RFM24W Datasheet, PDF (25/34 Pages) List of Unclassifed Manufacturers – ISM Transceiver module

RFM24W
5.2.1. Automatic Gain Control (AGC)
The AGC algorithm is implemented digitally using an advanced control loop optimized for fast response time. The
AGC occurs within a single bit or in less than 2 µs. Peak detectors at the output of the LNA and PGA allow for
optimal adjustment of the LNA gain and PGA gain to optimize IM3, selectivity, and sensitivity performance.
5.2.2. Auto Frequency Correction (AFC)
Frequency mistuning caused by crystal inaccuracies can be compensated by enabling the digital automatic
frequency control (AFC) in receive mode. There are two types of integrated frequency compensation, modem
frequency compensation, and AFC by adjusting the PLL frequency. With AFC disabled the modem compensation
can correct for frequency offsets up to ±0.25 times the IF bandwidth. When the AFC is enabled, the received signal
will be centered in the pass-band of the IF filter, providing optimal sensitivity and selectivity over a wider range of
frequency offsets up to ±0.35 times the IF bandwidth. When AFC is enabled, the preamble length needs to be long
enough to settle the AFC. In general, one byte of preamble is sufficient to settle the AFC.
5.2.3. Image Rejection and Calibration
Since the receiver utilizes a low-IF architecture the selectivity will be affected by the image frequency. The IF
frequency is 468.75kHz and the image frequency will be at 937.5kHz below the RF frequency. The native image
rejection of the RFM24W is 35dB. The calibration is performed during the initial cold boot and does not require an
external signal source. Also available in the Si4464/63 is the option to shift the IF frequency. With this option the IF
frequency can be shifted to the adjacent channel putting the image frequency in the alternate channel.
5.2.4. Received Signal Strength Indicator
The received signal strength indicator (RSSI) is an estimate of the signal strength in the channel to which the
receiver is tuned. The RSSI measurement is done after the channel filter so it is only a measurement of the in-band
signal power, desired or undesired. There are multiple options for reading the RSSI which are configured in
“MODEM_RSSI_CONTROL”. The RSSI can be set to update every bit or averaged over a four bit period. A latched
version of the RSSI may be saved and read after the packet. The current RSSI value or latched RSSI value are
read by readying the “GET_MODEM_STATUS” API command. The RSSI value can also be programmed into one
of the fast response registers. Clear channel assessment (CCA) may also be performed by programming an RSSI
threshold in “MODEM_RSSI_THRESH” and enabling this interrupt or programming a GPIO for this function.
To minimize the amount of time associated with reading the RSSI for frequency hopping applications automatic
hop control is available based on an RSSI threshold. Automatic hop features are available to hop based on the
availability of preamble or not, see the section for fast frequency hopping for more details on this feature.
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