On Thu, 23 Oct 1997 09:19:37 -0400 Tom Rogers writes: >Why not? Do the math... This is how surveying equipment like EDMs (Electronic Distance Measurement) work. They are kind of expensive but I suspect the cost is mostly in the optics, not electronics. The unit needs a perfectly reflecting target and medium-power telescope optics to obtain enough return signal to take a reading. If you can resolve the phase to 1 degree, the wavelength of the modulating signal needs to be (distance resolution / 720). The factor is 720, not 360 because the wave travels to the target and back, doubling the phase change for a given distance change. Using a 50 MHz signal (wavelength 6 meters), it could resolve about 1 cm. But the measurement repeats every 3m, so if the range is longer than that the measurement is ambiguous. So it is necessary to use additional modulating frequencies to resolve which cycle contains the target. >-----Original Message----- >From: Tjaart van der Walt >To: PICLIST@MITVMA.MIT.EDU >Date: Thursday, October 23, 1997 1:48 AM >Subject: Re: Distance measurement > > >>Why not modulate a laser diode with a sinus wave. Hook the receiver >onto >>a pll and measure the phase difference. By changing the modulation >>frequency, you can set the resolution and range. Ambient noise will >be >>less >>of a problem. The parts shouldn't be that expensive either. >