Y
Yannick
- Jan 1, 1970
- 0
As some will remember i developed a laser range finder last year for my
master thesis... it was a succes, got resolution of 1.5cm over 10 meter
range. I got 87% for it and placed second in the philips lighting
thesis contest for industrial engineering in belgium. I want to thank
all and especially Colin for his help on the receiver end, without him
and also Winfield Hill it would have costed me a lot more time and
probably not have worked as good...
Now i am trying to achieve mm resolution with optoelectronic mixing as
Colin also explained to me last year. I made a complete receiver were i
inject a frequenty into the bias voltage of the avalanche photodiode.
This injection occurs with a high voltage coupling capacitor right on
the kathode of the photodiode with a resistor of 50ohm to the dc
reverse bias power supply. The dc bias voltage is 207 V and the used
avalanche photodiode is the C30902E. Next i used a 100K smd resistor to
give sufficient signal at the input of my amplifier. The higher this
resistor the better for the signal-noise ratio(for thermal resistor
noise) (as noise increases with root of the resistance and the signal
lineair) but too much would decrease the bandwidth with a total
estimated input capacitance of about 20pf. Next i use a opa657 for now
(i am going to change this to the opa655 but i am waiting for the
samples) with voltage feedback(inverting amplifier) with a AC voltage
gain of 2000 (bandwidth 200KHz with feedback capacitor) and DC unity
gain (for exact offset at the output).The power supply is between 0 and
12V , and the signal is DC offsetted at 6V (7806 with 100K -> this give
50K small signal gain (2 times 100K parrallel)) after the amplifier i
use a one stage RC low pass filter too get high frequentie noise and
sum frequency out of the signal bandwidth.
Now i injected between 30-60Mhz signal with a amplitude between
100mV-3V on the reverse bias with a laser send frequency 0-300Khz
apart. On the output of my amplifier i clearly see the mid frequency of
0-300Khz with an amplitude of 8V pk-pk !!! if I use retroreflection and
increase the injection signal pk-pk upto 1.5V. But the output signal
is also very noisy => 1v pk-pk. I assume this is due the photodiode
shot noise wich is also amplified 2000 times. This would easily be
solved with using a tuned feedback but then rapid phase change would
occur for a small frequentie error wich would give a large distance
error (distance is measured through phase measurement).
Soo with using retroreflection signal / noise ratio is positive, but on
a white wall the signal is almost as low as the noise wich is 1V
pk-pk!!!!
Too better understand the reason where the noise came from i put a
spectrum analyser at the output of the photodiode and completly removed
the amplifier. Then it seems that the noise is really in function of
the DC reverse bias voltage(wich is perfectly normal as darkcurrent
increases and thus shot noise with reverse bias) and especially in the
lower frequency range 0-1Mhz. (It must be noted that the input
impedance of the spectrum analyser is 50ohms soo there is not much
signal voltage amplification) Also the DC reverse bias voltage was
lower then 180 V no extra noise occured (in comparison with no dc
voltage)soo this power supply i made with 2 zener stages and low pass
filtering is for noise no problem.
The big question is now, how should i get rid of the noise without
tuning , if u read this colin, did you also had much noise due the
large voltage gain of your amplifier wich amplifies the shot noise and
ifso how did you solve it???
Thx for any help on this one.
master thesis... it was a succes, got resolution of 1.5cm over 10 meter
range. I got 87% for it and placed second in the philips lighting
thesis contest for industrial engineering in belgium. I want to thank
all and especially Colin for his help on the receiver end, without him
and also Winfield Hill it would have costed me a lot more time and
probably not have worked as good...
Now i am trying to achieve mm resolution with optoelectronic mixing as
Colin also explained to me last year. I made a complete receiver were i
inject a frequenty into the bias voltage of the avalanche photodiode.
This injection occurs with a high voltage coupling capacitor right on
the kathode of the photodiode with a resistor of 50ohm to the dc
reverse bias power supply. The dc bias voltage is 207 V and the used
avalanche photodiode is the C30902E. Next i used a 100K smd resistor to
give sufficient signal at the input of my amplifier. The higher this
resistor the better for the signal-noise ratio(for thermal resistor
noise) (as noise increases with root of the resistance and the signal
lineair) but too much would decrease the bandwidth with a total
estimated input capacitance of about 20pf. Next i use a opa657 for now
(i am going to change this to the opa655 but i am waiting for the
samples) with voltage feedback(inverting amplifier) with a AC voltage
gain of 2000 (bandwidth 200KHz with feedback capacitor) and DC unity
gain (for exact offset at the output).The power supply is between 0 and
12V , and the signal is DC offsetted at 6V (7806 with 100K -> this give
50K small signal gain (2 times 100K parrallel)) after the amplifier i
use a one stage RC low pass filter too get high frequentie noise and
sum frequency out of the signal bandwidth.
Now i injected between 30-60Mhz signal with a amplitude between
100mV-3V on the reverse bias with a laser send frequency 0-300Khz
apart. On the output of my amplifier i clearly see the mid frequency of
0-300Khz with an amplitude of 8V pk-pk !!! if I use retroreflection and
increase the injection signal pk-pk upto 1.5V. But the output signal
is also very noisy => 1v pk-pk. I assume this is due the photodiode
shot noise wich is also amplified 2000 times. This would easily be
solved with using a tuned feedback but then rapid phase change would
occur for a small frequentie error wich would give a large distance
error (distance is measured through phase measurement).
Soo with using retroreflection signal / noise ratio is positive, but on
a white wall the signal is almost as low as the noise wich is 1V
pk-pk!!!!
Too better understand the reason where the noise came from i put a
spectrum analyser at the output of the photodiode and completly removed
the amplifier. Then it seems that the noise is really in function of
the DC reverse bias voltage(wich is perfectly normal as darkcurrent
increases and thus shot noise with reverse bias) and especially in the
lower frequency range 0-1Mhz. (It must be noted that the input
impedance of the spectrum analyser is 50ohms soo there is not much
signal voltage amplification) Also the DC reverse bias voltage was
lower then 180 V no extra noise occured (in comparison with no dc
voltage)soo this power supply i made with 2 zener stages and low pass
filtering is for noise no problem.
The big question is now, how should i get rid of the noise without
tuning , if u read this colin, did you also had much noise due the
large voltage gain of your amplifier wich amplifies the shot noise and
ifso how did you solve it???
Thx for any help on this one.
