Help needed designing a speedometer

Hi,

I have a project for a speedometer that can detect a vehicle speed between 0 - 999 kph. I have been doing some research but i seem to run into the same problem nomatter what I do.

The Basic principle I'm using is the Doppler effect. This gives me a change of frequency in response and from there i should be able to determine the speed.

Now the problem is the frequency response. First I chose to send a signal with a laser LED at a frequency of 100 MHz. Later I realised that I could improve the resolution by sending a pulse at 40 Khz (ultra sound) and then determine the response. The response from a 40KHz is then 40-218 Khz that i'm interested in. But in both cases, How can I determine a frequency change? I haven't done electronics for years and seem to get lost in various terms as PLL, VCO etc. Could I not simply use a Frequency to Voltage converter? If so, How?

My idea is then to normalise the given voltage, then convert it back to certain frequency range that in final stage goes in as a clock frequency into a counter. First I thought I could simply use the response as the clock frequency for the counter but because it's non-linear, the error increases dramatically on higher velocities.

Maybe i'm way of in my thinking and there is a better/simpler solution to this problem. Any help is appreciated.

Thanks

Indy

maybe you can use DSP to do it.
for example,the FSk is an algorithm on the frequency shift. maybe you can get idea from it. It's only my guess. maybe it does not work.

YOU MAY FACE PROB OF SYNCHONITION OF TXD AND RXD PULES.