Design of cascaded integrator-comb decimation filters for direct-RF sampling receivers

Cascaded integrator-comb (CIC) filters in a direct-RF sampling receiver reduce the quantization noise of an analog-to-digital converter (ADC) and out-of-band blockers being folded into the signal band. A higher order provides more attenuation for them but proportionally increases the power consumption and chip area. We present the procedure to determine the orders of a multistage CIC filter by using the analytical expressions of its output signal-to-noise ratio (SNR) and rejection in the signal band. The derived expressions first show a lower limit for the order to achieve the required SNR and then provide orders to satisfy the rejection requirements. A two-stage CIC filter consisting of 2nd- and 3rd-order CIC filters with a total decimation factor of 1024 is designed for a direct-RF sampling receiver complying with Bluetooth Low Energy (BLE) in MATLAB/Simulink. Simulations show that the CIC filter completely removes the folded quantization noise of a 2nd-order bandpass delta-sigma modulator and has a rejection of 63 dB for out-of-band blockers. The proposed procedure provides the lowest order of each CIC filter to enable the receiver to satisfy the SNR and out-of-band blocking requirements of BLE.