RFM210LW Datasheet, PDF (12/17 Pages) List of Unclassifed Manufacturers – Low-Cost Consumer Electronics Applications

RFM210LW
achieves over 25 dB image rejection ratio typically. The IF center frequency is dynamically adjusted to enable the IF filter to
locate to the right frequency band, thus the receiver sensitivity and out-of-band interference attenuation performance are kept
optimal despite the manufacturing process tolerances. The IF bandwidth is fixed at 200 kHz.
5.4.3 RSSI
The subsequent multistage I/Q Log amplifiers enhance the output signal from IF filter before it is fed for demodulation. Receive
Signal Strength Indicator (RSSI) generators are included in both Log amplifiers which produce DC voltages that are directly
proportional to the input signal level in both of I and Q path. The resulting RSSI is a sum of both these two paths. Extending from
the nominal sensitivity level, the RSSI achieves dynamic range over 66 dB.
The RFM210LW integrates a patented DC-offset cancellation engine. The receiver sensitivity performance benefits a lot from
the novel, fast and accurate DC-offset removal implementation.
5.4.4 SAR ADC
The on-chip 8-bit SAR ADC digitalizes the RSSI for OOK demodulation.
5.4.5 Crystal Oscillator
The crystal oscillator is used as the reference clock for the PLL frequency synthesizer and system clock for the digital blocks. A
crystal should be used with appropriate loading capacitors (C2 and C3 in Figure 9 of Page 9). The values of the loading
capacitors depend on the total load capacitance CL specified for the crystal. The total load capacitance seen between the XIN
and XOUT pin should equal CL for the crystal to oscillate at desired frequency.
CL =
1
+ Cparasitic
1
1
+ C3
C2
The parasitic capacitance is constituted by the input capacitance and PCB tray capacitance. The ESR of the crystal should be
within the specification in order to ensure a reliable start-up. An external signal source can easily be used in place of a
conventional XTAL and should be connected to the XIN pin. The incoming clock signal is recommended to have a peak-to-peak
swing in the range of 300 mV to 700 mV and AC-coupled to the XIN pin.
5.4.6 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 receive frequencies of 315
and 433.92 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 9. VCO Inductor for 315/433.92 MHz Frequency Band
LO Frequency Band (MHz)
VCO Inductor (nH)
315
433.92
33
22
Multiple subsystem calibrations are performed dynamically to ensure the frequency synthesizer operates reliably in any
working condition
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