In Reply to: Re: Car counter posted by Jim Drew on 05/30/00 at 12:19 PM:
O.K. I have a much better idea of what you are up to now. This may be a little complex in the decoding department in that you will need a separate receiver for each channel or do some hairy DSP to resolve the individual carrier frequencies. I have designed circuits like this and it's a little harder than it looks on the surface. With multiple transmitters being able to communicate all in close proximity channel spacing becomes important to avoid splatter or crosstalk. There's another issue with violating part 15 of FCC regs regarding intentional radiators in this band so you need to keep output power very low due to the fact that you are driving a coil with a harmonic generator (square wave) that will likely show up in many frequency slots. In fact, you just might swamp out your car's receiver with harmonics leading to unpredictable performance.
On to the circuit.
Trnasmitters can be simple xtal oscillator driven modulo dividers to obtain final frequency driving a NPN open collector driven coil. You could use a resonant tuned tank for the front end utilizing a high Q coil and cap combo tuned to the center frequency of the combined transmitters. Tank circuit math is very simple and can be found by doing a search. Use a standard opamp with decent bandwidth > 1mhz for your receiver front end and gain circuit followed by a phase lock loop (LM567)
With cars passing the loop at 100 MPH calculate your worst case carrier capture window and make sure that a PLL can respond quickly enough to lock and detect the carrier in time. Once the PLL gives you a phase locked indication you can take it from there. I would like to help more but it's a day after a Holiday and I am swamped !
Diagrams on just about everything you need is all over the net on Mfgr sites and hobbiest sites.
All the Best,