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RF Test Equipment Series: Software Defined Radios

Software Defined Radios (SDRs) were once a poorly understood niche device, and have essentially overtaken the architecture of most radio devices, superseding hardware defined radios (HDRs). Most common types of modern test equipment have some form of SDR capability, meaning that at least some of their physical-layer functions are determined by software instead of hardware. Moreover, this evolution has bread a new class of test and measurement equipment, also commonly referred to as SDRs.

Though the RF front-end (RFFE) hardware of SDRs is still determined by hardware parameters, digital-based RF synthesis and sampling circuits enable RF signals to be digital generated, sampled, and processed. This feature removes the need for hardware-based modulation/demodulation, filtering, amplification, and even frequency translation. With additional software-defined processing, such as with digital signal processors (DSPs), field-programmable gate arrays (FPGAs), or application-specific integrated circuits (ASICs), complex modulation/demodulation and signal analysis can be done in real-time or signal data can be stored and processed more intensely later with CPUs and GPUs.

Having physical-layer functions defined by software is extremely useful in test and measurement settings where it may be beneficial to test components, devices, and assemblies in nearly real-world signal conditions. This is especially true with modern and extremely complex communication protocols, where actual device or system performance may be poorly represented by testing with signals that aren’t comparable to real-world use cases. This also applies to modern radar and signal intelligence (SIGINT), electronic warfare (EW), and other critical applications where a precise understanding of a device or system performance under specific conditions is necessary.

Many SDRs are also implemented on modular platforms, where the RFFE hardware can be swapped, or customized, to enable operation at other frequencies, wider bandwidths, or following other criteria. In this case, the only necessary changed with the SDR is to connect the new RFFE hardware and modify the SDR software to facilitate the new frequencies and RF signal specifics.

Like other types of RF test equipment, SDRs are often accessorized with external adapters, attenuators, couplers, coaxial cables, and antennas, depending upon the testing applications. For over-the-air (OTA) type testing were multiple SDRs may be used to prototype a communications platform, large antennas with power amplifiers, low noise amplifiers, attenuators, and high gain antennas may be used. In the case of using an SDR with much higher frequencies than its RF synthesis and sampling circuitry can handle, an external mixer may be used to upconverter or downconvert DUT frequencies.

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