How to caracterize transfer function of signal chain

Dsp1000 wrote:

   ...

> A laser range finder is a laser distance (range) measuring device.
> It will give you distance to an object you shine light on as a binary or
> analog output. There are two main methods to use: Time of flight of the
> laser beam and phase difference detection. The latter is better on short
> distances but has some challenges associated with it. It is a more advanced
> version of the latter I am looking at. 
> 
> All experts on this list: anybody with laser range finder experience?

Not every distance-measuring device is a range finder. Laser 
interferometers, while capable of measuring movements significantly 
smaller than a wavelength, do not make absolute measurements. There is a 
phase uncertainty of half a wavelength in distance (a whole wavelength 
in the round trip.

Every laser has a coherence length. Interferometry ceases to function 
when the round-trip distance approximated the coherence length.

These two issues make time of flight attractive.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

>Dsp1000 wrote:
>
>   ...
>
>> A laser range finder is a laser distance (range) measuring device.
>> It will give you distance to an object you shine light on as a binary
or
>> analog output. There are two main methods to use: Time of flight of
the
>> laser beam and phase difference detection. The latter is better on
short
>> distances but has some challenges associated with it. It is a more
advanced
>> version of the latter I am looking at. 
>> 
>> All experts on this list: anybody with laser range finder experience?
>
>Not every distance-measuring device is a range finder. Laser 
>interferometers, while capable of measuring movements significantly 
>smaller than a wavelength, do not make absolute measurements. There is a

>phase uncertainty of half a wavelength in distance (a whole wavelength 
>in the round trip.
>
>Every laser has a coherence length. Interferometry ceases to function 
>when the round-trip distance approximated the coherence length.
>
>These two issues make time of flight attractive.
>
>Jerry
>-- 
>Engineering is the art of making what you want from things you can get.
>�����������������������������������������������������������������������
>

Ok regarding tine of flight contra phase. However the distance that is
going to be measured here is down to 10 cm and up to approx 30 meters. I
think the pulse delay will be so short that it might be difficult to
detect? 

How would you solve this problem? The laser must be eye safe, low cost
(laser pointer type). The detector should use a photodiode or avalanche
photodiode. There might be a CPU running DSP algorithms in the system. The
range will be 10cm to 30 meters. The height must be measured in 1cm
resolution.  

Mike

Dsp1000 wrote:
>> Dsp1000 wrote:
>>
>>   ...
>>
>>> A laser range finder is a laser distance (range) measuring device.
>>> It will give you distance to an object you shine light on as a binary
> or
>>> analog output. There are two main methods to use: Time of flight of
> the
>>> laser beam and phase difference detection. The latter is better on
> short
>>> distances but has some challenges associated with it. It is a more
> advanced
>>> version of the latter I am looking at. 
>>>
>>> All experts on this list: anybody with laser range finder experience?
>> Not every distance-measuring device is a range finder. Laser 
>> interferometers, while capable of measuring movements significantly 
>> smaller than a wavelength, do not make absolute measurements. There is a
> 
>> phase uncertainty of half a wavelength in distance (a whole wavelength 
>> in the round trip.
>>
>> Every laser has a coherence length. Interferometry ceases to function 
>> when the round-trip distance approximated the coherence length.
>>
>> These two issues make time of flight attractive.
>>
>> Jerry
> 
> Ok regarding tine of flight contra phase. However the distance that is
> going to be measured here is down to 10 cm and up to approx 30 meters. I
> think the pulse delay will be so short that it might be difficult to
> detect? 

Yes.

> How would you solve this problem? 

A laser interferometer can count fringes and so make differential 
measurements. (It takes careful thought to make this work in the 
presence of even minute vibration, but that's an implementation detail.) 
If you have some way to establish a base-line distance (contact, or 20 
cm, or whatever) You're good to go.

> The laser must be eye safe, low cost
> (laser pointer type). The detector should use a photodiode or avalanche
> photodiode. There might be a CPU running DSP algorithms in the system. The
> range will be 10cm to 30 meters. The height must be measured in 1cm
> resolution. 

The basic resolution of an interferometer is half a wavelength, but you 
should be able to halve that and maybe halve it again. Fine enough? :-)

You will need at least two photodiodes to get direction information, 
just as with a quadrature shaft encoder. Counting fringes is useless 
without knowing whether the distance is increasing or decreasing. The 
fringes will be going by much too fast for the processor to count them. 
You'll need some external hardware.

I'm not at all sure that a laser pointer has enough coherence length for 
a 60-meter round trip.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Jerry Avins wrote:


> The basic resolution of an interferometer is half a wavelength, but you 
> should be able to halve that and maybe halve it again. Fine enough? :-)

Actually the resolution of a reasonably good HeNe laser interferometer 
is at the order of one angstrom (1e-10 meters) or so. You can see the 
effects like the Brownian motion of the macroscopic objects.

