I am developening a board for detection of car speed until 180Km/h. I am using a Colpits oscillator where the loop at the street surface is part of the inductor (parallel). Only for information the circuit uses a ARM7 (70MHz) microprocessor and a 100 MHz crystal oscillator and a set of counter 74AHC or 74VHC family in order to detect the speed with 1% precision.
I tested at lab the best frequencies and that results in frequencies between 80 and 120 KHz.
But the doubt is the characteristics of the transformer between the line feed until the loop, including the loop, and the Colpitts circuit.
All the informations that I found shows that the transformer is necessary. But what is the best relation between the primary and secundary, and where is the primary (oscillator side or loop side). Some boards use 2:1 where 2 is the oscillator side. Others use 3:1 where 3 is the oscillator side. Others say 1:1.
I would apreciate to discuss that with a member of the group.
Thanks
Marcos Olandoski
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