NI USRP-2901 Sampling Rate

Updated Jan 4, 2021

Reported In

Hardware

  • USRP-2944
  • USRP-2942
  • USRP-2943
  • USRP-2945
  • USRP-2940
  • USRP-2955
  • USRP-2952
  • USRP-2953
  • USRP-2950
  • USRP-2954

Issue Details

  • I know that NI USRP-294x and USRP-295x (except USRP-2945 and USRP-2955) has fixed sampling rate at the hardware level. In Specification document of USRP-2901, in footnotes 5 and 10 it is mentioned the following:
    • The DAC rate changes with the sample rate.
    • The ADC rate changes with the sample rate.

What does it mean?
 

  • How does USRP-2901 sets its sampling rate?

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 Architecture

Different 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.

Additional Information

In case if we are using USRP-2945, then setting sampling rate will not effect on its ADC/DAC IC's actual sampling rates.
Instead, it will be adjusted by DDCs/DUCs.