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Chapter 4. Implementation 42

RFM22B is a low-cost transceiver with a large functional range. Basically, it consists of the IC

SI4432 from SiLabs with a preceding ﬁltering circuitry. During normal operation, the conﬁguration

and data transmissions are done via SPI; however, in our case the module is put into the so-called

"Direct Mode", which allows direct data transfer through IO pins. Some of these IO pins can serve

as an output for interrupt notiﬁcations or important status messages. However, the direct mode is

not the operation mode the module was designed for, therefore many of the features of the SI4432

cannot be used, such as automatic frequency adjustment and automatic packet handling. As a

result, additional eﬀort is required, as those features have to be implemented manually. Figure 4.7

depicts the signal chain for receiving data.

config bus

data out

MCU

deglitching

processing

RFM22B

rx clock

Figure 4.7.: The signal chain in detail: The RFM22B receives incoming data and sends it to

the MCU, where it is ﬁltered by a deglitching algorithm and ﬁnally processed. The

conﬁguration bus is composed of a SPI and several IO-pins and used for controlling

the RFM22B.

The signal path consists of three parts: The data reception and its basic ﬁltering within the

RFM22B, the advanced ﬁltering by the deglitching algorithm and the ﬁnal processing. These

parts are elaborated in the following sections.

The ﬁrst part of the signal chain is the RFM22B: It receives all incoming signals and performs

basic ﬁltering operations with its internal ﬁlter circuitry (ﬁgure 4.8 on the following page). The

signal is ﬁrst ampliﬁed and then ﬁltered by a low-pass ﬁlter to remove the carrier frequency as

well as high frequency distortion. A comparator continuously compares the signal with a reference

voltage and outputs a binary data stream. The reference voltage is generated by integrating

the signal with given parameters like rising time, fall oﬀ time and gain, which are set during

the initialisation of the module. Due to the dynamic reference voltage, the overall sensitivity

is strongly increased allowing both strong and weak signals to be outputted properly. Both the

minimum and the maximum allowable signal strength depend on the rising and falling time, which

are therefore critical parameters and must be carefully tuned. All parameters were ﬁrst generated

by the Wireless Development Suite

and improved manually by ﬁne-tuning. In practice, signals

can range from very strong to very weak, for example a transmitter in close proximity will cause

a strong signal and one that is far away will cause only minimal signal strength. Therefore, the

dynamic range of received signals can be greater than the dynamic range of the RFM22B. As a

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RFXCOM RFXtrx433 Spezifikationen Seite 49