| ??? 05/08/11 15:07 Read: times |
#182201 - chopping... Responding to: ???'s previous message |
Go to this site:
http://translate.google.de/translate...fault.aspx When you scroll down, you will find a schematic of such a sensor circuitry. You will see, that R1, R2 and C are components of this circuitry. In the text, they tell something about a modulation frequency of 1200Hz. This is the frequency by which the light is flickering. The sensor does not generate this frequency, but does only amplify this frequency, which comes from the modulated light, which is given to the sensor. So, the sensor is not exposed to constant light, but light that is modulated, for instance by the help of a chopper wheel, like shown here: http://www.google.de/imgres?im...-wbf9c3SAQ This helps to eliminate the influence of DC-offsets and low frequency noise. Mehdi said:
Thanks,I guess you are right,because when i put a potentiometer instead of R1, at some pot places the sensor output pulsed with some frequensies arround 180khz, I think, this is some sort of unwanted oscillation and has nothing to do with the original signal. Mehdi said:
so if i need to capture 100 khz input light, The electronics inside such a sensor, especially if it contains a phototransistor, isn't very fast. So, I think the modulation of light should be arround 1kHz or even below. Mehdi said:
1-what are typical values for best performance for r1,r2 and c? I guess, a gain of 10 is realistic, maybe even more. I guess, a starting point could be R2=4k7, R1=47k and C=100nF. This gives a corner frequency of high pass filter of around 340Hz and should be suited for a modulating frequency of 1kHz. Mehdi said:
2-can i place a lowpass filter in sensor output before the signal conditioning stage? if yes , what is corner frequency of this lowpass filter? Of course, you can put a low pass filter into the signal chain. And if the modulation frequency of light is 1kHz, than the low pass filter corner frequency could be about 3kHz. But take care, the signal processing of such signals is usually way more sophisticated and contains lock-in amplifiers and such stuff. It highly depends on your application... Mehdi said:
Kai,i really appreciate you,and i don't know, how i can recompense your kindness There is nothing to recompense, dear Mehdi... Kai Klaas |
| Topic | Author | Date |
| Datasheet question???? | 01/01/70 00:00 | |
| where | 01/01/70 00:00 | |
| High pass filter... | 01/01/70 00:00 | |
| pulsed light | 01/01/70 00:00 | |
| "typo" | 01/01/70 00:00 | |
| Thanks Andy | 01/01/70 00:00 | |
| chopping... | 01/01/70 00:00 | |
| DC Offset | 01/01/70 00:00 | |
| But gain is different for AC and DC! | 01/01/70 00:00 | |
| But gain is different for AC and DC? | 01/01/70 00:00 | |
| C reduces the gain for DC | 01/01/70 00:00 | |
I understood much more about this sensor | 01/01/70 00:00 | |
| Thank You Very Much | 01/01/70 00:00 |