Actually you can make it even simpler than that. You begin with a known frequency as your transmit frequency. Nothing non-linear here. You send little bursts of this frequency out. While the transmitter is quiet, the bounced and doppler-shifted echo is amplified and mixed with the original output frequency. The resulting low frequency "beat" frequency represents the doppler-shifted component only. (Ignore the higher frequency beat). Once you know the original frequency and the low beat frequency, it just takes a little bit of math to calculate the relative speed of the object producing the echo. If you can isolate the receiver from the transmitter, then you can forget about the bursts and just send out a continuous signal from the transmitter. In general the receiver needs to be high gain and tuned to respond to a very narrow frequency band. Some laser type doppler-shift device works along the same principles. It is very easy to maintain isolation between transmitter and receiver since the laser beam is very narrow and does not diverge much. So these are almost always run in continuous mode. A form of interferometer mixes a small sample of the outgoing reference signal, and 100% of the received signal (which is usually quite small). The "beat" frequency shows up as light and dark bands. By scanning these fluctuations optically an electrical signal can be produced that can then be measured with a simple counter to reveal the optical "beat" frequency. A little math and the velocity is determined. Fr. Tom McGahee -----Original Message----- From: Dennis Gearon To: PICLIST@MITVMA.MIT.EDU Date: Saturday, January 22, 2000 10:35 AM Subject: Re: [OT] Ground SpeedOmeter >The cops measure speed with a radar. What they do is use a non linear >xtal to mix the incoming with the outgoing reference. It outputs RF from >the mixing! The rest is easy. Laser doesn't work as well in bad >conditions, but your target selection is more accurate. > >-- >----------------------------------------------------------------- >Time Spent With Your Dog(Kid) is Time Well Spent >----------------------------------------------------------------- > Sincerely, Dennis Gearon >