Abstract
This guide explains both basic usage of the USRP as well as how to expand it.
Table of Contents
The Universal Software Radio Peripheral, or USRP (pronounced "usurp") is designed to allow general purpose computers to function as high bandwidth software radios. In essence, it serves as a digital baseband and IF section of a radio communication system. In addition, it has a well-defined electrical and mechanical interface to RF front-ends (daughterboards) which can translate between that IF or baseband and the RF bands of interest
The basic design philosophy behind the USRP has been to do all of the waveform-specific processing, like modulation and demodulation, on the host CPU. All of the high-speed general purpose operations like digital up- and downconversion, decimation and interpolation are done on the FPGA.
It is anticipated that the majority of USRP users will never need to use anything other than the standard FPGA configuration. However, for those users that wish to, the FPGA design may be changed or replaced. All of the interfaces are well defined and documented.
Figure 1. USRP with Daughterboards
This USRP has 2 BasicTX and 2 BasicRX boards mounted on it. Notice that the boards on the left are rotated 180 degrees.
The USRP has 4 high-speed analog to digital converters (ADCs), each at 12 bits per sample, 64 million samples per second. There are also 4 high-speed digital to analog converters (DACs), each at 14 bits per sample, 128 million samples per second. These 4 input and 4 output channels are connected to an Altera Cyclone EP1C12 FPGA. The FPGA, in turn, connects to a USB2 interface chip, the Cypress FX2, and on to the computer. The USRP connects to the computer via a high speed USB2 interface only, and will not work with USB1.1.
The first step in using your USRP system is to get all of GNU Radio installed. This can sometimes be a daunting process, as there are several other libraries which will need to be installed first.
SWIG
We use SWIG (Simple Wrapper Interface Generator) to tie together the C++ and Python code in the GNU Radio system. We require that you have version 1.3.24 or newer. You'll probably have to compile it from source, which you can find here: SWIG
FFTW
FFTW is the library which GNU Radio uses for FFTs. GNU Radio requires version 3.0.1 or newer, and it must be compiled for single precision. You can get it from the FFTW Homepage
Boost Library
Boost provides several low-level structures used in our C++ code. If it is not included in your OS distribution, you can get it here: Boost
CPP Unit
CPPUnit provides our unit-testing framework. This creates automated tests to insure that code does not break when changes are made. Get it at the CPP Unit Homepage
There are several packages of software which make up GNU Radio and the USRP support software. Links to the latest versions of each can be found on the GNU Radio Wiki at Download Links. Gr-build can greatly simplify the installation process, and its use it highly recommended.
Development for the USRP proceeds very quickly at times, so some users may want to keep up with the latest by following the CVS trees. There are three separate software repositories which contain various parts of the USRP system.
USRP-HW, containing the hardware and FPGA designs.
All of the schematics in this repository were created in gEDA. The board layouts were created in PCB. Verilog designs are compiled in Quartus II Web Edition from Altera.
USRP-SW, USRP-SW, containing firmware and host drivers for the USRP
Host side drivers and firmware which runs in the USB2 interface chip on the board.
GNU Radio/gr-usrp which contains the GNU Radio interface to the USRP
The USRP ships with a complete set of standoffs, nuts and bolts. There are 20 standoffs, M3x10mm M-F, of which 4 are intended to be used as "feet" for the USRP. Place them in the 4 corner holes on the main board, inserting the male part from below. The remaining 16 are used to hold the daughterboards in place. Four of them should be connected to the male portion of the 4 standoffs already inserted from below. The remaining 12 should be connected to the board with the 12 M3x6mm screws from below. At this point there should be 16 standoffs on the board with the male ends up to serve as a guide for the daughterboards. The 16 M3 nuts are used to fasten the daughterboards down to the main board.
The USRP accomodates 2 TX and 2 RX daughterboards. The placement of the standoffs is designed to prevent the accidental incorrect connection of daughterboards. The 2 sides of the USRP have their daughterboard slots rotated 180 degrees. The USRP should not be operated without standoffs, and daughterboards should never be connected or removed while power is applied.
The USRP is powered by a 6V 4A power converter included in the kit. The converter is capable of 90-260 Vac, 50/60 Hz operation, and so should work in any country. If there is a need to use another power supply, the connector is a standard 2.1mm/5.5mm DC power connector. The USRP itself only needs 5V at 2A, but a 6V supply was chosen to accomodate future daughterboards. Extra power supplies are available from Ettus Research.
The included USB cable should be connected to a USB2-capable socket on a computer. The USRP does not support USB 1.1 operation at this time.
