Re: Designing a APD based receiver for use with a TOF laser range finder


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:9tug53ll3foc09hau0qd3af0jss5b42fgf@xxxxxxxxxx
On Sat, 26 May 2007 18:16:25 GMT, "colin"
<colin.rowe1@xxxxxxxxxxxxxxxxxx> wrote:

That binary sampler looked interesting, im trying to average a time
interval
signal down to sub picosecond resolution, but my data's standard deviation
is nearly 10 nanoseconds. I do however have ~10 million points per day to
play with.

It looks to me like it effectivly does the same thing as creating a line
of
best fit with equal number of points above and below, as opposed to line
with minimum absoulute error or square error. have I got this right ?


Yes. I argued with MM over this for some number of years, and gave up.
You saw it right away.

hmm i see.

however im not familiar with mathmatical techniques to find such a line of
best fit, is there any code around for doing such techniques ?

its just a one off physics measurement experiment im doing. At the moment
im
just doing a fft to find the signal im looking for wich shows up as
modulation of time interval of <1ps with a period of many hours. I'm
playing
about with stuff like rejecting 10% of the points at the edge of the
standard deviation. I gues I could just limit the error rather than reject
points. I havent realy looked into error reduction yet as im still trying
to
reduce the source of the error.

The noise is from mechanical system so there are many things like short
and
long term drift, and sudden change in offsets, spurious spikes etc.

However if there is a way to reduce noise in proportion to the number of
samples rather than the sqrt this would make a big difference.

Life doesn't allow that. MM would have a Nobel by now if it did.

dam. how sure are you ? it seemed to claim that it did, im not convinced
either way yet.
I was going to test it on my test data just need to make a different stat
analysis algorithm.
I was realy hopefull :/

im not the right person to enjoy doing a proveable mathmatical analasys to
find how the noise reduction equates to the number of samples with this so
called binary aproach. any mathmatical types here ? its probably already
been done somewhere anyway, not that I would know exactly where to look for
it.

I gues a test program would suffice however.

As for heterodyne aproach for LIDAR I tried to do this with swept ~2ghz
modulated laser and an APD with a modulated bias voltage with a freq
offset
or IF of 455khz. the difficulty was keeping ghz VCOs 455khs apart from
eachother, they always tried to lock. however any phase noise could easily
be compensated for with a null control path.

I managed to see resolutions down to less than a millimeter with no signal
proceesing. I hope to revisit this project and apply some DSP like the
above
too wich might help acheive my goal of detecting 1um change, idealy it
would
be able to replace a mechanical dial indicator guage, advantage being non
contact of a rotating shaft for example. maybe DDS of 1ghz wil be possible
by then.

im not sure about how well it would be possible to account for multiple
targets on the same path.

However I also saw an interesting method of determining absolute distance
using laser interference, by modulating the laser frequency and doing
correlation on the change in the resulting interference signal.
claimed resolution of the wavelength of light over distances >10M. it
doesnt
even need a fast detector.

A good laser interferometer could do what you want. But if you want to
use tof, be aware that it takes extreme measures to get a signal chain
like yours down to 1 ps per degree C drift. The tof chip you suggest
using is going to be far, far worse, as will the laser+driver, the
pin/apd amplifier, and whatever comparator you use.

What's the physics here? Could you use an incremental, as opposed to
absolute, position measurement system? Could you use some other
distance measuring scheme, capacitance maybe?

I just don't think you can hold < 1 ps for any usable amount of time
using tof as described.

John

for me its 2 seperate projects, the <1ps timing is not from a distance
measuring.
although they do have similar needs for precise timing wich is why i
mentioned both of them.

trying to hold the <1ps for any length of time ... hell yeah tel me about it
!!
fortunatly I can null out any offset error every few seconds.
however ive found if the table tilts a bit it gives ~1ns offset error wich
isnt so easy to null out.

My LIDAR is for general purpose for machining/robotic sensing so realy needs
to be absolute.
The corelating interferometer is as acurate as an ordinary inteferometer,
but does absolute too.

thanks
Colin =^.^=


.

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