[time-nuts] First success with very simple, very low cost GPSDO
bob at evoria.net
Fri Apr 11 05:39:49 UTC 2014
I did a poor job of explaining this. So, consider the "phase point" as where you are on the PPS coming in compared to your oscillator. That PPS is not corrected for sawtooth. So, when you have a regular sawtooth PPS, then at the "phase point" you will see the count bouncing back and forth. But, then a ramp or a hanging bridge comes along and you no longer know where the phase is WRT where where you are. You also don't know how long it's going to be there, or even whether you were at the actual phase center point and it's now on a hanging bridge, or that you were on a hanging bridge on one side and it's now gone to center, a bridge on the other side, or back to a sawtooth that averages to zero. That's probably still pretty poorly said, but it's way past my bedtime.
> From: Bob Stewart <bob at evoria.net>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>Sent: Friday, April 11, 2014 12:20 AM
>Subject: Re: [time-nuts] First success with very simple, very low cost GPSDO
>The only thing I need to know, in fact the only thing available to me, is the fact that a phase crossing has occurred. Since 10MHz is 0x989680, then all I need to know is whether my timer delta is above, equal to, or below 0x9680, without regard to anything else. None of the other 0x9680 points are within the range of my oscillator. In fact, they are way out of range. There are two conditions that can happen with this type of counter which someone else spoke about. In the one case, you are sitting right on the phase point, and the timer will bounce back and forth, from 0x967F to 0x9681; i.e. you will have a limited amount of phase information. In the other, you are not near the phase point, and you will get just the frequency result.
>The problem with sitting right on the phase point is that it tricks you into thinking you are getting more out of the system than you actually are. But, you are only getting a few nanoseconds, i.e. a few degrees, worth of phase information. Once the system drifts out of that little phase band, then all you know is which way it drifted, not how quickly or how far. I spent far too many tens of hours thinking I could solve the problem. I couldn't. There's just not enough information available to you. You have to resort to guessing, based on what you know about your oscillator. But you can't be right enough in your guess to make a reliable phase control. So, at least for me, it sits on phase for quite awhile, then it drifts off, your PLL stops, and you wait till you drift back to the phase point. Rinse, repeat, fail. But, it's always possible that you can see something in the data that I didn't.
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