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

>
> 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.
>

more because of my lack of knowledge, i think i understand it now all
much better then 6 months ago.

> 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)

now 3mw, i dont want to go higher due safety reasons.

> 2) larger lense (cheap and easy)

is already quite large:)

> 3) APD cant remember if your using an avalnche detector or not (quite
> expensive)

yes i use the same as you, C30902E at 200-220V reverse bias and cooled
with a peltier element.

> 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)
>

you mentioned this 6 months ago but then i didnt understand it because
i didnt know much about mixing signals, now i think i understand.Did
you came up with it yourself or did you read it somewhere? But i think
it has to be very difficult because you have to sit in the exponential
part (but still in reverse bias) of the Id-Vak curve of the photodiode
otherwise it wont work. but i wonder if it still works for high
frequenties because the current wich flows due the light on the APD
will always see the input capacitance soo the 400Khz wich you get out
is indeed a lower frequency but will also be small if the main
frequency is very high (the carrier). The improvement will be on the
side of the feedback resistor wich can be much larger because the
bandwidth has only to be 400Khz of the transimpedance. Is this correct
or am i missing the point?

> 5) use a simple amplifier with a single ended input stage and much lower
> noise then an op amp, such as one of the latest mosfets.

i might do this with the BF998 orso..

> 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.
>

I meant with signal averaging not just take phase averages , this is
what i do now in my microcontroller but i mean to detect the signal if
it is lower then noise.If your signal is periodic and you know the
frequency you can easily integrate it out of the noise , thats what i
meant with signal averaging. I am gonne do this later when i will
change my phase detector AD8302 with a fast ADC and a DSP (blackfin).
Now i cant due the clipping wich occurs inside the AD8302.

thanks for the good info again, are you still working on yours and
whats the resolution now and the maximum distance? I don't know if you
are cooling your acalanche photodiode but i think it also will give a
*2-3 improvement. The downside is the power consumption of a peltier
element soo battery applications is not possible.


Yannick
.

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