When first powered up, an LED on the USRP should be flashing at about 3-4x per second. This indicates that the processor is running, and has put the device in a low power mode. Once firmware has been downloaded to the USRP, the LED will blink at a slower rate. If there is no blinking LED, check all power connections, and check for continuity in the power fuse (F501, near the power connector). If the fuse needs replacement, it is size 0603, 3 amps.
In the standard fpga configuration, usrp_std, all samples sent over the USB interface are in 16-bit signed integers in IQ format. When there are multiple channels (up to 4), the channels are interleaved. For example, with 4 channels, the sequence would be I0 Q0 I1 Q1 I2 Q2 I3 Q3 I0 Q0, etc.
The USRP can operate in full duplex mode. When in this mode, the transmit and receive sides are completely independent of one another. The only consideration is that the combined data rate over the bus must be 32 Megabytes per second or less. The multiple RX channels (1,2, or 4) must all be the same data rate (i.e. same decimation ratio). The same applies to the 1,2, or TX channels, which each must be at the same data rate (which may be different from the RX rate).
On the RX side, each of the 4 ADCs can be routed to either of I or the Q input of any of the 4 downconverters. This allows for having multiple channels selected out of the same ADC sample stream.
The digital upconverters (DUCs) on the transmit side are actually contained in the AD9862 CODEC chips, not in the FPGA. The only transmit signal processing blocks in the FPGA are the interpolators. The interpolator outputs can be routed to any of the 4 CODEC inputs.
Daughterboards are provided with clean regulated 3.3V for the analog and digital sections. Additionally there is a 6V connection straight from the wall supply which is intended to supply a 5V LDO regulator. All daughterboards may draw a combined total of 1.5 A.
There are slots for 2 TX daughterboards, labeled TXA and TXB, and 2 corresponding RX daughterboards, RXA and RXB. Each daughterboard slot has access to 2 of the 4 high-speed data converter analog signals (DAC outputs for TX, ADC inputs for RX). This allows each daughterboard which uses real (not IQ) sampling to have 2 independent RF sections, and 2 antennas (4 total for the system). If IQ sampling is used, each board can support a single RF section, for a total of 2 for the whole system.
No antialias or reconstruction filtering is provided on the USRP motherboard. This allows for maximum flexibility in frequency planning for the daughterboards. The analog input bandwidth of the ADCs is over 200 MHz, so IF frequencies up to that high may be chosen. If several decibels of loss is tolerable, and IF frequency as high as 500 MHz can be used.
Every daughterboard has an I2C EEPROM (24LC024 or 24LC025) onboard which identifies the board to the system. This allows the host software to automatically set up the system properly based on the installed daughterboard. The EEPROM may also store calibration values like DC offsets or IQ imbalances. If this EEPROM is not programmed, a warning message is printed every time USRP software is run.
Each RX daughterboard has 2 differential analog inputs (VINP_A/VINN_A and VINP_B/VINN_B) which are sampled at a rate of 64 MS/s. The input impedance is approximately 1Kohm. The motherboard has a software-controllable programmable gain amplifier on these inputs, with 0 to 20 dB of gain. With gain set to zero, full scale inputs are 2 Volts peak-to-peak differential. When set to 20 dB, only .2 V pk-pk differential is needed to reach full scale.
If signals are AC-coupled, there is no need to provide DC bias as long as the internal buffer is turned on. It will provide an approximately 2V bias. If signals are DC-couple, a DC bias of Vdd/2 (1.65V) should be provided to both the positive and negative inputs, and the internal buffer should be turned off. VREF provides a clean 1 V reference.
Each TX daughterboard has a pair of differential analog outputs which are updated at 128 MS/s. The signals (IOUTP_A/IOUTN_A and IOUTP_B/IOUTN_B) are current-output, each varying between 0 and 20 mA. Since they are high-impedance, they can be converted into differential voltages with a resistor.
In addition to the high-speed signals, each daughterboard has exclusive access to 2 low-speed ADC inputs (labeled AUX_ADC_A and AUX_ADC_B) which can be read from software. These are useful for sensing RSSI signal levels, temperatures, bias levels, etc. Additionally, each board has shared access to 4 low-speed DAC signals, labeled AUX_DAC_A through AUX_DAC_D. RXA and TXA share one set of these 4 lines, and RXB and TXB share their own independent set. These signals are useful for controlling gain of variable-gain amplifiers, for example. AUX_ADC_REF provides a reference level for gain setting if it is necessary.