Update on laser distance meter project (master thesis) and question

[Yannick]

the beauty is that if your signal is to strong you can reduce the bias

yes but for a high dynamix range you sometimes have to reduce with
40-50Volts soo then the capacitance will change too much and the phase
delay will change significantly. Shouldn't it be better to reduce the
apmlitude of the AC signal on the reverse bias. Lets say this is 500mv
and if you lower it to 50mv this will give a much(more then 10 times , because of the exponential curve) lower signal at
the input of the amplifier but the capacitance will be the same (450mv
wont change much).

after thinking this would also not be possible because to change the
amplitude of the ac signal without altering its phase is almost
impossible. And if it is possible you could do it with the photodiode
amplifier soo forget what i wrote.

Yannick

 

[colin]

Yes but say your carrier is 20Mhz generated with a DDS. your frequency
superponated on the reverse bias of 200V will be 20.4 Mhz. Now what
about the phase between the two because i work with different
frequenties (carriers in this aproach) the 20Mhz from the 20.4 must be
in phase with the 20Mhz send frequency. otherwise you always measure
an other phase delay because the second dds will never load up at the
exact same time as the first.

ah ok, yes theres more i need to explain, its quite clever ... obviously the
two signals of 20 amd 20.4 cant be in phase however the signal you get from
the mixer/detector is 400khz and this is what is now your signal so you need
a reference signal of 400khz to compare it to. you get this from mixing the
two original signals of 20 and 20.4 mhz in a standard mixer stage, as you
increase the frequency you use say 30 and 30.4 mhz the result is still
400khz in both cases.

the clever part is that if you do the maths or just think about it you see
that the phase change in the reflected high frequency signal is actualy
directly contained in the 400khz signal. ie the actual delay in time is
multiplied by the diference in frequency.

yes ofcourse, thats the part i still didnt got, i was still thinking
of mixing due the diode characteristic but you actually mix with the
voltage/gain characteristic.

a mixer is a just a multiplier, the photomultiplier gain is determined by
the bias voltage and so the output from the APD is the light signal
multiplied by the bias voltage. its handy that it has a high slope at point
of use so you dont need to superimpose a high ac voltage.

yes but for a high dynamix range you sometimes have to reduce with
40-50Volts soo then the capacitance will change too much and the phase
delay will change significantly. Shouldn't it be better to reduce the
apmlitude of the AC signal on the reverse bias. Lets say this is 500mv
and if you lower it to 50mv this will give a 10 times lower signal at
the input of the amplifier but the capacitance will be the same (450mv
wont change much).

yes you definatly need to keep the bias voltage wel over 140 volt or the
capacitance change is phenominal. and yes as you can easily change your ac
voltage within your DDS it would be better, its dificult to design a
analogue variabl gain stage that has a totaly constant phase, this has
hindered me quite a bit, i found this solution worked reasonably well.
i have found a few volts change in bias voltage cuases a very wide change in
gain with minimal change in delay, dont forget the apd capacitance is very
smal anyway, and at 400khz a slight change wil have minimal efect.

however to get an extremly wide agc range you may find your ac bias voltage
needs to be a few uv youl find this cuases considerably problems. changing
both at the same time is of course an option wich i would probaly try to
use, you can also atempt to make erros cancel eachother out.

another option is to use an optical shuter such as a lcd

you need to adjust the bias voltage to some extent to acount for changes in
temperature. and to set the optimum gain.

soo just amplify the voltage over the bias resistor. I am thinking now
about non linearity due the exponential APD characteristic but for
small ac signal amplitude it wont give a problem.

the 400khz signal you get should be close enough to a sinewave to give good
acuracy, narowband filter wil of course take away any distortion. you can do
the maths for mixer stages with non ideal multiplication charectersitcs, u
just asume the input signals are distorted and fed into an ideal mixer, then
u multiply al the distortion products too, but i doubt you wil find it
cuases any problem.

but if you work with a narrow band filter the phase change will be
rapid due any frequency error (drift).Isnt it better to use a filter
with low quality(Q factor) soo ok more noise but less phase change, a
compromise between the two must exist.

yes drift is potential problem you have to assess the point at wich its
aceptable. you can get ceramic IF filters used in old radios about 440khz
although i found it surprisngly hard to get hold of one from any handy
stockists.

yes i suppose but this way is easy

i c now that you are thinking of using a digitiser and doing away with the
detector altogether. i realy dont like the detector you are using its not
designed as a detector at all. i didnt realy look at it before im surprised
you geting such good results with it actualy. im wondering how big your
lense is ??

of course at 400khz your digitiser is a whole lot easier.

