[time-nuts] TIC model

Bob Stewart bob at evoria.net
Sun Feb 16 01:19:34 UTC 2014


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.walenius at hotmail.com>
>To: "time-nuts at febo.com" <time-nuts at 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
>
>


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