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measure phase difference in square waves (one of which is noisy)

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uoficowboy

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Hi - I'd like to try building a laser range finder that uses phase difference to measure distance to an object. In other words - I'd send out a square wave and then compare the received square wave to the transmitted wave and look for the phase shift. The received signal would of course be quite noisy.

Can anybody give me a starting point for how I would accomplish such a circuit?

Thanks so much!
 

Depends on the frequency and amount of noise. A couple of comparators which may need hysteresis if the signal is very noisy. Then measure each cycle and do a lot of averaging.

Keith
 

keith1200rs said:
Depends on the frequency and amount of noise. A couple of comparators which may need hysteresis if the signal is very noisy. Then measure each cycle and do a lot of averaging.

Keith
Click to expand...

Hi Keith - I would like to drive the laser at at least 1MHz, potentially significantly faster.

I was assuming I'd have to put a PLL or something on the received signal, to get a clean signal out of it. Would that not make sense?
 

I don't think a PLL will help. A PLL consists of a VCO and a phase comparator so you need a decent phase comparator to make a PLL.

I have done it simply by filtering the incoming signal and putting the resultant sine through a zero crossing detector (comparator). By getting a measurement every cycle and averaging it you can remove a considerable amount of the effects of noise.

Keith.
 

keith1200rs said:
I don't think a PLL will help. A PLL consists of a VCO and a phase comparator so you need a decent phase comparator to make a PLL.

I have done it simply by filtering the incoming signal and putting the resultant sine through a zero crossing detector (comparator). By getting a measurement every cycle and averaging it you can remove a considerable amount of the effects of noise.

Keith.
Click to expand...

Hi Keith - what sort of filter do you think would make sense here? I'm hoping to be able to modify the frequency as well as duty cycle (sorry - I should have mentioned this earlier). So a simple bandpass filter would not work well I think.

Also, I think the return signal will be fairly weak, so I may need to do something like bandpass it, amplify it, and then compare. Does that make sense?
 

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