SI4463 Datasheet, PDF (30/53 Pages) Silicon Laboratories – HIGH-PERFORMANCE

Si4463/61/60-C
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 468.75 kHz (Fxtal/64), and the image frequency will be at 937.5 kHz (2 x Fxtal/64) below the RF
frequency. The native image rejection of the Si446x family is 40 dB. Image rejection calibration is available in the
Si446x to improve the image rejection to more than 55 dB. The calibration is initiated with the IRCAL API
command. The calibration uses an internal signal source, so no external signal generator is required. The initial
calibration takes 250 ms, and periodic re-calibration takes 100 ms. Recalibration should be initiated when the
temperature has changed more than 30 °C.
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 two methods for reading the RSSI value and several
different options for configuring the returned RSSI value. The fastest method for reading the RSSI is to configure
one of the four fast response registers (FRR) to return a latched RSSI value. The latched RSSI value is measured
once per packet and is latched at a configurable amount of time after RX mode is entered. The fast response
registers can be read in 16 SPI clock cycles with no requirement to wait for CTS. The RSSI value may also be read
out of the GET_MODEM_STATUS command. In this command, both the current RSSI and the latched RSSI are
available. The current RSSI value represents the signal strength at the instant in time the GET_MODEM_STATUS
command is processed and may be read multiple times per packet. Reading the RSSI in the
GET_MODEM_STATUS command takes longer than reading the RSSI out of the fast response register. After the
initial command, it takes 33 μs for CTS to be set and then the four or five bytes of SPI clock cycles to read out the
respective current or latched RSSI values.
The RSSI configuration options are set in the MODEM_RSSI_CONTROL API property. The latched RSSI value
may be latched and stored based on the following events: preamble detection, sync detection, or a configurable
number of bit times measured after the start of RX mode (minimum of 4 bit times). The requirement for a minimum
of four bit times is determined by the processing delay and settling through the modem and digital channel filter. In
MODEM_RSSI_CONTROL, the RSSI may be defined to update every bit period or to be averaged and updated
every four bit periods. If RSSI averaging over four bits is enabled, the latched RSSI value will be delayed to a
minimum of seven bits after the start of RX mode to allow for the averaging. The latched RSSI values are cleared
when entering RX mode so they may be read after the packet is received or after dropping back to standby mode.
If the RSSI value has been cleared by the start of RX but not yet latched, a value of 0 will be returned if it is
attempted to be read.
The RSSI value read by the API may be translated into dBm by the following linear equation:
RF_Input_Level_dBm = (RSSI_value / 2) – MODEM_RSSI_COMP – 70
The MODEM_RSSI_COMP property provides for fine adjustment of the relationship between the actual RF input
level (in dBm) and the returned RSSI value. That is, adjustment of this property allows the user to shift the RSSI vs
RF Input Power curve up and down. This may be desirable to compensate for differences in front-end insertion loss
between multiple designs (e.g., due to the presence of a SAW preselection filter, or an RF switch). A value of
MODEM_RSSI_COMP = 0x40 = 64d is appropriate for most applications.
Clear channel assessment (CCA) or RSSI threshold detection is also available. An RSSI threshold may be set in
the MODEM_RSSI_THRESH API property. If the Current RSSI value is above this threshold, an interrupt or GPIO
may notify the host. Both the latched version and asynchronous version of this threshold are available on any of
the GPIOs. Automatic fast hopping based on RSSI is available. See “5.3.1.2. Automatic RX Hopping and Hop
Table”.
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