RFM22
2. Functional Description
The RFM22 is a wireless transceiver module with continuous frequency tuning over the complete
240–930 MHz band. The wide operating voltage range of 1.8–3.6 V and low current consumption makes the
RFM22 and ideal solution for battery powered applications.
The RFM22 operates as a time division duplexing (TDD) transceiver where the device alternately transmits and
receives data packets. The device uses a single-conversion, image-reject mixer to downconvert the 2-level
FSK/GFSK/OOK modulated receive signal to a low IF frequency. Following a programmable gain amplifier (PGA)
the signal is converted to the digital domain by a high performance △∑ ADC allowing filtering, demodulation, slicing,
error correction, and packet handling to be performed in the built-in DSP increasing the receiver‘s performance and
flexibility versus analog based architectures. The demodulated signal is then output to the system MCU through a
programmable GPIO or via the standard SPI bus by reading the 64-byte RX FIFO.
A single high precision local oscillator (LO) is used for both transmit and receive modes since the transmitter and
receiver do not operate at the same time. The LO is generated by an integrated VCO and △∑ Fractional-N PLL
synthesizer. The synthesizer is designed to support configurable data rates, output frequency, frequency deviation,
and Gaussian filtering at any frequency between 240–930 MHz. The transmit FSK data is modulated directly into
the △∑ data stream and can be shaped by a Gaussian low-pass filter to reduce unwanted spectral content.
The PA output power can be configured between +8 and +17 dBm in 3 dB steps. The PA incorporates automatic
ramp-up and ramp-down control to reduce unwanted spectral spreading. The +17dBm power amplifier can also be
used to compensate for the reduced performance of a lower cost antenna or antenna with size constraints due to a
small form-factor. Competing solutions require large and expensive external PAs to achieve comparable
performance.
The RFM22 is designed to work with a microcontroller to create a very low cost system as shown Figure 1. Voltage
regulators are integrated on-chip which allow for a wide range of operating supply voltage conditions from +1.8 to
+3.6 V. A standard 4-pin SPI bus is used to communicate with the microcontroller.
Three configurable general purpose I/Os are available for use to tailor towards the needs of the system. A more
complete list of the available GPIO functions is shown in "8. Auxiliary Functions" but just to name a few,
microcontroller clock output, POR, and specific interrupts.
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