User Manual

24
Mini-RFM - Reference Guide November ’00
Figure 10 shows an example of how to supply the RF module in a battery supplied
reading unit where short lines are possible.
Figure 11 shows an example of how to wire a stationary application where long sup-
ply lines (over 15 cm) are used. The capacitor C_E1 between the VSP and GNDP
pins is necessary to reduce the RF ripple voltage and EMI on the supply line and
must have low impedance at the transmit frequency (ESR = Equivalent Serial Resis-
tor). In addition the common mode choke coil (choke) and the capacitor C_E2 also
reduce the EMI during transmit time. These components should be placed as closely
as possible to the RF module connector ST1.
The capacitor values shown in Figure 10 and Figure 11 are for general use and can
vary according to different application requirements.
Regardless of which kind of power supply you use, we recommend that you build in
fuses or electronic current limitations in between the RF module and the power sup-
plies to avoid any damage in case of short circuit. The value of the fuse depends very
much on the application, for example: the current consumption of the module which
in turn depends on several factors, such as: the antenna used, whether or not a ca-
pacitor is connected and its size.
Figure 10: Supply Line Wiring for Short Supply Lines
Note:
The fuse load for VSP under normal conditions is less than 1 A.
However, if the Mini-RFM is connected into an application where the
power is permanently on (hot” connecting), the load could surge by
several amps due to the current in capacitor CS1 inside the Mini-
RFM.