A basic SDR architecture looks relatively simple, at least "on paper", just digitise the amplified RF signal directly, without the need for down conversion and a local oscillator, or any hardware-driven tuning, and then process the digitised results using various algorithms. However, for hardware and software engineers who want to investigate its potential, evaluate its suitability, and even develop products for their application, there's been a challenge: getting all the hardware and software pieces in place and ready is difficult and frustrating. The demands on the components of the analogue signal chain are quite stringent in terms of bandwidth, dynamic range, number of bits, and other performance parameters.
The ability to provide the SDR platform has been complicated by need for overlapping discrete channels in the analogue front end (AFE) to support coverage across the very wide RF spectrum of interest, which can span from around 100 MHz to several GHz. As a result, the component cost in board space, number of devices, power dissipation, and in money terms, for a wideband SDR implementation can outweigh any potential SDR benefits. Further, without adequate tools and support, even having that basic development hardware and software is insufficient.
An analogue IC changes the situation
These were the issues Ettus Research, active in SDR for over a decade, faced when they embarked on the design of their Universal Software Radio Peripheral (USRP), a fully integrated, direct-conversion transceiver which can be configured for multiple formats, protocols, and frequencies. Their goal was to produce an efficient, cost-effective, easy-to-use device for a low-cost market.
They also wanted to support MIMO (Multiple Input/Multiple Output), an advanced technique which is getting increasing attention as a path to increased performance, higher data rates, and lower bit error rates. Their primary hardware challenge was meeting the front-end requirements in a single, wideband, high-performance signal chain, since using spectrum-overlapping AFE channels in parallel would have been too complex and costly.
Ettus's need coincided with the availability of an RF IC which is specifically tailored for this SDR requirement, the AD9361 RF Agile Transceiver from Analog Devices. This 10 × 10 mm device with dual independent channels, Figure 1, has user-tunable RF bandwidth from 200 kHz to 56 MHz, and 12-bit resolution, along with other features and performance which are needed to build a signal chain spanning 70 MHz to 6 GHz. Using this component reduced the entire AFE into a single, relatively simple circuit.
Figure 1: The AD9361 from Analog Devices is optimised for the user-tunable wideband, low noise needs of a flexible SDR design.
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