Hi guys

I am looking at the datasheet here: (DS00VQ100 chip, laser range finder chip)
http://forums.parallax.com/attachment.php?attachmentid=84096&d=1313511811

The chip is only running at 12.8MHz, by my calculation, it has a resolution of 156.25ns. So how does it get 60ps - 400ps resolution? (Counter is used to measure time of flight of the laser pulse)

I am guessing something like over sampling uses in ADC. so if I am right, what this technique calls, I want to find out more about it.

Thanks guys!

 

This is just a guess, but what you are seeing is just the reference frequency. It may be part of a phase locked loop in which an oscillator with a much higher frequency is divided down and compared to the reference frequency. Then using a control voltage the higher frequency (VCO) is adjusted up or down in order lock it to the reference frequency.

 

Hi,

I found this: (ref: http://forum.arduino.cc/index.php?topic=110549.30)

Q: how does a slower than light microcontroller measure a speed of light signal?
A: The short answer is DOWN CONVERSION. The long answer is in the DS00 manual on the lightware website;
basically it takes lots and lots of readings and these build up to make a signal, kind of like when we make a charcol rubbing off and old grave stone; one pass is not readable but many passes gives us an image of the faded head stone.
Another way to look at it is simnilar to the "beat frrequency" when tuning two guitar strings relative to each other (in the old days guitars had strings, ask somebody old, possibly with long hair, to show you).

But I still don't understand, any more guess?

 

Hi,

I think it explains (sort of) how to get a higher resolution in the datasheet from page 12, but I still don't get it. Any thoughts?

 

I agree with you that the data sheet doesn't tell us how the part works.

I am just guessing here but I think they are dithering the crystal oscillator frequency. This is why they need a crystal that can be pulled in frequency. By varying the oscillator frequency they can do a lot of averaging of the received signal timing and get improved time resolution. They also seem to be comparing the frequency of the varying oscillator frequency to a stable oscillator by mixing (down converting). The source of the signal that varies the frequency may be controlled the Noise register.

Here is a newer version of the data sheet.

 

I agree with you that the data sheet doesn't tell us how the part works.

I am just guessing here but I think they are dithering the crystal oscillator frequency. This is why they need a crystal that can be pulled in frequency. By varying the oscillator frequency they can do a lot of averaging of the received signal timing and get improved time resolution. They also seem to be comparing the frequency of the varying oscillator frequency to a stable oscillator by mixing (down converting). The source of the signal that varies the frequency may be controlled the Noise register.

Here is a newer version of the data sheet.

Thanks for the new datasheet, it gives me a bit more info, which is good. It looks like some sort of DSP happening in that FPGA, I guess I need more googling.

 

Thanks for the new datasheet, it gives me a bit more info, which is good. It looks like some sort of DSP happening in that FPGA, I guess I need more googling.

You are very welcome. Please let us know what you find out.

 

You are very welcome. Please let us know what you find out.

Hi,

I got some good replies over at EEVBlog, but too much for me to understand. I am slowly working on the new concepts (to me) that they mentioned, and hopefully I have a basic understand of it soon. if you want to follow, here is the link:
http://www.eevblog.com/forum/projec...tion-with-two-oscillator/msg664635/#msg664635

 

For the people have the same question, this is a very good read:
http://www.ko4bb.com/~bruce/TDC.html (point out by user: krenzo @ eevblog forum)