# Relationship between sampling rate and bandwidth in gnu radio

I wondering something. While working with adalm pluto sdr in gnu radio, the sample rate I enter in the source block is actually bandwidth. I know that bandwidth must be half of the sampling rate because of the Nyquist rate. How can GNU radio taking a 4MHz sample rate and 4MHz bandwidth.Also, can we sample 20 MHz bandwidth with 4 MHz sampling rate in GNU radio with pluto sdr? Because in continuous RX mode max sample rate of adalm pluto is 4-5 MHz.