> You will need at least two photodiodes to get direction information, 
> just as with a quadrature shaft encoder. Counting fringes is useless 
> without knowing whether the distance is increasing or decreasing. The 
> fringes will be going by much too fast for the processor to count them. 
> You'll need some external hardware.

Yes, this is one of the ways to do it.

> I'm not at all sure that a laser pointer has enough coherence length for 
> a 60-meter round trip.

Sure it hasn't, and the noise of the laser sets the limit also. The 
common laser rangefinders use the on-off keying of the beam with the 
frequency of ~50MHz or so, and look for the phase shift at the receiver.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

>Dsp1000 wrote:
>>> Dsp1000 wrote:
>>>
>>>   ...
>>>
>>>> A laser range finder is a laser distance (range) measuring device.
>>>> It will give you distance to an object you shine light on as a
binary
>> or
>>>> analog output. There are two main methods to use: Time of flight of
>> the
>>>> laser beam and phase difference detection. The latter is better on
>> short
>>>> distances but has some challenges associated with it. It is a more
>> advanced
>>>> version of the latter I am looking at. 
>>>>
>>>> All experts on this list: anybody with laser range finder
experience?
>>> Not every distance-measuring device is a range finder. Laser 
>>> interferometers, while capable of measuring movements significantly 
>>> smaller than a wavelength, do not make absolute measurements. There is
a
>> 
>>> phase uncertainty of half a wavelength in distance (a whole wavelength

>>> in the round trip.
>>>
>>> Every laser has a coherence length. Interferometry ceases to function

>>> when the round-trip distance approximated the coherence length.
>>>
>>> These two issues make time of flight attractive.
>>>
>>> Jerry
>> 
>> Ok regarding tine of flight contra phase. However the distance that is
>> going to be measured here is down to 10 cm and up to approx 30 meters.
I
>> think the pulse delay will be so short that it might be difficult to
>> detect? 
>
>Yes.
>
>> How would you solve this problem? 
>
>A laser interferometer can count fringes and so make differential 
>measurements. (It takes careful thought to make this work in the 
>presence of even minute vibration, but that's an implementation detail.)

>If you have some way to establish a base-line distance (contact, or 20 
>cm, or whatever) You're good to go.
>
>> The laser must be eye safe, low cost
>> (laser pointer type). The detector should use a photodiode or
avalanche
>> photodiode. There might be a CPU running DSP algorithms in the system.
The
>> range will be 10cm to 30 meters. The height must be measured in 1cm
>> resolution. 
>
>The basic resolution of an interferometer is half a wavelength, but you 
>should be able to halve that and maybe halve it again. Fine enough? :-)
>
>You will need at least two photodiodes to get direction information, 
>just as with a quadrature shaft encoder. Counting fringes is useless 
>without knowing whether the distance is increasing or decreasing. The 
>fringes will be going by much too fast for the processor to count them. 
>You'll need some external hardware.
>
>I'm not at all sure that a laser pointer has enough coherence length for

>a 60-meter round trip.
>
>Jerry
>-- 
>Engineering is the art of making what you want from things you can get.
>�����������������������������������������������������������������������

Hi, what I have in mind is not an optical intererometer. More a
transmitter that has a modulated beam (modulation on top of the coherent
laser beam. For example amplitude modulation). The modulation could have 25
MHz frequency or more. 

The detector will detect the modulation and measure phase difference on
that signal. 

A more advanced version will use a pseudo random sequence to modulate the
laser. A correlator will give a peak and indicate delta T. 

Any idea how to characterize the transmission / reception chain? 

Mike

>Dsp1000 wrote:
>>> Dsp1000 wrote:
>>>
>>>   ...
>>>
>>>> A laser range finder is a laser distance (range) measuring device.
>>>> It will give you distance to an object you shine light on as a
binary
>> or
>>>> analog output. There are two main methods to use: Time of flight of
>> the
>>>> laser beam and phase difference detection. The latter is better on
>> short
>>>> distances but has some challenges associated with it. It is a more
>> advanced
>>>> version of the latter I am looking at. 
>>>>
>>>> All experts on this list: anybody with laser range finder
experience?
>>> Not every distance-measuring device is a range finder. Laser 
>>> interferometers, while capable of measuring movements significantly 
>>> smaller than a wavelength, do not make absolute measurements. There is
a
>> 
>>> phase uncertainty of half a wavelength in distance (a whole wavelength

>>> in the round trip.
>>>
>>> Every laser has a coherence length. Interferometry ceases to function

>>> when the round-trip distance approximated the coherence length.
>>>
>>> These two issues make time of flight attractive.
>>>
>>> Jerry
>> 
>> Ok regarding tine of flight contra phase. However the distance that is
>> going to be measured here is down to 10 cm and up to approx 30 meters.
I
>> think the pulse delay will be so short that it might be difficult to
>> detect? 
>
>Yes.
>
>> How would you solve this problem? 
>
>A laser interferometer can count fringes and so make differential 
>measurements. (It takes careful thought to make this work in the 
>presence of even minute vibration, but that's an implementation detail.)

