CF45538NSRG4 Datasheet, PDF (15/19 Pages) Texas Instruments – TIRIS RF-Module IC for Automotive

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To FSK signal demodulator
Typical Antenna Circuit Configuration
RI45538NS
From Antenna
1
A3OP
2
A3IN
3
A2OP
4
A2IN
5
A1OP
6
A1IN
R1
C2
R2
C4
R3
R4
C1
C3
C5
D2
C6
D1
In general, the FSK signals sent from a remote TIRIS transponder are found within a band of 120 kHz to
140 kHz, mainly due to dispersion of workmanship during manufacture and ambient temperature
fluctuation during transmitting operation. Therefore, by damping signals that fall out of the above band
range as much as possible, noise suppression performance can be improved. In the illustrated circuit
configuration, amplifier input coupling capacitors C2, C4 and C6 are used to reduce extremely
low-frequency noise signals, and amplifier output load capacitors C1, C3 and C5 to reduce high-frequency
noises. If you desire to enhance the noise resistance of the circuit still more, it is necessary to install a
required number of external wide band amplifiers with high amplification factor and add an active band
pass filter, LC resonance circuit, etc.
D1, D2 and R4 for input into the first amplifier (A1) form a circuit to prevent high-voltage signals for power
transmission and similar signals from entering the IC, thereby keeping from occurrence of latch-up or
other adverse situations. This circuit or otherwise, an equivalent protector, must be inserted without fail.
13 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 (134.2KHz) = ¹
2π LC
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.
SCBU036 – December 1996
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TIRIS RF-Module IC for Automotive
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