SYSTEM DEVELOPMENT
Precision Clocks used in Software Defined Radios (SDRs) By Brendon McHugh, Per Vices Introduction It is no secret that digital electronics dominates almost every aspect of modern technology, from logical computation to signal processing and radio frequency (RF) applications. In fact, most of the RF systems being developed today have already replaced most of the onboard analog functions with software-based elements, which provides much more flexibility, reconfigurability, and immunity to environmental conditions. The fundamental component of digital-based RF systems is the software-defined radio (SDR). SDRs are mostly based on digital electronics and therefore carries the burden of being completely dependent on a clock system to regulate timing, synchronicity, analog-to-digital conversion, and speed of digital functions. Robust clocking systems found on time boards in these SDRs are especially critical in multiple-input multiple-output (MIMO) applications, which have to deal with a huge amount of very fast data transmission. Oven-controlled crystal oscillators (OCXO) are the foundation of precise timing solutions, providing a very stable and accurate clock signal, using a temperature-controlled chamber to maintain the quartz temperature constant, thus preventing frequency drifts due to environmental conditions. In an SDR, the OCXO is responsible for defining the frequency of radio transmitters, base stations, test and measurement devices, and military applications with tremendous accuracy. In this article, we discuss how OCXOs are applied in high-end SDRs, providing up to 10 MHz clock signals, and how this clock is distributed over the many components of the SDR, including phase-locked loops (PLL), analog to digital convertor/digital to analog convertor (ADC/ DAC) JESD interfaces, frequency synthesizers, and the field programmable array (FPGA) in the digital domain. The OCXO signal can also be outputted for external synchronization with other instruments. We will discuss how the timing solutions are calibrated to ensure a known phase and deterministic behavior of the clock 20
COTS Journal | June 2022
signal, improving the performance of the SDR. We will conclude with an overview about how the precision clock allows the many functionalities of SDR, including up/down-conversion, MIMO synchronicity, and frequency tuning mechanisms. What is an SDR? The term SDR refers to any RF device that has most of its signal processing and managing system based on software. The motivation for using SDRs instead of conventional analog radios is flexibility; the flexibility of moving functionality into the software domain enabling easier implementation of digital signal processing (DSP) functions in software-based systems, such as PCs and FPGAs when compared to customized hardware. This allows for a commercial-off-theshelf (COTS) SDR to be utilized in a wide variety of applications because of the flexibility and re-
Figure 1 - SDRs with the time board
configurability of the digital board. SDRs come in different size, weight, and power (SWaP) configurations, ranging from small and portable low-power transceivers to immense high-power base stations. The basic architecture of an SDR consists of a radio front-end (RFE) and a digital backend (DBE). The RFE is responsible for both the receive (Rx) and transmit (Tx) functions, often providing more than one channel in MIMO SDRs. The channels operate with a very wide tuning range with very high bandwidths; the highest performance SDRs operate across a range from near DC to 18GHz with up to 3 GHz of instantaneous bandwidth and multiple channels, each one with a dedicated ADC/DAC interface. The digital backend consists of an FPGA, which reads the amplified and filtered received signals from the Rx RFE and can generate trans-