I am going to write a whole chapter on this one (optoelectronic
mixing) for my book (thesis) because this is the way to go... to make
it work in practice in one month i dont gonne try, but next year on my
own i will try, i am really curious what this will give in resolution
when i step uy my carrier to 100Mhz orzo (with a better DDS)

good i always fancy writing a book but doubt itl ever hapen, its good to be
able to at least write about it here. oh and please feel free to mention my
name if you want (Colin P. Rowe)

i sugest you look up superhet radio operation if you havnt covered it
already.

it al depends how easy it would be to add a second DDS to your project (and
a simple mixer), hardly any other significant changes are required. this may
be a whole lot easier than ading a digitiser anyway, as your curent phase
measuering wil be adequate with a much cleaner signal.

can you get dual DDS ?

its a challenging project, i am really happy i did it, its far more
difficult i ever could expect but thats good because i learned a lot.

yes it is surprisingly chalenging isnt it, but thats what makes things so
exciting.
my latest project is a bit of a spin off trying to measure smal changes in
speed of light to investigate the validity of the Silvertooth experiment
wich suposedly contradicted the Michelson-Morely experiment. its probably
more chalenging as i hope to make it simple enough for it to be easily
reproduced so the results cvan be easily demonstrated. if it actualy works i
make no prediction about the outcome of the experiment tho as this would be
rather contreversial anyway (im realy not sure, wich is why i want to try
it).

Colin.

 

[Yannick]

i also did my Master Thesis (1999) on a Laser rangefinder where the

result was sampled, averaged and stored to an old C25 TI DSP.
Exciting project. Isn't it? I gather that you used a CW LASER. As
you discover, I found the photodiode/preamp can be quite tricky.

Yes its a very exciting project, did you do it on your own or in
cooperation with a company? What kind of resolution did you get and
maximum distance ?

Indeed i use a 3mw CW laserdiode (655nm) and yes the photodiode
preamplifier is the greatest limitation in my current prototype as is
the phase detector. I will however change in a later state to a DSP
solution like u did. But first i am going to try optoelectronic mixing
like described by Colin.

Yannick

 

[Yannick]

ah ok, yes theres more i need to explain, its quite clever ... obviously the

two signals of 20 amd 20.4 cant be in phase however the signal you get from
the mixer/detector is 400khz and this is what is now your signal so you need
a reference signal of 400khz to compare it to. you get this from mixing the
two original signals of 20 and 20.4 mhz in a standard mixer stage, as you
increase the frequency you use say 30 and 30.4 mhz the result is still
400khz in both cases.

Yes ofcourse.

the clever part is that if you do the maths or just think about it you see
that the phase change in the reflected high frequency signal is actualy
directly contained in the 400khz signal. ie the actual delay in time is
multiplied by the diference in frequency.

ok i got that part.

another option is to use an optical shuter such as a lcd

yes but this will always polarise the light in one way , soo you get
inevitble 50% loss.But yes if you use it at the laser diode you could
use a higher power (6mw)laser soo after the lcd it's still 3mw, and do
the gain control with the laser power.

you need to adjust the bias voltage to some extent to acount for changes in
temperature. and to set the optimum gain.

Yes i know, but this will be done in the final stages , are you
already doing this?

i c now that you are thinking of using a digitiser and doing away with the
detector altogether. i realy dont like the detector you are using its not
designed as a detector at all. i didnt realy look at it before im surprised
you geting such good results with it actualy. im wondering how big your
lense is ??

its 80mm , you mean my phase detector the ad8302? what's soo bad
about it , only the clipping is not good i thaught. Maybe a normal
gilbert cell was far better and then just using the cos-1 function in
my microcontroller to get the phase difference. I actually was
thinking, if no clipping occurs you don't have to use a DSP to detect
signals out of noise because if you multiply the received signal with
the reference signal(with the mixer) and then doing ad conversion of
the result wereafter you integrate the result in the microcontroller.
Then this would be equal to fouriertransformation and this is actually
a correlation soo if you do it long enough the signal has to rise out
from the noise. I was always thinking it had to do this with a fase
ADC and a DSP processor but actually you can do the correlation analog
at first sight.

good i always fancy writing a book but doubt itl ever hapen, its good to be
able to at least write about it here. oh and please feel free to mention my
name if you want (Colin P. Rowe)

Ofcourse i will do, i will mention it at the references for the final
chapter and in my thank word.

can you get dual DDS ?

hmm never saw any, will look it up.

yes it is surprisingly chalenging isnt it, but thats what makes things so
exciting.
my latest project is a bit of a spin off trying to measure smal changes in
speed of light to investigate the validity of the Silvertooth experiment
wich suposedly contradicted the Michelson-Morely experiment. its probably
more chalenging as i hope to make it simple enough for it to be easily
reproduced so the results cvan be easily demonstrated. if it actualy works i
make no prediction about the outcome of the experiment tho as this would be
rather contreversial anyway (im realy not sure, wich is why i want to try
it).

