3.4.1 Overview
Two separate LNA inputs, one for Low-Band and one for High-Band, are provided to obtain optimum performance matching for each frequency range and to allow multi-band applications. A radio frequency (RF) level detector at the LNA output and a switchable damping included into the single-pole double-trough (SPDT) switch is used in the presence of large blockers to achieve enhanced system-blocking performance.
The mixer converts the received RF signal to a low intermediate frequency (IF) of about 250 kHz. A double-quadrature architecture is used for the mixer to achieve high image rejection. Additionally, the third-order suppression of the local oscillator (LO) harmonics makes receiving without a front-end SAW filter less critical, such as in a car key fob application.
An IF amplifier provides additional gain and improves the receiver sensitivity by 2-3 dB. Because of built-in filter function, the in-band compression is degraded by 10 dB, while the out-of-band compression remains unchanged.
The ADC converts the IF signal into the digital domain. Due to the high effective resolution of the ADC, the channel filter and received signal strength indicator (RSSI) can be realized in the digital signal domain. Therefore, no analog gain control (AGC) potentially leading to critical timing issues or analog filtering is required in front of the ADC. This leads to a receiver front end with excellent blocking performance up to the 1 dB compression point of the LNA and mixer, and a steep digital channel filter can be used.
The RX DSP performs the channel filtering and converts the digital output signal of the ADC to the baseband for demodulation. Due to the digital realization of these functions, the RX DSP can be adapted to the needs of many different applications. Channel bandwidth, data rate, modulation type, wake-up criteria, signal checks, clock recovery and many other properties are configurable. The RSSI value is realized completely in the digital signal domain, enabling very high relative and absolute accuracy that is only deteriorated by the gain errors of the LNA, mixer and ADC.
Two independent receive paths, A and B, are integrated in the RX DSP after the channel filter, and allow the use of different data rates, modulation types and protocols without the need to power up the receive path more than once to determine which signal will be received. This results in a reduced polling current in several applications.
The integration of different applications (remote access, temperature sensors and more) into one module is simplified because completely different protocols can be supported and a low polling current is achieved. It is even possible to configure different receive RF bands for different applications by using the two LNA inputs.