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3. The A431 Radio Module
3.1.
About the A431 Radio Module
The A431 was specially designed for narrow-band FM data communication. It exhib-
its a relatively flat response in the audio band from 10 Hz to 2.5 kHz, both on send
as well as on receive paths. Additionally, the receiver's group delay is very low.
The module operates in the 430 to 470 MHz range, making it compatible with most
radio communication regulations in the world. The output power is 0.5 W, while the
modulation is narrow-band FM (12.5, 20 or 25 kHz channel spacing). The power con-
sumption in receive mode is remarkably low (under 32 mA), in spite of the good IP3
characteristics and high sensitivity. The switching from receive to transmit is fast, un-
der 20 mS.
3.2.
Functional description
The following functional description refers to the schematic diagram in Figure 14.
3.2.1.
Receiver Section
The antenna signal is fed through the common low-pass filter for both receive and
transmit and applied to the U3 antenna switch. From the J3 port of the switch, it is
then fed to the FL1 helical filter and amplified by the LNA built with Q2 that assures
a high amplification factor (over 20 dB). Another helical filter follows (FL2), giving a
total of more than 70 dB attenuation of the first image frequency.
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