Wow, you are also interested in the whole thing about speed of light
and relativity i see. I didnt hear about this silvertooth experiment
however i am a bit sceptic because what reference clock will u use as
if the speed of light would change (wich would be indeed very
controversial , einstein would not be happy if this should be the
case ) then the atoms in the reference clock will do the same and
the clock will tick accordingly matching the difference out.

I always wondered why the speed of light is independed of the speed of
the reference point. Once you take this as true the rest follows like
time goes slower when you move due acceleration.It's very very
interesting these things...
I am curious how you will try to do this...

Yannick

 

[colin]

yes but this will always polarise the light in one way , soo you get

inevitble 50% loss.But yes if you use it at the laser diode you could
use a higher power (6mw)laser soo after the lcd it's still 3mw, and do
the gain control with the laser power.

good point and idea, although this will be noticable as the laser is useful
for tergeting.
there are other devices wich dont polarise the light, some1 sugested here
some time ago but i forget what it was now, but it wld be rather bulky.

Yes i know, but this will be done in the final stages , are you
already doing this?

yes i find that the agc alone sets the bias voltage, with no signal the bias
voltage rises until the dark noise reaches the agc level, this is a
reasonable method another, way would be to interupt the light path
mechanicaly, or in your case you could digitaly adjust both the transmited
signal and the reference signal amplitude.

its 80mm

thats a lot biger then mine. i stll used the 15mm.

you mean my phase detector the ad8302? what's soo bad
about it , only the clipping is not good i thaught. Maybe a normal
gilbert cell was far better and then just using the cos-1 function in
my microcontroller to get the phase difference. I actually was
thinking, if no clipping occurs you don't have to use a DSP to detect
signals out of noise because if you multiply the received signal with
the reference signal(with the mixer) and then doing ad conversion of
the result wereafter you integrate the result in the microcontroller.
Then this would be equal to fouriertransformation and this is actually
a correlation soo if you do it long enough the signal has to rise out
from the noise. I was always thinking it had to do this with a fase
ADC and a DSP processor but actually you can do the correlation analog
at first sight.

yes the cliping is the bad part, if you use 2 gilbert cells and adjust one
signal by 90' to each of them you get two signals out wich you can compare
and get full 360' measurment otherwise the signal strength from 1 cell
dominates the output voltage. (hence the cliping to make it simpler). using
2 gilbert cells in this way and analyizing the filtered then digitised
reults in you mcu would get rid of most of the noise. or as someone sugested
in this thread, just using one cell, rotate the phase of your reference
signal through 360' and note the null.

Wow, you are also interested in the whole thing about speed of light
and relativity i see. I didnt hear about this silvertooth experiment
however i am a bit sceptic because what reference clock will u use as
if the speed of light would change (wich would be indeed very
controversial , einstein would not be happy if this should be the
case ) then the atoms in the reference clock will do the same and
the clock will tick accordingly matching the difference out.

I always wondered why the speed of light is independed of the speed of
the reference point. Once you take this as true the rest follows like
time goes slower when you move due acceleration.It's very very
interesting these things...
I am curious how you will try to do this...

hmm maybe shld start another thread, but i dont actualy measure the speed of
light absolutly i just measure any tiny diference between two oposite
directions. i intend to use 2 matched clocks initialy synchronized (atomic
ones would be nice but im gona try make do with state of the art crystal
ones) 1 meter apart then receive the signal in the middle. then il turn the
aparatus thru 180' and note any phase diference. the clocks wil always be
moving at the same velocity relative to eachother so should not experience
any diference between them.

for simplicity leds will indicate positive and negative diference and so you
wil be able to see the orientaion that has maximum diference as they light
up as it all rotates.

