Direct RF Facilitates Wideband Signal Conversion Without Multiple RF Tuners or Upconverters

Direct RF is quickly becoming the technology of choice for electronic warfare, communication systems architecture and radar applications at the edge. As direct RF continues to evolve, it will shape the architecture of RF acquisition, processing and software-defined radio and communications. It will affect A/D and D/A roadmaps, approaches for high-speed data movement and processing solutions, all of which are required to match the high-bandwidth data common with direct RF converters.

Direct RF Architecture

Coupling new wideband Direct RF signal data converters with the latest FPGAs affords significant system performance advantages over previous architectures. Unlike traditional systems that use costly analog frequency conversion hardware, Direct RF technology allows the direct processing of broadband signals. In a traditional architecture, after the receiver receives the signal at RF frequencies, it downconverts the signal to a lower intermediate frequency (IF) where it is digitized, filtered and then demodulated. The RF front end involved consists of a bandpass filter, low noise amplifier, mixer and local oscillator (LO).

A Direct RF sampling receiver architecture consists of a low-noise amplifier, the appropriate filters, and the ADC. Instead of using mixers and LOs, the ADC digitizes the RF signal directly and sends it to a processor. This streamlined architecture reduces the total system size and cost while increasing flexibility. This can be important when building systems such as fully active phased-array radars, which form beams by phase shifting the signals emitted from up to hundreds or more antennas. With so many RF signal generators and analyzers in the same system, size and cost per channel become an important factor.

Prior to the advancement of converter technologies in recent years, direct sampling architectures weren’t practical because of the limitations in converter sample rates and resolution. Semiconductor companies have been able to expand resolution at higher sampling frequencies using new techniques to reduce noise within the converter. With the availability of much higher-speed converters featuring increased resolution, you can now directly convert RF input signals up to several gigahertz.

Because most RF input signal frequencies far exceeded the capabilities of early analog-to digital converters (ADCs), software radios required an RF tuner stage to translate RF signals to lower IF frequencies before they could be digitized. With ADCs fast enough to digitize RF signal frequencies directly, the RF tuner section can be eliminated, resulting in the direct RF receiver. This dramatically shrinks the size of the electronics housing so it can often fit directly behind the antenna, a major trend in new system architectures.

In addition to size, weight and power reduction, the simplified architecture removes potential sources of noise, images and other errors, such as LO leakage within the RF instrument itself. Because each element needs its own unique transmit and receive signal, direct RF ADCs and DACs also significantly reduce the system cost of phased-array systems by eliminating the RF frequency translation stages in each signal channel.

Direct RF Data Converters

The industry is resonating with a new generation of Direct RF data converters. Most with sampling rates above 10 GS/sec are available as discrete packaged devices or as silicon die known as “chiplets,” suitable for attaching directly to other die in a multichip module.

Analog Devices offers its Apollo MxFE (mixed-signal front end) family of direct RF ADCs and DACs. A member of this series is the AD9084 featuring four 20 GS/sec, 12-bit ADCs and four 28 GS/sec, 16-bit DACs using monolithic 16 nm CMOS technology. FPGA compute can be offloaded to the Apollo MxFE on-chip digital signal processing (DSP) circuit saving size, weight, power and cost. The DSP is all dynamically reconfigurable without requiring recalibration, offering a highly flexible platform to enable innovations in aerospace and defense, instrumentation, and communications.

Jariet Technologies line-up includes the Electra-MA direct RF data converter chip with two 64 GS/sec, 10-bit ADCs and two 64 GS/sec, 10-bit DACs. With a usable analog bandwidth up to 32 GHz for both transceiver channels, the Electra-MA supports Ka-band applications that are becoming increasingly critical for defense systems.

Intel offers the Stratix 10AX and the Agilex 9. These combine FPGA core die, fabricated with the Intel 14 nm and 10 nm SuperFin manufacturing processes respectively, with function specific and general-purpose I/O tiles using Intel’s chiplet-based semiconductor packaging technologies. The Agilex 9 devices use Intel’s 7-nm process with 2,693 logic elements, over 17k multipliers, 287 Mbits of RAM and PCIe Gen4 interfaces.

AMD’s Versal ACAP (Adaptive Compute Acceleration Platform) devices are based on its 7-nm silicon process and consist of a series of six SoC architectures, each with different processing engines, high bandwidth memory and peripherals. For radar processing, merging vector-based DSP engines with AI engines in a small form factor enables advanced radars such as active electronically scanned arrays.

The Mercury Systems DRF3182 Direct RF Processing Module is purpose built for the aerospace and defense industry. It leverages Intel’s Stratix 10 AX SoC FPGA, which adds a key capability to the Mercury Processing Platform by enabling the direct digitization and processing of broadband RF signals. Mercury Systems also has announced an early access program for its RFS1140 direct RF System-in-Package (SiP). The RFS1140 is said to be the first multi-chip module to combine the processing power of the AMD Versal AI Core series adaptive SoC along with Jariet Technologies’ high speed data converters and Micron LPDDR4 NOR flash memory.

The early access program for the RFS1140 includes product documentation and support as well as priority delivery for evaluation hardware as it becomes available this year.

The FlexRIO IF transceiver is the first NI instrument to take advantage of direct RF sampling. This 12-bit, 6.4 GS/sec, 2-Channel PXI transceiver enables direct RF acquisition up to 6 GHz and generation up to 3 GHz with 12 bits of resolution. It is well-suited for applications that require wideband acquisition and generation with multichannel synchronization, including radar, electronic warfare and communications.

CAES has expanded its product offerings with a new direct-to-digital RF converter designed to meet the bandwidth and form factor requirements of radar, electronic warfare and C4ISR (Command, Control, Communications, Computers (C4) Intelligence, Surveillance and Reconnaissance (ISR)) mission applications.

The CAES TORNADO converter, the latest in CAES’ investment in direct RF sampling architectures, has 8x transmit and 8x receive channels paired with the Intel Agilex 9 FPGA Direct RF Series in a 3U, SOSA-aligned configuration. The Sensor Open Systems Architecture (SOSA) Consortium grew out of a U.S. Department of Defense initiative to define open standard electronic architectures to ensure component interoperability and reduce costs.

The CAES TORNADO converter will be ready for delivery this year.