In a method for demodulating an analog FSK signal, a current sample ;Idi) of the downconverted and digital inphase component is multiplied with a previous sample ;Qdi1) of the downconverted an digital orthogonal phase component. The method comprises first charging a capacitor to a predetermined voltage during a precharge stage. The method of the present invention has advantages of minimizing the numbers and the amounts of byproducts by using a complex reducing agent and producing organic-transition metal hydride safely without denaturation under more moderate reaction conditions. The tag portion can be configured to be attached to animal in any method known in the art.