Well the single board experiment is a bust. Doesn't look like it is possible.
Oh well. I tried....
Did you try a 4060 (or similar SO8 74HCT6323, for lower divide ratios) ?
Also, by my maths, you need a ppm difference in frequencies, that allows all time-slots to be covered in the sample time.
( otherwise your reading is less stable, and more quantized)
So some deliberate capacitor skewing will be beneficial, to get some tens of ppm offset.
"... At any point in space, the electric field corresponds not to the condition of the electric energy flow at that moment, but to that of the flow at a moment earlier. "
I should have something to post tomorrow, but right now it appears able to have about 50 picoSecond resolution. But I just got it working a couple minutes ago, so that may change.
...
Stay tuned...
Another thread reminded me of this - Did you ever get more measurements on the usable ps resolution, and what sample counts are needed ?
It seems to me you can predict the measurement samples needed, and the "virtual timer" LSB, and perhaps even vary those to suit users requirements ?
Doing some maths on your earlier values of 1MHz and 19ppm, we have a 'time walk' of ~18.9996ps per cycle, which gives 52632.578 samples to cover all time-slots in a 1us aperture.
If you are measuring phase, this also gives a ceiling on 'run lengths' per phase
Wow, for a while now I have been pondering on how to measure the speed of light. Why? Well just because I think it's something I should be able to do. We it back in uni with big and complicated arrangement of a laser and rotating octagonal mirror. Seemed to me that with todays MCU's laser diodes and such we should be able to do it much more directly.
Perhaps you have the solution there. Just run the signal through a laser/photo diode set up.
Anyway the speed of signals through wires might be close enough for me:)
In 1849, Armand Fizeau sent a beam of light through a rotating wheel with a large number
of teeth around the outside.
A mirror on the other side reflected the beam each time a gap appeared in the path of the
light. Fizeau realized that if the wheel rotated fast enough, the return beam would be blocked
by the next tooth as it came around. So he varied the speed of the wheel until the reflected
beam disappeared, performed a bit of math, and got a result of 315,000 km/second
(195,732 miles/second)certainly in the ballpark. Meanwhile, Foucault was working on a different but equally clever technique, which he
demonstrated the following year. Foucaults method was to shine a sharply focused beam
of light onto a rotating mirror, and from there onto a fixed mirror. Once the light hit the fixed
mirror, it bounced back onto the rotating mirror and then back toward the source.
But because the mirror was rotating, the angle at which it was positioned had changed slightly
by the time the beam made its return trip. Consequently, the reflected beam did not line up
precisely with the original. Foucault could easily measure the angle between the original light
source and the reflected beam, and along with known constants (the distances between the
various surfaces and the speed of the mirrors rotation), it was a matter of a few straightforward
calculations to convert that small angle into a representation of speed. Using this technique,
Foucault produced a measurement of 298,000 km/second (185,167 miles/second), which is
shockingly close to the modern measurement of 299,792 km/second (186,282 miles/second),
keeping in mind that the latter figure applies only in a vacuum; light travels more slowly in air.
As for the tuning fork Foucault used this to regulate the speed of the rotating mirror.
The apparatus that turned the mirror made a sound that varied with its speed; when the sound
exactly matched that of the tuning fork, Foucault knew precisely how many revolutions per
second it was making.
One easy way of measuring the speed of "light" is to use the network of chirp sounders around the world. These are radio transmitters that transmit across a wide range of frequencies (chirp) and are locked to GPS. Using them, it is easy to measure the time of flight of the radio wave.
Comments
Did you try a 4060 (or similar SO8 74HCT6323, for lower divide ratios) ?
Also, by my maths, you need a ppm difference in frequencies, that allows all time-slots to be covered in the sample time.
( otherwise your reading is less stable, and more quantized)
So some deliberate capacitor skewing will be beneficial, to get some tens of ppm offset.
Thanks for providing detail on this. I tend to forget the "like water in a pipe" comparison is just a model.
What about the "1/2 per hour" estimate? How does one determine the actual "net drift"? Is by statistics?
http://en.wikipedia.org/wiki/Drift_velocity#Numerical_example
Duane
I can feel my brain gettting bigger!!!
Thanks!
Another thread reminded me of this - Did you ever get more measurements on the usable ps resolution, and what sample counts are needed ?
It seems to me you can predict the measurement samples needed, and the "virtual timer" LSB, and perhaps even vary those to suit users requirements ?
Doing some maths on your earlier values of 1MHz and 19ppm, we have a 'time walk' of ~18.9996ps per cycle, which gives 52632.578 samples to cover all time-slots in a 1us aperture.
If you are measuring phase, this also gives a ceiling on 'run lengths' per phase
In 1849, Armand Fizeau sent a beam of light through a rotating wheel with a large number
of teeth around the outside.
A mirror on the other side reflected the beam each time a gap appeared in the path of the
light. Fizeau realized that if the wheel rotated fast enough, the return beam would be blocked
by the next tooth as it came around. So he varied the speed of the wheel until the reflected
beam disappeared, performed a bit of math, and got a result of 315,000 km/second
(195,732 miles/second)certainly in the ballpark. Meanwhile, Foucault was working on a different but equally clever technique, which he
demonstrated the following year. Foucaults method was to shine a sharply focused beam
of light onto a rotating mirror, and from there onto a fixed mirror. Once the light hit the fixed
mirror, it bounced back onto the rotating mirror and then back toward the source.
But because the mirror was rotating, the angle at which it was positioned had changed slightly
by the time the beam made its return trip. Consequently, the reflected beam did not line up
precisely with the original. Foucault could easily measure the angle between the original light
source and the reflected beam, and along with known constants (the distances between the
various surfaces and the speed of the mirrors rotation), it was a matter of a few straightforward
calculations to convert that small angle into a representation of speed. Using this technique,
Foucault produced a measurement of 298,000 km/second (185,167 miles/second), which is
shockingly close to the modern measurement of 299,792 km/second (186,282 miles/second),
keeping in mind that the latter figure applies only in a vacuum; light travels more slowly in air.
As for the tuning fork Foucault used this to regulate the speed of the rotating mirror.
The apparatus that turned the mirror made a sound that varied with its speed; when the sound
exactly matched that of the tuning fork, Foucault knew precisely how many revolutions per
second it was making.
http://itotd.com/articles/284/measuring-the-speed-of-light/
http://www.qsl.net/zl1bpu/IONO/chirps.htm
I am sure that this could be the start of a nice Propellor application for someone!
They take about 138ms to go right round the earth and on a good day I have seen three complete circuits.
Regards
Richard