>If you have some way to establish a base-line distance (contact, or 20 
>cm, or whatever) You're good to go.
>
>> The laser must be eye safe, low cost
>> (laser pointer type). The detector should use a photodiode or
avalanche
>> photodiode. There might be a CPU running DSP algorithms in the system.
The
>> range will be 10cm to 30 meters. The height must be measured in 1cm
>> resolution. 
>
>The basic resolution of an interferometer is half a wavelength, but you 
>should be able to halve that and maybe halve it again. Fine enough? :-)
>
>You will need at least two photodiodes to get direction information, 
>just as with a quadrature shaft encoder. Counting fringes is useless 
>without knowing whether the distance is increasing or decreasing. The 
>fringes will be going by much too fast for the processor to count them. 
>You'll need some external hardware.
>
>I'm not at all sure that a laser pointer has enough coherence length for

>a 60-meter round trip.
>
>Jerry
>-- 
>Engineering is the art of making what you want from things you can get.
>�����������������������������������������������������������������������

Hi, what I have in mind is not an optical intererometer. More a
transmitter that has a modulated beam (modulation on top of the coherent
laser beam. For example amplitude modulation). The modulation could have 25
MHz frequency or more. 

The detector will detect the modulation and measure phase difference on
that signal. 

A more advanced version will use a pseudo random sequence to modulate the
laser. A correlator will give a peak and indicate delta T. 

Any idea how to characterize the transmission / reception chain? 

Mike

Dsp1000 wrote:
>> Dsp1000 wrote:
>>>> Dsp1000 wrote:
>>>>
>>>>   ...
>>>>
>>>>> A laser range finder is a laser distance (range) measuring device.
>>>>> It will give you distance to an object you shine light on as a
> binary
>>> or
>>>>> analog output. There are two main methods to use: Time of flight of
>>> the
>>>>> laser beam and phase difference detection. The latter is better on
>>> short
>>>>> distances but has some challenges associated with it. It is a more
>>> advanced
>>>>> version of the latter I am looking at. 
>>>>>
>>>>> All experts on this list: anybody with laser range finder
> experience?
>>>> Not every distance-measuring device is a range finder. Laser 
>>>> interferometers, while capable of measuring movements significantly 
>>>> smaller than a wavelength, do not make absolute measurements. There is
> a
>>>> phase uncertainty of half a wavelength in distance (a whole wavelength
> 
>>>> in the round trip.
>>>>
>>>> Every laser has a coherence length. Interferometry ceases to function
> 
>>>> when the round-trip distance approximated the coherence length.
>>>>
>>>> These two issues make time of flight attractive.
>>>>
>>>> Jerry
>>> Ok regarding tine of flight contra phase. However the distance that is
>>> going to be measured here is down to 10 cm and up to approx 30 meters.
> I
>>> think the pulse delay will be so short that it might be difficult to
>>> detect? 
>> Yes.
>>
>>> How would you solve this problem? 
>> A laser interferometer can count fringes and so make differential 
>> measurements. (It takes careful thought to make this work in the 
>> presence of even minute vibration, but that's an implementation detail.)
> 
>> If you have some way to establish a base-line distance (contact, or 20 
>> cm, or whatever) You're good to go.
>>
>>> The laser must be eye safe, low cost
>>> (laser pointer type). The detector should use a photodiode or
> avalanche
>>> photodiode. There might be a CPU running DSP algorithms in the system.
> The
>>> range will be 10cm to 30 meters. The height must be measured in 1cm
>>> resolution. 
>> The basic resolution of an interferometer is half a wavelength, but you 
>> should be able to halve that and maybe halve it again. Fine enough? :-)
>>
>> You will need at least two photodiodes to get direction information, 
>> just as with a quadrature shaft encoder. Counting fringes is useless 
>> without knowing whether the distance is increasing or decreasing. The 
>> fringes will be going by much too fast for the processor to count them. 
>> You'll need some external hardware.
>>
>> I'm not at all sure that a laser pointer has enough coherence length for
>> a 60-meter round trip.
>>
> Hi, what I have in mind is not an optical intererometer. More a
> transmitter that has a modulated beam (modulation on top of the coherent
> laser beam. For example amplitude modulation). The modulation could have 25
> MHz frequency or more. 
> 
> The detector will detect the modulation and measure phase difference on
> that signal. 
> 
> A more advanced version will use a pseudo random sequence to modulate the
> laser. A correlator will give a peak and indicate delta T. 
> 
> Any idea how to characterize the transmission / reception chain? 

Characterize? With the specs on the tender ans receiver, that's easy. 
Otherwise. assemble and test it. If it's just a theoretical exercise. 
make any assumptions you like.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������