RFM66W Datasheet, PDF (30/87 Pages) List of Unclassifed Manufacturers – 868 & 915MHz Ultra Low Power High Link Budget Integrated UHF Transceiver

WIRELESS & SENSING
RFM66W
DATASHEET
3.5.4. RSSI
The RSSI value reflects the incoming signal power provided at antenna port within the receiver bandwidth. The signal
power is available in RssiValue. This value is absolute and its unit is in dBm with a resolution of 0.5dB. The formula
hereafter gives the relationship between the register value and the absolute input signal level in dBm at antenna port:
RssiValue = −2 ⋅ RF level [dBm]+ RssiOffset [dB]
The RSSI value can be compensated for to take into account the loss in the matching network or the gain of an additional
LNA, by using RssiOffset. The offset can be chosen in 1dB steps from -16 to +15dB. When compensation is applied, the
effective signal strength is read as follows:
RSSI [dBm] = − ^RssiValue
2
The RSSI value is smoothed on a given number of measured RSSI samples. The precision of the RSSI value is related to
the number of RSSI samples used. RssiSmoothing selects the number of RSSI samples from a minimum of 2 samples up
to 256 samples in increments of power of 2. Table 17 hereafter gives the estimation of the RSSI accuracy for a 10dB SNR
and the response time versus the number of RSSI samples selected in RssiSmoothing.
Table 17 RssiSmoothing Options
RssiSmoothing
„000‟
„001‟
„010‟
„011‟
„100‟
„101‟
„110‟
„111‟
Number of Samples
2
4
8
16
32
64
128
256
Estimated Accuracy
± 6dB
± 5dB
± 4dB
± 3dB
± 2dB
± 1.5dB
± 1.2dB
± 1.1dB
Response Time
2 ^{(RssiSmoothing +1)}
4 ⋅ RxBw[kHz]
[ms]
The RSSI is calibrated, up the RFI pin, when Image and Rssi calibration is launched; please see section 3.5.13 for details.
3.5.5. Channel Filter
The role of the channel filter is to filter out the noise and interferers outside of the channel. Channel filtering on the RFM66W
is implemented with a 16-tap Finite Impulse Response (FIR) filter, providing an outstanding Adjacent Channel Rejection
performance, even for narrowband applications.
Note to respect oversampling rules in the decimation chain of the receiver, the Bit Rate cannot be set at a higher value
than 2 times the single-side receiver bandwidth (BitRate < 2 x RxBw)
The single-side channel filter bandwidth RxBw is controlled by the parameters RxBwMant and RxBwExp in RegRxBw:
RxBw = ------------------------^F----^X----^O----^S----^C-------------------------
RxBwMant × 2^{RxBwExp + 2}
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