SI4464 datasheet(31/56 Pages) SILABS

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Si4464/63/61/60

Rev 1.2

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 below the RF frequency. The

native image rejection of the Si446x family is 35 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. Re-calibration 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

desired or undesired in-band signal power. There are two different methods for reading the RSSI value and several

different options for configuring the RSSI value that is returned. 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 will take 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 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 7 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 latched yet, a value of 0 will be returned if it is attempted to

be read.

The RSSI value read by the API could be translated to dBm by the following linear equation:

RSSI (in dBm) = (RSSI_value /2) – RSSIcal

RSSIcal in the above formula depends on the matching network, modem settings, and external LNA gain (if

present). The RSSIcal value can be obtained by a simple calibration with a signal generator connected at the

antenna input. Without external LNA, the value of RSSIcal is around 130 ±30.

During packet reception, it may be useful to detect whether a secondary interfering signal (desired or undesired)

arrives. To detect this event, a feature for RSSI jump detection is available. If the RSSI level changes by a

programmable amount during the reception of a packet, an interrupt or GPIO can be configured to notify the host.

The level of RSSI increase or decrease (jump) is programmable through the MODEM_RSSI_JUMP_THRESH API

property. If an RSSI jump is detected, the modem may be programmed to automatically reset so that it may lock

onto the new stronger signal. The chip may also be configured to automatically reset the receiver upon jump

detection in order to acquire the new signal. The configuration and options for RSSI jump detection are

programmed in the MODEM_RSSI_CONTROL2 API property. By default, RSSI jump detection is not enabled.

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