Re: Leica laser range finder , someone opened it?


Hi,

Hi Colin:)

I used the same APD device but I find it works best as a mixer closer to or
above vbr (~250v for mine) where the gain/voltage curve is much steeper,
of course you need a bit of careful control (and current limit), the amount
of extra noise from the apd at low light is a good indication of the
operating point on the curve.

Yes i was planning to do this but first wanted to get rid of the noise
somehow...

what frequency and level of modulation have you got on the apd ?
and what is the IF frequency ?

50Mhz and also 100Mhz worked fine, after this my laser modulator wasnt
fast enough (used a transistor with ft of 100Mhz) , i have build a new
one which will go to 500Mhz. Will try it soon.

The IF is between 10Khz and 100Khz. The amplitude between 0.5V and 3V
(after this not more signal gain seen, about 2V seems best for S/R
ratio) at 217V reverse bias.


the main problem I am finding now is that at >1ghz I cant fully modulate the
cheap laser diode I have and so the constant light cuases more shot noise.
Im trying to get it to go at ~2ghz (and looking for a better laser)


Were can you find lasers with datasheets, i am still using sone
laserdiode from a cheap laser pen:)

I use a discrete DG mosfet to amplify the signal from the APD with a gain of
~20 (using a pnp current source and 1k load impedance), a sheilded inductor
provides a DC path and tunes out the capacitance - you actually need a
capacitor to ground to provide an ac path for the APD bias modulation
current,

I dont really understand your schematic and how you bias it but an
inductor must be really really large to be high impedant at these low
IF frequencies...

the filtering of the bias frequency happens mostly due to the large
gain and GBP , but i was thinking of just adding a capacitor at the
input to reject it before... dont know if it is going to make a lot
difference...



I find the noise is purly shot noise from the apd wich increases with
increasing light, when totaly dark the output is very quiet.

Ok, thats also the case, when its dark the noise is a factor 2 less but
still about 100mV after a total amplification of 5K * 5000 voltage
gain...

My signal amplitude on a white wall at about 3 meters is 3.5V pk-pk.
but AC laser power is still not optimal... maybe a factor 2-3 more is
possible

If i short input, noise is all gone...(mostly :)) if i remove
photodiode it doesnt do much,just a factor 2...

Therefore I think something is wrong if your amplifier/resister noise is
greater than your apd noise, I had a 10k resistor initialy but like you
found DC bias current was too high for this, an inductor needs to be
shielded well, a good alternative is to use a common base npn transistor to
provide a DC feedback path.


A common base is very low impedant , the signal would source away, i
think i do not understand the schematic:)

I have seen a different type of laser range finder wich uses interference
yet still measures absolute distance, it uses a dual cavity arangement where
the light from the target is reflected back into the laser and an external
cavity is modulated by a piezo element. The internal cavity is also affected
by the distance to target and as the variable cavity length is changed it
also affects the operating wavelength of the laser and so alters the
interference cuased by the reflected light, the resultant change in
brightness due to inteference is actually detected by the photodiode in the
laser diode itself and some correlation method comparing brightness to
external cavity length is used to determine absoulute actual distance down
to a few nm.

Ive no idea wich method the Leica uses, it sounds very usefull.


we will see, will buy one in the coming weeks...

I still find it interesting enough to drive me to see how fast I can get it
going with optoelectronic mixing.

Me too, think 2Ghz is the max due to diffusion current speed is limited
to these periods...

Yanick

Colin =^.^=

.

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