SLOS757G December 2011 – March 2020 TRF7962A
The TRF7962A has two receiver inputs: RX_IN1 (pin 8) and RX_IN2 (pin 9). Each of the inputs is connected to an external capacitive voltage divider to ensure that the modulated signal from the tag is available on at least one of the two inputs. This architecture eliminates any possible communication holes that may occur from the tag to the reader.
The two RX inputs (RX_IN1 and RX_IN2) are multiplexed into two receivers–the main receiver and the auxiliary receiver. Only the main receiver is used for reception; the auxiliary receiver is used for signal quality monitoring. Receiver input multiplexing is controlled by bit B3 in the Chip Status Control register (address 0x00).
After start-up, RX_IN1 is multiplexed to the main receiver which is composed of an RF envelope detection, first gain and band-pass filtering stage, second gain and filtering stage with AGC. Only the main receiver is connected to the digitizing stage which output is connected to the digital processing block. The main receiver also has an RSSI measuring stage, which measures the strength of the demodulated signal (subcarrier signal).
The primary function of the auxiliary receiver is to monitor the RX signal quality by measuring the RSSI of the demodulated subcarrier signal (internal RSSI). After start-up, RX_IN2 is multiplexed to the auxiliary receiver. The auxiliary receiver has an RF envelope detection stage, first gain and filtering with AGC stage and finally the auxiliary RSSI block.
The default MUX setting is RX_IN1 connected to the main receiver and RX_IN2 connected to the auxiliary receiver. To determine the signal quality, the response from the tag is detected by the "main" (pin RX_IN1) and "auxiliary" (pin RX_IN2) RSSI. Both values measured and stored in the RSSI level register (address 0x0F). The MCU can read the RSSI values from the TRF7962A RSSI register and decide if swapping the input signals is preferable or not. Setting B3 in the Chip Status Control register (address 0x00) to 1 connects RX_IN1 (pin 8) to the auxiliary receiver and RX_IN2 (pin 9) to the main receiver.
The main and auxiliary receiver input stages are RF envelope detectors. The RF amplitude at RX_IN1 and RX_IN2 should be approximately 3 VPP for a VIN supply level greater than 3.3 V. If the VIN level is lower, the RF input peak-to-peak voltage level should not exceed the VIN level.