Rev. 3.5

RI-RFM-006A

Page 16 of 19

! Typical Antenna Circuit Configuration

The antenna circuit consisting of L and C illustrated earlier in this manual (see the section describing

“Applied Circuit Configuration – Example”) is designed to work as an LC serial resonance circuit in

which impedance drops in the presence of resonance frequencies during sending operation, and as an LC

parallel resonance circuit in which impedance increases in the presence of resonance frequencies during

receiving operation. The relationship between L and C incorporated in the resonance circuit can be

defined according to the following expression. Each value is calculated using this expression:

f

LC

()

134. 2 KHz

=

1

2

π

The higher the Q value (quality factor) is, the higher transmission power the antenna L obtains and also

the higher the receiving gain becomes, thus allowing the system to have a greater available data

communication distance. If, however, at switch-over from power transmission mode to the receiving

mode, damping of the power transmission signal would not be completed before the remote TIRIS

transponder sends its ID code back to the IC, the signals sent from the transponder could not be received

properly. And the higher the Q value is, the longer the decay time of this power transmission signal will

be. It has been revealed by experimental testing that an antenna with its maximum Q value of around 30 is

usable in the circuit as is as illustrated in the “Applied Circuit Configuration – Example” section. If it is

desired to use an antenna having a higher Q value, some measures must be devised and added to this

circuit.

Additionally, the characteristics and efficiency of the resonance circuit used here greatly depend not only

on the antenna L but also on the capacitor C and the MOSFETs which drive them. Therefore, the

application of the lowest possible impedance at the frequency f(134.2 kHz) to them will permit a higher

transmission power, and as a result, it will allow the system to have a greater available data

communication distance with the remote transponder.