Responder Circuit

[PerilBrain]

Hi,
I want to make a circuit which receives a prespecified frequency and returns back a response. Can any one help me..??
I want the circuit to be as tiny as 2X2 cm.
My idea was to put a an oscillator in fm range but i dont know how to incorporate a switch which recieves the frequency and switch on the oscillator.....

Can any exprienced guy give me suggestion on my this plan:-
ANTENNA--->AMPLIFIER--->FREQUENCY DOUBLER-->BACK TO ANTENNA

So when the circuit receives it doubles the frequency and retransmits back through the same antenna.................thanks

 

[MikeMl]

What range are you trying to get? This is sort of like what they put in library books. Those devices have a range of only a foot or two, and use a huge loop antenna for the interogator.

 

[PerilBrain]

range is about 1.2 km (~.8mile).........