversion can be triggered > directly by an external signal. > >Bruce > > > What Bruce says is really important. > > > For the ATmega328 the datasheet says 14pF sampling capacitance and nothing > about temperature coefficient. > > It also specifies a series resistance 1..100k. So not very precise. If it > is 100k the time constant is 1400ns! > > I have tested several boards and they seem to behave similar with my 1nF > NPO capacitor. With a 47pF I guess it is more uncertain. > > > I also recommend you to test your model in the real world. I have used two > good OCXOs and/or rubidiums with a small offset. Say 1E-9 offset that gives > 1nS per sec. One of the channels have had a divider for example HC390s or > the excellent PICDIVs from Tom Van Baak to output 1PPS. > > > I have also applied a heat gun near the circuit to test that the circuit > doesn´t drift with temperature. A reasonable goal is to have less than 1LSB > drift with a couple of degrees change. This test I have done with the same > source for both 10MHz and 1PPS and a high reading from the ADC. > > > What Bruce says about interrupts is also worth to check in real life as > "jitter" due to unexpected interrupts or different timing may give problem. > In the Arduino GPSDO the timer1 overflow interrupt may delay the 1PPS > interrupt about 3us and delay the ADC conversion 3us. This is not so > critical as it sounds as the ADC input is not changing at this time. For me > this jitter gives more problem with the timer1 Reading. This jitter is not > so easy to to test as it in the Arduino GPSDO program only happens every > 1024secs and if you are (un)lucky it may not be seen at all depending on > startpoint of timer1 relative to the 1PPS. > > > Lars > _______________________________________________ > 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. > _______________________________________________ > 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. -- Chris Albertson Redondo Beach, California _______________________________________________ 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.
