Understanding SDRs and their RF Test Requirements, Part 1

Moving up in frequency, ultrahigh-frequency (UHF) radios span 225 to 512 MHz as part of both terrestrial line-of-sight (LOS) links and satellite-communication (satcom) systems. To use smaller antennas in tactical designs, UHF radios require relatively high-power transmit amplifiers and low-noise amplifiers (LNAs) prior to the receive electronics. Guidelines for these requirements are listed in MIL-STD-188-181B. Often, directional antennas are employed with UHF radios to increase system gain and data rates on both receive and transmit links. For high data throughput, UHF radio hardware must provide fast transmit-to-receive switching and fast frequency hop rates, requiring the use of agile frequency synthesizers such as DDS sources. Often the choice of radio synthesizer is a tradeoff between switching speed and phase noise.

SDR Architecture Three types of radios (HF, VHF, and UHF), along with coverage of higher frequencies, are encompassed in the definition of an SDR. While current hardware SDR designs vary (covering the entire 2 to 2000 MHz band continuously or splitting the band into two switched segments, for example), the software portion of the radio is clearly defined by the Software Communications Architecture (SCA).

In an SDR, the software defines radio operation from the physical layer through higher-level protocol layers. The SCA is an open-architecture framework that aims for the portability, reusability, and scalability of the software and hardware developed under its guidelines to ensure that radios and software from one vendor work with the hardware and software from another vendor.

The SCA was promoted during the development of the JTRS program, and has been adopted by the US military as the standard software radio architecture for military communications systems. The SCA will likely be the guiding force for compatibility in commercial SDR designs as well. For example, the SCA now forms the basis of the SDR Forum's Software Radio Architecture (SRA).

The current SCA specification includes two methods for communications between SDR modules: using the middleware Common Object Request Broker Architecture (CORBA) and the Hardware Abstraction Layer (HAL) for high-demand communications between embedded hardware. CORBA interfaces are defined using Interface Definition Language (IDL) code, which is programming language independent and can be compiled into programming languages such as C++.

The SCA ultimately provides an Operating Environment (OE) for the SDR system. The OE combines the set of Core Framework (CF) services, interfaces, board support packages, operating system, and middleware services to host an SCA application. A typical SDR waveform includes all radio functions from the user input to the RF output: the combination of components, interconnections, and software needed to make the SDR behave in a certain way.

An ideal SDR receiver would digitize signals from the antenna so that received information spends the majority of its time in the digital realm. But current analog-to-digital converter (ADC) technology lacks the combination of bandwidth and bit resolution needed for this "direct-to-digital" receiver architecture. As a result, the analog front ends of SDRs still resemble the superheterodyne architectures of their analog HF, VHF, and UHF counterparts, albeit with the possibility of all three bands being handled by a single set of components (Figure 2).

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Analog signals are downconverted in frequency in an SDR's receiver front end, then converted to a digital IF using an ADC. Switchable analog filters select a desired radio channel, but filtering and signal processing at IF and baseband are implemented by means of digital signal processing (DSP) and sharply defined digital filters to remove images and interference. In the transmitter, digital baseband/IF signals are converted to the analog realm by means of digital-to-analog converters (DACs) and subsequently translated to the desired transmit frequencies by means of analog frequency upconverters.

References
1. Quoted from sdrforum web site at http://www.sdrforum.org/pages/brussels_05_drop_box/yearbook/9780000000002text.001.pdf

About the Author
Eric Hakanson is a Product Marketing Manager in the Microwave Measurements Division of Anritsu Company. He has worked in the Test & Measurement industry for over 25 years, in various marketing roles. Eric received his Bachelor of Science in Electrical Engineering from Virginia Tech.

Part 2 examines Test Challenges, Sample SDRs, and Test Hardware