RFM219SW

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5.5.6 Frequency Synthesizer

A fractional-N frequency synthesizer is used to generate the LO frequency for the down conversion I/Q mixer. The frequency

synthesizer is fully integrated except the VCO tank inductor which enables the ultra low-power receiver system design. Using the

26 MHz reference clock provided by the crystal oscillator or the external clock source, it can generate any receive frequency

between 300 to 960 MHz with a frequency resolution of 24.8 Hz.

The VCO always operates at 2x of LO frequency. A high Q (at VCO frequency) tank inductor should be chosen to ensure the

VCO oscillates at any conditions meanwhile burns less power and gets better phase noise performance. In addition, properly

layout the inductor matters a lot of achieving a good phase noise performance and less spurious emission. The recommended

VCO inductors for different LO frequency bands are shown as bellow.

Table 11. VCO Inductor for Common Used Frequency Bands

LO Frequency Band (MHz)

315

433.92

868.35

915

VCO Inductor (nH)

33

22

3.9

3.9

Multiple subsystem calibrations are performed dynamically to ensure the frequency synthesizer operates reliably in any working

conditions.

5.5.7 LPOSC

An internal 1 kHz low power oscillator is integrated in the RFM219SW. It generates a clock to drive the sleep timer to

periodically wake the device up from sleep state. The Sleep Time can be configured from 3 to 134,152,192 ms (more than 37

hours) when the device works in duty-cycle receive mode. Since the frequency of the LPOSC drifts when the temperature and

supply voltage change, it is automatically calibrated during the PUP state, and is periodically calibrated since then. The

calibration scheme allows the LPOSC to maintain its frequency tolerance to less than ±1%.

5.5.8 OOK Demodulation

The OOK demodulation is done by comparing the RSSI to a demodulation threshold. The threshold is an 8-bit binary value that is

comparable to the 8-bit digitized RSSI. There are two methods of OOK demodulation supported: Fixed TH and Peak TH. The

symbol rate range for the OOK demodulation is from 0.1 to 40 ksps. More details of the OOK demodulation can be found in the

5.5.9 (G)FSK Demodulation

High-performance (G)FSK demodulation is supported. The symbol rate range for the (G)FSK demodulation is from 0.1 to 100

ksps. The device supports a wide range of deviations. The deviation is the maximum instantaneous difference between the

modulated frequency and the nominal carrier frequency Fo.

Deviation

Deviation

Figure 12. (G)FSK Deviation

A proper selection of the deviation is regarding to the modulation index and the frequency error between the TX and the RX. The