Solution
USRP stands for
Universal Software Radio Peripheral and is a software defined radio (SDR) used for RF applications. There fore USRP hardware implements a direct conversion analog front end with high-speed analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) featuring a fixed-personality FPGA for the digital downconversion (DDC) and digital upconversion (DUC) steps. All USRP devices use the same general architecture as shown in Picture 1 below.
Picture 1. General USRP ArchitectureDifferent SDR configurations, waveforms, and applications require different sample rates. For example, a user may wish to monitor 100 MHz of instantaneous RF bandwidth, but their host PC may only be capable of analyzing 20 MHz of real-time bandwidth. For this and other cases,
USRP Hardware Driver (UHD) allows users to set various sample rates to meet their custom application. Within the FPGA, UHD includes decimation and interpolation blocks in order to perform these sample rate translations.
Considering all this, it follows that sampling rate can be set either by using hardware: by setting ADC/DAC IC's sampling rate, or software: by using DDC/DUC when sampling rate of ADC/DAC IC's will remain the same.
In our case USRP-2901 uses first option to adjust its sampling rate. If we take a look on
Mapping Between ER-USRP and NI-USRP Product Numbers table we will find out that USRP-2901 corresponds to B210 Ettus Research (A National Instruments Brand) model.
Here we can see that it uses
Analog Devices AD9364 RFIC direct-conversion transceiver, whose ADCs and DACs has adjustable sample rates.