Hi Tom, I appreciate the offer. I may wind up taking you up on it. But, I'm more interested in doing the project than getting an interpolator that works; if you get my meaning. So I'll just plod along at my own speed on this. I may wind up with questions in the future. I'll address them to the list, you, or Richard as seems appropriate, if that's OK.
Bob >________________________________ > From: Tom Van Baak <t...@leapsecond.com> >To: Discussion of precise time and frequency measurement <time-nuts@febo.com> >Sent: Saturday, February 15, 2014 7:33 PM >Subject: Re: [time-nuts] TIC model > > >Bob, > >I have versions of the picPET that capture an ADC reading along with a >timestamp. I used this with one of Richard's simpler interpolator designs. >There's some info at http://leapsecond.com/pic/picpet2.htm and the projects >aren't final yet but you can contact me or Richard directly. > >/tvb > >----- Original Message ----- >From: "Bob Stewart" <b...@evoria.net> >To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> >Sent: Saturday, February 15, 2014 5:19 PM >Subject: Re: [time-nuts] TIC model > > >> Hi Lars, >> >> I'm lucky in that I'm starting with a PIC running internally at 40MHz. So I >> think timing is not going to be a real problem. But after getting a response >> from Richard, I'm concerned about flaws in the spice chip models. I'm also a >> bit concerned about breadboarding, but come to think of it, in that link you >> gave me the guy used dead-bug assembly. So maybe that's not a big concern. >> >> I suppose the next thing I need to do is experiment with using the ADC to >> grab the voltage from the incoming 1PPS pulse. Mine stays high for 50ms, so >> it should be an easy place to start. In fact, there are lot of things I can >> do with the 1PPS and an RC to get some experience working with the ADC. >> >> Bob >> >> >> >> >> >>>________________________________ >>> From: Lars Walenius <lars.walen...@hotmail.com> >>>To: "time-nuts@febo.com" <time-nuts@febo.com> >>>Sent: Saturday, February 15, 2014 5:13 PM >>>Subject: Re: [time-nuts] TIC model >>> >>>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.
