[time-nuts] TIC model

[Chris Albertson]

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.walenius at 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 at 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 at 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