of course the efect of moving even at 100,000kmh has a small efect on the
speed of light, with 2 1ghz clock 1 meter apart i need to resolve a few uv
from the phase detector and of course the clocks have to behave like perfect
clocks for the period of rotation. hopefuly any jitter wil average out over
the the period of rotation. i wil probably put the clocks in a phase locked
loop but with a response many times slower than that of rotation. i might
have to move the clocks further apart but would be hard to rotate them, of
course the earth is rotating so i could put them several thousand miles
apart but that would cuase problems, not least is the clocks would not stay
synchronised for 24 hrs.

the Sagnac efect makes use of the diference in speed of light in a circular
fibre to detect rotation movement and is used in aviation guidance, but its
harder to do it linearly. the explanatin Silvertooth gave why his experimant
gave a result but previous ones didnt is beyond me to see why it aplies to
one but not the other. I think the important thing to do is not to try
measure the speed averaged over the forward and reverse directions as this
any change wil be imeasurably small, maybe this is the diference.

Another way to syncrhronise the two clocks would be a very high speed
rotating shaft with encoders on the ends. but the problems here are far more
mechanicaly orienteted. I have heard about a rusian scientist used a mothd
involving a long rotating shaft with slited discs to determine the speed of
light and he published complete with unexplained variations in the speed of
light, but this remained hiden for almost 200 years.

again i like to point out i make no prediction about the outcome although i
suspect my design wil need some improvement before it is sensitive enough.

Colin =^.^=

 

[LRCR]

In both cases. On my own for my Master Thesis and for my former
employer, Hughes Danbury Optical Sysytems, CT.

1. My Thesis used a coarsed resolution Laser rangefinder:

Resolution: 3.44ft (due to the DSP sampling rate)
Max distance: About 44ft., (Laser Power and optical telescpoe Limited)
Laser : CW Diode at 826nm (100mW) Collimated (Melles Giot)
telescope: Refractor (Gold plated mirrored telescope would have been
better, but can't afford it).
Mixer: made by minicircuit for the heterdyne part of my design
local oscillator: 1.64MHz
Output from the mixer is 30kHz
APD detector used

2. From my work:


Resolution: 0.5ft
Max. Distance: 1,000 ft.
Laser: 1.5Watt CW Diode laser (Very Expensive) Collimated (Melles
Giot)
Telescope: Gold plated mirrors Cassegrain telescope
Also an heterdyne Mixer design. Local oscillator much higher (can't
remember now) than the Thesis version in order to achieve the
resolution it has.
APD detector used


The great thing about this project is that you learn about different
areas in engineering(Analog and Digital Electronics, Optics and
Optical Geometry, Power, Laser, Detector, filters, Rf, DSP, HV for the
APD, etc.). Also, cuting and drilling metal parts (thanks to what
tools can find in Sears and Al metals from my local HW store) was fun.

This was a fun project and I believe it helped in my engineering
carrer both directly and indirectly..

 

[Yannick]

good point and idea, although this will be noticable as the laser is useful

for tergeting.
there are other devices wich dont polarise the light, some1 sugested here
some time ago but i forget what it was now, but it wld be rather bulky.

hmm thats interesting, i didnt know it existed.

yes i find that the agc alone sets the bias voltage, with no signal the bias
voltage rises until the dark noise reaches the agc level, this is a
reasonable method another, way would be to interupt the light path
mechanicaly, or in your case you could digitaly adjust both the transmited
signal and the reference signal amplitude.

that's what i inted to do but my dds doesnt allow me to adjust the
output amplitude digital. I am going to use the ad603 variable gain
amplifier after my dds.

thats a lot biger then mine. i stll used the 15mm.

yes but you are using the optoelectronical mixing soo you dont need
such a big signal at the input like me. what were your results without
the mixing?

yes the cliping is the bad part, if you use 2 gilbert cells and adjust one
signal by 90' to each of them you get two signals out wich you can compare
and get full 360' measurment otherwise the signal strength from 1 cell
dominates the output voltage. (hence the cliping to make it simpler). using
2 gilbert cells in this way and analyizing the filtered then digitised
reults in you mcu would get rid of most of the noise. or as someone sugested
in this thread, just using one cell, rotate the phase of your reference
signal through 360' and note the null.

that's a good idea if i understand it correctly u would use 2 gilbert
cells because the detected signal is lower in amplitude as the
reference signal and soo to get the normalised 0.5cos(dphase) output
you compare the output of two gilbert cells wich are in quadrature.

hmm maybe shld start another thread, ...

i am goig to read it carefull but not much time right now, it sounds
interesting...

Yannick