Re: Update on laser distance meter project (master thesis) and question
- From: "colin" <no.spam.for.me@xxxxxxxxxxxx>
- Date: Sun, 03 Apr 2005 18:40:44 GMT
"Yannick" <yannick_de_wit@xxxxxxxxxx> wrote in message
news:cc50d220.0504011305.180f3939@xxxxxxxxxxxxxxxxxxxxx
> Maybe some remember i was developing a laser range finder for my
> master thesis. Now 2 months from the end i can say it is a succes. I
> can measure distances up to 10meters from white objects. The
> resolution is good the first few meters (1-2cm) and afterwards worse
> and worse (10-15cm for 10 meter). The problems lies on several things
> , i still use a dds frequency generator with 125Mhz clock , for the
> higher frequenties (20-30Mhz) the sinus is far from perfect. The
> second problem is ofcourse the photodiode preamplifier wich is still
> designed around the opa657 in the most simple configuration. The
> thirth bottle neck is that i use a phase detector ic from analog
> devices (AD8302) wich does clipping on the signal before multiplying.
> Soo signals lower then noise cant be detected, this will be solved due
> using a DSP processor (ad's blackfin) and with signal averaging i can
> measure signals lower then noise (it will only take more time).
>
> There is still something strange with the photodiode preamplifier , i
> tried many things including the R-C-R trick described by winfield
> hill, although simulation in microcap gave me stunning results and
> huge bandwidth improvement, in practice it didnt work, i the best case
> i had exact the same results and mostly worse. Then i thought of
> something simpler, just using a resistor tree network to reduce
> resistor values and still have the same total feedback resistor (68K
> in my case). This is described in photodiode amplifiers from gerald
> greame and i did it exactly like he pointed out. i use for R1 = 22K ,
> R2=270 and R3 to mass = 100 soo i will have a total feedback
> resistance of Rfeq=R1*(1+R2/R3) = 81.4K.
>
> In simulation this worked wonderfull,more gain, more bandwidth then
> with my singe 68K feedback resistance. But in practice the result was
> worse. What can this be? everything is made in smd components with a
> open ground plane to reduce capacitance to ground and everything is
> shielded in a RFI case.
>
> I want to thank Colin,Winfield hill and al the others for their help
> in the beginning (6 months ago) of this project , it made everything a
> lot easier for me.
>
> greetings,
>
> Yannick
Hi, I was wondering how you were getting on, and glad to of been some help
although i mentioned so many things i feel i might have confused things a
bit.
As we discued before ultimately the SNR for a given signal is limited by the
total capacitance at the detector and the amplifier input noise, regarldes
of any feedback network, wich can only degrade it further if you are not
carefull and might explain why its worse, or as before you experienced high
frequcncy instabliity wich you could not see on your scope.
there are several things to consider to make things better, starting at one
end and working through ..
1) stronger light source (probably not cheap or safe)
2) larger lense (cheap and easy)
3) APD cant remember if your using an avalnche detector or not (quite
expensive)
4) use a technique of downshifting the frequcncy using the photodiode itself
as a multiplier/mixer (for example 400khz), the result is that the signal is
no longer swamped by the efect of the capacitance and hence a great deal
stronger at the input to the amplifier, yet the actual phase shift in the
high frequcncy signal is maintained. you would need to generate a frequcncy
400khz lower or higher than your transmision signal and aply this to the
bias voltage of the detector, the folowing amplifier would only see signal
of 400khz so could easily be tuned further reducing the efect of the
capacitance. this may easily give a several thousandfold improvement in SNR,
its the technique i used in the end with dual transmision frequencies.
(not particularly easy but by far the most dramatic improvment and it
greatly simplifies the amplifier so may wel be worth it)
5) use a simple amplifier with a single ended input stage and much lower
noise than an op amp, such as one of the latest mosfets.
6) im not sure why you are focusing on constant bandwidth so much when you
are cliping the signal anyway, it seems like this is cuasing you a lot of
hardwork for no aparent gain.
what i would think is most important by far is its noise performance and
obviously that it has suficient gain and also phase delay that is
predictable and stable and that you can take this into acount in your phase
calculations.
reducing the feedback network to a bare minimum at the expense of flatness
of response might make things better - more stable and beter noise
performance. a constant 90' phase lag over the frequncy range you are using
might be quite easy to compensate for in your calculations, and extremly
easy to build a simple amplifier with much beter noise performance.
7) averaging over time is always posible, i asume you have tried taking
readings and averaging them by hand ? however i suspect that once the signal
drops below the noise floor at the clipper it is iretrevably lost.
8) im realy not sure to what extent the noise from the DDS is causing a
problem or how you would improve it, i suspect it has little efect, did i
sugest before a switched bank of sevaral crystal oscilators ? i have decided
to move away from PLL etc as its so hard to completly eliminate jitter from
creeping in through the supply ect, and use a series of dividers and
frequcncy multipliers to derive each frequency from one master clock, but i
only use 6 frequcnies in total.
Colin =^.^=
.
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