RFID system with controlled charge

TLDR: In this paper, the transponder response signal indicates that it has received enough power from the interrogation signal, represented as a voltage stored on a capacitor located in the Transponder.

Abstract: Since the distance from the interrogator to the transponder cannot be controlled in many RF-ID applications, a time or power control over the interrogation pulse is provided to eliminate transponder heat-up. The RF_ID system transmits a narrow interrogation pulse, for example of 5 msec duration and then monitors for an even shorter duration, for example 1 msec, for a transponders response signal. The monitoring duration is maintained a minimum to create the most efficient charge-up system. Upon the absence of the transponders response signal, the interrogator increases the interrogation pulse duration in steps, until the receiver in the interrogator detects a response signal. A response signal indicates that the transponder has received enough power from the interrogation signal, represented as a voltage stored on a capacitor located in the transponder. This system can be supported by transponders which have no discharge function, to allow the transponders to achieve the final charge-up voltage level with a series of combined short duration power pulses, or by transponders which have discharge functions and which must wait for an adequate duration interrogation pulse to receive enough voltage. Alternatively, instead of increasing the duration of the interrogation pulses, the amplitude of the interrogation pulses can be increased to produce a similar increase in the amount of interrogation power transmitted.