CF45538NSRG4 Datasheet, PDF (16/19 Pages) Texas Instruments – TIRIS RF-Module IC for Automotive

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Precautions for Mounting and Actual Use
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.
14 Precautions for Mounting and Actual Use
Described in this section are the precautions to be taken at mounting and actual use of the RI-RFM-006A
while designing and manufacturing a TIRIS Read-Write System using this IC, especially, critical issues as
may affect the operating performance of the IC and, in particular, the system communication performance
with a remote TIRIS transponder.
14.1 Power Supply Line
When the signals returned from a remote TIRIS transponder are amplified by sequentially using the three
CMOS inverter type amplifiers incorporated in this RF-module IC, an undesirable feedback loop is formed
from the third amplifier toward the first one through parasitic L, R and C whose formation is not avoidable
because of the structure of this IC and its internal power supply line (see the diagram below) as CMOS
devices are inevitably bi-directional. If this feedback loop is left as is, it normally leads to an oscillation;
particularly, when the frequency band of each amplifier is limited so that it matches that of the transponder
return signals, oscillation occurs at a frequency within this band which counts on high gains from the very
nature of things, in consequence, adversely affecting the demodulating performance of the received FSK
signals.
Positive power supply (Vcc) line
Bypass capacitor
16
15
14
13
Internal power supply line
12
11
Feedback loop
Amplifier 3
Amplifier 2
Amplifier 1
1
2
3
4
5
6
From Antenna
Equivalent circuit for internal power supply line feedback loop
This situation does not a little occur when a multiple number of high-gain amplifiers are integrated in a
CMOS device. Formation of this undesirable feedback loop in the amplifier band can be avoided by
minimizing the impedance of the power supply line through optimization of the printed circuit boards and
using a suitable bypass capacitor. It is extremely difficult to obtain true values for these parasitic L, R and
C forming the internal power supply line feedback loop, but it can be said from the empirical viewpoint that
it is possible to inhibit the said oscillation using a bypass capacitor with a capacity of 1 µF and having a
sufficiently low impedance within the said amplifier frequency band provided that appropriate wiring
patterns are defined for the power supply line on the printed circuit boards and in the peripheries of
external parts for the amplifiers.
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TIRIS RF-Module IC for Automotive
SCBU036 – December 1996
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