Technical Manual

6
Wireless Sensor Interface A720 (addIT™)
2. Hardware
Most of the electronics are situated on the main board, while an interface board is
used to connect the unit to the outside world. The main board contains a radio unit,
a low speed modem interface, a microcontroller and a power management sub-
system.
For further details you may want to consult the schematic diagram located on page 7.
2.1. The Radio Unit
It consists of a RF transceiver, that conforms or surpasses the ETSI 300 220 specifica-
tions. The receiver is a double superheterodyne type, first oscillator being synthe-
sized. The transmitter is also synthesized, only the PLL chip and the frequency
reference being common (there are two separate VCOs, one for the receiver’s local
oscillator and another for the transmitter).
2.1.1. Receiver Section
The antenna signal is fed through a low pass filter and the antenna switch (D1/D2)
to a helical band-pass filter (it has a 20 MHz passband at 3dB), which attenuates the
first image and the oscillator fed-through signal. A cascode chip (U3) is used for RF
signal preamplification and first mixer, the LO being fed via the second emitter of the
cascode. The resulting IF on 45 MHz is filtered through XF1 which in this case pro-
vides the image attenuation for the second mixer.
After a pre-amplification by means of the transistor Q11, the 45 MHz IF signal is ap-
plied to U10, which provides the second frequency change to 455 kHz, pre-amplifi-
cation, limiting and FM demodulation. The second oscillator is also built into this
Radio Unit
Antenna
µController
Modem
Interface
Power
Management
System Supply
Battery
External Power
ADC
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4
Analog Inputs
Digital I/O