RFM23
2. Functional Description
The RFM23 is a ISM wireless receiver 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 RFM23 and ideal
solution for battery powered applications.
The RFM23 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 +13 dBm in 3 dB steps. The PA incorporates automatic
ramp-up and ramp-down control to reduce unwanted spectral spreading. The RFM23 supports frequency hopping,
TX/RX switch control, and antenna diversity switch control to extend the link range and improve performance.
Antenna diversity is completely integrated into the RFM23 and can improve the system link budget by 8–10 dB,
resulting in substantial range increases depending on the environmental conditions.
The RFM23 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, Antenna
Diversity, POR, and specific interrupts. A limited number of passive components are needed to match the LNA and
PA.The application shown in Figure 1 is designed for a system with an TX/RX direct-tie configuration without the use
of a TX/RX switch. Most lower power applications will use this configuration.
For applications seeking improved performance in the presence of multipath fading antenna diversity can be used.
Antenna diversity includes a switch to select the optimal antenna between a pair of antennas for improved
performance. The Antenna Diversity Control Algorithm is completely integrated into the module.
An application example with a separate RX and TX antenna is shown in Figure 31, ―Split RF I/Os with Separated
TX and RX Connectors—Schematic,‖. This example is used for testing of the TX and RX paths in a lab environment
and shows, conceptually, the matching of the TX and RX antennas.
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